0001564590-21-054369 | 10-Q | XBRL Viewer

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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2021	Oct. 29, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Trading Symbol	KOP
Entity Registrant Name	KOPPERS HOLDINGS INC.
Entity Central Index Key	0001315257
Entity Current Reporting Status	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		21,306,315
Title of 12(b) Security	Common Stock
Security Exchange Name	NYSE
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity File Number	1-32737
Entity Tax Identification Number	20-1878963
Entity Address, Address Line One	436 Seventh Avenue
Entity Address, City or Town	Pittsburgh
Entity Address, State or Province	PA
Entity Address, Postal Zip Code	15219
City Area Code	412
Local Phone Number	227-2001
Document Quarterly Report	true
Document Transition Report	false
Entity Incorporation, State or Country Code	PA

Condensed Consolidated Statement of Operations and Comprehensive (Loss) Income (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Net sales	$ 424.8	$ 437.5	$ 1,273.3	$ 1,276.0
Cost of sales	348.9	329.8	1,012.1	1,004.8
Depreciation and amortization	13.4	12.9	43.4	39.7
Gain on sale of assets	(0.0)	(0.0)	(7.8)	(0.0)
Impairment and restructuring charges	0.1	1.6	2.2	5.5
Selling, general and administrative expenses	37.8	34.6	110.6	104.1
Operating profit	24.6	58.6	112.8	121.9
Other income, net	0.9	0.9	2.7	1.9
Interest expense	10.2	11.8	30.5	38.6
Income from continuing operations before income taxes	15.3	47.7	85.0	85.2
Income tax provision	4.8	8.6	22.4	14.8
Income from continuing operations	10.5	39.1	62.6	70.4
Income (loss) from discontinued operations, net of tax benefit of $0.0, $0.4, $0.0 and $1.4	0.0	0.6	0.1	(3.8)
(Loss) gain on sale of discontinued operations, net of tax benefit (expense) of $0.2, $(8.3), $0.3, $(8.3)	(0.5)	35.8	0.0	35.8
Net income	10.0	75.5	62.7	102.4
Net loss attributable to noncontrolling interests	(0.2)	(0.1)	(0.3)	(1.0)
Net income attributable to Koppers	$ 10.2	$ 75.6	$ 63.0	$ 103.4
Basic -
Continuing operations	$ 0.50	$ 1.86	$ 2.96	$ 3.37
Discontinued operations	(0.02)	1.73	0.01	1.56
Earnings per basic common share	0.48	3.59	2.97	4.93
Diluted -
Continuing operations	0.49	1.83	2.87	3.33
Discontinued operations	(0.02)	1.70	0.00	1.55
Earnings per diluted common share	$ 0.47	$ 3.53	$ 2.87	$ 4.88
Comprehensive (loss) income	$ (17.4)	$ 103.3	$ 45.7	$ 125.9
Comprehensive (loss) income attributable to noncontrolling interests	(0.2)	0.9	(0.2)	(0.1)
Comprehensive (loss) income attributable to Koppers	$ (17.2)	$ 102.4	$ 45.9	$ 126.0
Weighted average shares outstanding (in thousands):
Basic	21,322	21,047	21,253	20,968
Diluted	21,947	21,380	21,949	21,227

Condensed Consolidated Statement of Operations and Comprehensive (Loss) Income (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Income (loss) from discontinued operations, tax benefit (expense)	$ (0.0)	$ 0.4	$ (0.0)	$ 1.4
Tax benefit (expense) on (Loss) gain on sale of business	$ 0.2	$ (8.3)	$ 0.3	$ (8.3)

- Definition

Amount of tax expense (benefit) related to a discontinued operation. Includes, but is not limited to, tax expense (benefit) related to income (loss) from operations during the phase-out period, tax expense (benefit) related to gain (loss) on disposal, tax expense (benefit) related to gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and tax expense (benefit) related to adjustments of a prior period gain (loss) on disposal.

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+ Details

Condensed Consolidated Balance Sheet - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets
Cash and cash equivalents, including restricted cash (Note 4)	$ 44.9	$ 38.5
Accounts receivable, net of allowance of $3.3 and $2.6	198.5	175.1
Inventories, net	295.1	295.8
Derivative contracts	51.9	38.5
Other current assets	23.8	16.6
Total current assets	614.2	564.5
Property, plant and equipment, net	460.9	409.1
Operating lease right-of-use assets	95.2	102.5
Goodwill	296.1	297.8
Intangible assets, net	135.6	149.8
Deferred tax assets	16.5	18.4
Non-current derivative contracts	10.4	31.9
Other assets	23.3	24.6
Total assets	1,652.2	1,598.6
Liabilities
Accounts payable	159.9	154.1
Accrued liabilities	80.7	106.7
Current operating lease liabilities	21.6	21.2
Current maturities of long-term debt	4.6	10.1
Total current liabilities	266.8	292.1
Long-term debt	802.6	765.8
Accrued postretirement benefits	44.6	46.2
Deferred tax liabilities	20.3	21.3
Operating lease liabilities	73.8	81.3
Other long-term liabilities	43.6	45.9
Total liabilities	1,251.7	1,252.6
Commitments and contingent liabilities (Note 18)
Equity
Senior Convertible Preferred Stock, $0.01 par value per share; 10,000,000 shares authorized; no shares issued	0.0	0.0
Common Stock, $0.01 par value per share; 80,000,000 shares authorized; 23,993,214 and 23,688,347 shares issued	0.2	0.2
Additional paid-in capital	246.2	234.1
Retained earnings	278.8	215.8
Accumulated other comprehensive loss	(33.0)	(15.9)
Treasury stock, at cost, 2,686,899 and 2,589,803 shares	(95.8)	(92.5)
Total Koppers shareholders’ equity	396.4	341.7
Noncontrolling interests	4.1	4.3
Total equity	400.5	346.0
Total liabilities and equity	$ 1,652.2	$ 1,598.6

Condensed Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Accounts receivable, allowance	$ 3.3	$ 2.6
Senior Convertible Preferred Stock, par value	$ 0.01	$ 0.01
Senior Convertible Preferred Stock, shares authorized	10,000,000	10,000,000
Senior Convertible Preferred Stock, shares issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common Stock, shares authorized	80,000,000	80,000,000
Common Stock, shares issued	23,993,214	23,688,347
Treasury stock, shares	2,686,899	2,589,803

Condensed Consolidated Statement of Cash Flows (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Cash provided by (used in) operating activities:
Net income	$ 10.0	$ 75.5	$ 62.7	$ 102.4
Adjustments to reconcile net cash provided by (used in) operating activities:
Depreciation and amortization			43.4	39.7
Stock-based compensation			9.8	8.5
Change in derivative contracts			2.7	(4.2)
Non-cash interest expense			2.0	2.0
Gain on sale of assets			(7.8)	(35.8)
Insurance proceeds			(3.6)	(0.7)
Deferred income taxes			1.0	(3.2)
Change in other liabilities			3.8	(0.5)
Other - net			2.6	1.8
Changes in working capital:
Accounts receivable			(26.7)	(35.3)
Inventories			(4.2)	31.3
Accounts payable			7.7	(43.6)
Accrued liabilities			(29.4)	6.7
Other working capital			(4.4)	(3.6)
Net cash provided by operating activities			59.6	65.5
Cash (used in) provided by investing activities:
Capital expenditures			(87.6)	(43.8)
Insurance proceeds received			3.6	0.7
Cash provided by sale of assets			5.3	78.1
Net cash (used in) provided by investing activities			(78.7)	35.0
Cash provided by (used in) financing activities:
Net increase (decrease) in credit facility borrowings			37.1	(85.6)
Repayments of long-term debt			(7.6)	(7.5)
Issuances of Common Stock			2.1	0.8
Repurchases of Common Stock			(3.3)	(1.2)
Payment of debt issuance costs			(0.0)	(0.2)
Net cash provided by (used in) financing activities			28.3	(93.7)
Effect of exchange rate changes on cash			(2.8)	(0.3)
Change in cash and cash equivalents of discontinued operations held for sale			0.0	0.7
Net increase in cash and cash equivalents			6.4	7.2
Cash and cash equivalents at beginning of period			38.5	32.3	$ 32.3
Cash and cash equivalents at end of period	$ 44.9	$ 39.5	44.9	39.5	$ 38.5
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash outflow from operating leases			23.2	23.6
Supplemental disclosure of non-cash investing and financing activities:
Right-of-use assets obtained in exchange for new operating lease liabilities			10.7	5.9
Non-cash investing activities
Accrued capital expenditures			$ 8.6	$ 3.7

Basis of Presentation

9 Months Ended

Sep. 30, 2021

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Basis of Presentation

1. Basis of Presentation

The accompanying unaudited condensed consolidated financial statements and related disclosures have been prepared in accordance with accounting principles generally accepted in the United States applicable to interim financial information and with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by generally accepted accounting principles for complete financial statements. In the opinion of management, all adjustments considered necessary for a fair presentation of Koppers Holdings Inc.’s and its subsidiaries’ (“Koppers”, “Koppers Holdings”, the “Company”, “we” or “us”) financial position and interim results as of and for the periods presented have been included. All such adjustments are of a normal recurring nature unless disclosed otherwise. Because our business is seasonal, results for interim periods are not necessarily indicative of those that may be expected for a full year. The Condensed Consolidated Balance Sheet as of December 31, 2020 has been summarized from the audited balance sheet contained in the Annual Report on Form 10-K as of and for the year ended December 31, 2020. Certain prior period amounts in the consolidated financial statements and notes to the consolidated financial statements have been reclassified to conform to the current period’s presentation as a result of reporting discontinued operations. See Note 4 – “Discontinued Operations.”

The financial information included herein should be read in conjunction with our audited consolidated financial statements and related notes included in our Annual Report on Form 10-K for the year ended December 31, 2020.

COVID-19 Assessment

In March 2020, the World Health Organization categorized the current coronavirus disease (“COVID-19”) as a pandemic. COVID-19 continues to impact the United States and other countries across the world, and the duration and ultimate severity of its effects are currently unknown. This current level of uncertainty over the economic and operational impacts of COVID-19 means the related future financial impact cannot be reasonably estimated at this time. Our condensed consolidated financial statements presented herein reflect certain estimates and assumptions made by management that affect the reported amounts of assets and liabilities and disclosure of such assets and liabilities at the date of the condensed consolidated financial statements and reported amounts of revenue and expenses during the reporting periods presented.

Such estimates and assumptions affect, among other things, our goodwill, long-lived asset and intangible asset valuation; inventory valuation; assessment of the annual effective tax rate; valuation of deferred income taxes; the allowance for doubtful accounts; and measurement of cash incentive plans. Events and changes in circumstances arising after September 30, 2021, including those resulting from the impacts of COVID-19, will be reflected in management’s estimates for future periods.

New Accounting Pronouncements

9 Months Ended

Sep. 30, 2021

Accounting Changes And Error Corrections [Abstract]

New Accounting Pronouncements

2. New Accounting Pronouncements

In March 2020, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2020-04: Reference Rate Reform (Topic 848) Facilitation of the Effects of Reference Rate Reform on Financial Reporting. This update provides temporary optional expedients and exceptions to U.S. GAAP on contract modifications, hedging relationships, and other transactions affected by reference rate reform to ease entities' financial reporting burdens as the market transitions from the London Interbank Offered Rate (LIBOR) and other interbank offered rates to alternative reference rates. The guidance was effective upon issuance and may be applied prospectively to contract modifications made, hedging relationships entered into, and other transactions affected by reference rate reform, evaluated on or before December 31, 2022, beginning during the reporting period in which the guidance has been elected. Our debt agreements include the use of alternate rates when LIBOR is not available and we do not maintain hedging relationships applicable to this ASU. We do not expect the application of this update to have a material impact on our financial statements and, to the extent we enter into modifications of agreements that are impacted by the LIBOR phase-out, we will apply such guidance to those contract modifications.

- Definition

The entire disclosure for change in accounting principle. Includes, but is not limited to, nature, reason, and method of adopting amendment to accounting standards or other change in accounting principle.

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-Name Accounting Standards Codification
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Reference 33: http://www.xbrl.org/2003/role/disclosureRef
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Reference 34: http://www.xbrl.org/2003/role/disclosureRef
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-Publisher FASB
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+ Details

Plant Closures and Divestitures

9 Months Ended

Sep. 30, 2021

Discontinued Operations And Disposal Groups [Abstract]

Plant Closures and Divestitures

3. Plant Closures and Divestitures

Over the past seven years, we have been restructuring our Carbon Materials and Chemicals (“CMC”) segment in order to concentrate our facilities in regions where we believe we hold key competitive advantages to better serve our global customers. Recent closure activities include:

	•	In June 2021, we sold a subsidiary related to our closed facility located in Uithoorn, the Netherlands and we recorded a gain on sale of $0.3 million. In April 2014, we had ceased coal tar distillation activities at the facility.
	•	In February 2021, we sold our closed Follansbee, West Virginia coal tar distillation facility and we recorded a gain on sale of $5.7 million, consisting of $2.6 million from cash proceeds in addition to the assumption of certain liabilities by the buyer.
	•	In September 2020, we sold Koppers (Jiangsu) Carbon Chemical Company Limited (“KJCC”). Refer to Note 4 – “Discontinued Operations” for more details.
	•	In October 2018, we sold our closed Clairton, Pennsylvania coal tar distillation facility. In the first quarter of 2021, certain post-sale conditions were achieved and the buyer of the property released cash held in escrow to us resulting in a gain on sale of $1.8 million.

Other closure and divestiture activity relates to our Railroad Utility Products and Services (“RUPS”) segment. Most recently, we discontinued production activities at our crosstie treating plant located in Denver, Colorado in the third quarter of 2020. In October 2021, we sold the facility and will recognize a gain on the transaction of approximately $23 million during the three months ended December 31, 2021.

Details of the restructuring activities and related reserves are as follows:

	Severance and employee benefits			Asset Retirement			Other			Total
(Dollars in millions)
Reserve at December 31, 2019	$	0.9		$	0.7		$	2.4		$	4.0
Accrual		0.5			2.9			3.4			6.8
Cost charged against assets		0.0			0.0			(3.4	)		(3.4	)
Reversal of accrued charges		(0.3	)		0.0			0.0			(0.3	)
Cash paid		(0.2	)		(0.8	)		(0.3	)		(1.3	)
Currency translation		0.0			0.0			0.2			0.2
Reserve at December 31, 2020	$	0.9		$	2.8		$	2.3		$	6.0
Accrual		0.0			0.0			2.4			2.4
Cost charged against assets		0.0			0.0			(2.4	)		(2.4	)
Reversal of accrued charges		(0.2	)		(0.7	)		0.0			(0.9	)
Cash paid		(0.7	)		(1.9	)		0.0			(2.6	)
Sale of subsidiary		0.0			(0.2	)		(2.3	)		(2.5	)
Reserve at September 30, 2021	$	0.0		$	0.0		$	0.0		$	0.0

Discontinued Operations

9 Months Ended

Sep. 30, 2021

Discontinued Operations And Disposal Groups [Abstract]

Discontinued Operations

4. Discontinued Operations

On September 30, 2020, we sold KJCC to Fangda Carbon New Material Co., Ltd and C-Chem Co., Ltd., a subsidiary of Nippon Steel Chemical & Material Co., Ltd. KJCC was located in Pizhou, Jiangsu Province, China and was a 75 percent-owned coal tar distillation company which was part of our CMC segment. Included in the cash proceeds is restricted cash of $2.3 million which is being held in an escrow account and is recorded within cash and cash equivalents as of September 30, 2021 to cover potential customary indemnity claims by the buyers for a remaining period of six months. In addition, an amount of $6.1 million and $5.6 million is recorded in accrued liabilities as of September 30, 2021 and December 31, 2020, respectively, in anticipation of final post-closing working capital adjustments payable to the buyers and related withholding taxes. This final adjustment was paid to the buyers in October 2021.

The sale of KJCC represented a strategic shift that had a major effect on our operations and financial results in 2020 and were, therefore, classified as discontinued operations in our condensed consolidated financial statements.

Net sales and operating loss from discontinued operations for the three and nine months ended September 30, 2020 consisted of the following amounts:

	Three Months Ended September 30,		Nine Months Ended September 30,
	2020		2020
(Dollars in millions)
Net sales	$	8.8	$	31.6
Operating income (loss)		0.3		(5.0	)

In addition, we ceased carbon black production at our CMC facility located in Kurnell, Australia during 2011. Costs associated with this closure are also included in income (loss) from discontinued operations on the condensed consolidated statement of operations and comprehensive (loss) income.

Fair Value Measurements

9 Months Ended

Sep. 30, 2021

Fair Value Disclosures [Abstract]

Fair Value Measurements

5. Fair Value Measurements

Carrying amounts and the related estimated fair values of our financial instruments as of September 30, 2021 and December 31, 2020 are as follows:

	September 30, 2021				December 31, 2020
	Fair Value		Carrying Value		Fair Value		Carrying Value
(Dollars in millions)
Financial assets:
Investments and other assets	$	1.2	$	1.2	$	1.2	$	1.2
Financial liabilities:
Long-term debt (including current portion)	$	828.5	$	813.5	$	799.2	$	784.2

Investments and other assets – Represents the broker-quoted cash surrender value on universal life insurance policies. This asset is classified as Level 2 in the valuation hierarchy and is measured from values received from financial institutions.

Debt – The fair value of our long-term debt is estimated based on the market prices for the same or similar issuances or on the current rates offered to us for debt of the same remaining maturities (Level 2). The fair value of our Credit Facility approximates carrying value due to the variable rate nature of this instrument.

Comprehensive (Loss) Income and Equity

9 Months Ended

Sep. 30, 2021

Equity [Abstract]

Comprehensive (Loss) Income and Equity

6. Comprehensive (Loss) Income and Equity

Total comprehensive (loss) income for the three and nine months ended September 30, 2021 and 2020 is summarized in the table below:

	Three Months Ended September 30,					Nine Months Ended September 30,
	2021			2020		2021			2020
(Dollars in millions)
Net income	$	10.0		$	75.5	$	62.7		$	102.4
Changes in other comprehensive (loss) income:
Currency translation adjustment		(11.1	)		12.3		(14.5	)		3.1
Unrealized (loss) gain on cash flow hedges, net of tax benefit (expense) of $6.1, $(5.3), $1.2 and $(6.9)		(16.6	)		15.3		(3.3	)		19.8
Unrecognized pension net loss, net of tax expense of $0.1, $0.1, $0.3 and $0.2		0.3			0.2		0.8			0.6
Total comprehensive (loss) income		(17.4	)		103.3		45.7			125.9
Comprehensive (loss) income attributable to noncontrolling interests		(0.2	)		0.9		(0.2	)		(0.1	)
Comprehensive (loss) income attributable to Koppers	$	(17.2	)	$	102.4	$	45.9		$	126.0

Amounts reclassified from accumulated other comprehensive loss to net income consist of amounts shown for changes in or amortization of unrecognized pension net loss. This component of accumulated other comprehensive loss is included in the computation of net periodic pension cost as disclosed in Note 13 – “Pensions and Post-Retirement Benefit Plans.” Other amounts reclassified from accumulated other comprehensive loss related to derivative financial instruments, net of tax, of $11.1 million and $31.0 million for the three and nine months ended September 30, 2021, respectively, and $1.3 million and $2.1 million for the three and nine months ended September 30, 2020, respectively.

The following tables present the change in equity for the three months ended September 30, 2021 and 2020, respectively:

(Dollars in millions)	Common Stock		Additional Paid-In Capital		Retained Earnings			Accumulated Other Comprehensive Loss			Treasury Stock			Noncontrolling Interests			Total Equity
Balance at June 30, 2021	$	0.2	$	242.7	$	268.8		$	(5.5	)	$	(94.4	)	$	4.3		$	416.1
Net income (loss)		0.0		0.0		10.2			0.0			0.0			(0.2	)		10.0
Issuance of common stock		0.0		0.3		0.0			0.0			0.0			0.0			0.3
Employee stock plans		0.0		3.3		0.0			0.0			0.0			0.0			3.3
Other comprehensive income (loss)
Currency translation adjustment		0.0		0.0		(0.3	)		(11.1	)		0.0			0.0			(11.4	)
Unrealized loss on cash flow hedges		0.0		0.0		0.0			(16.6	)		0.0			0.0			(16.6	)
Unrecognized pension net loss		0.0		0.0		0.0			0.3			0.0			0.0			0.3
Repurchases of common stock		0.0		0.0		0.0			0.0			(1.4	)		0.0			(1.4	)
Balance at September 30, 2021	$	0.2	$	246.2	$	278.8		$	(33.0	)	$	(95.8	)	$	4.1		$	400.5

(Dollars in millions)	Common Stock		Additional Paid-In Capital		Retained Earnings		Accumulated Other Comprehensive Loss			Treasury Stock			Noncontrolling Interests			Total Equity
Balance at June 30, 2020	$	0.2	$	228.0	$	121.6	$	(81.8	)	$	(92.1	)	$	10.4		$	186.3
Net income (loss)		0.0		0.0		75.6		0.0			0.0			(0.1	)		75.5
Issuance of common stock		0.0		0.3		0.0		0.0			0.0			0.0			0.3
Employee stock plans		0.0		2.9		0.0		0.0			0.0			0.0			2.9
Sale of discontinued operations		0.0		0.0		0.0		0.0			0.0			(7.2	)		(7.2	)
Other comprehensive income
Currency translation adjustment		0.0		0.0		0.0		11.3			0.0			1.0			12.3
Unrealized gain on cash flow hedges		0.0		0.0		0.0		15.3			0.0			0.0			15.3
Unrecognized pension net loss		0.0		0.0		0.0		0.2			0.0			0.0			0.2
Balance at September 30, 2020	$	0.2	$	231.2	$	197.2	$	(55.1	)	$	(92.1	)	$	4.1		$	285.5

The following tables present the change in equity for the nine months ended September 30, 2021 and 2020, respectively:

(Dollars in millions)	Common Stock		Additional Paid-In Capital		Retained Earnings		Accumulated Other Comprehensive Loss			Treasury Stock			Noncontrolling Interests			Total Equity
Balance at December 31, 2020	$	0.2	$	234.1	$	215.8	$	(15.9	)	$	(92.5	)	$	4.3		$	346.0
Net income (loss)		0.0		0.0		63.0		0.0			0.0			(0.3	)		62.7
Issuance of common stock		0.0		2.2		0.0		0.0			0.0			0.0			2.2
Employee stock plans		0.0		9.9		0.0		0.0			0.0			0.0			9.9
Other comprehensive income (loss)
Currency translation adjustment		0.0		0.0		0.0		(10.2	)		0.0			0.1			(10.1	)
Cumulative translation adjustment loss on sale of subsidiary		0.0		0.0		0.0		(4.4	)		0.0			0.0			(4.4	)
Unrealized loss on cash flow hedges		0.0		0.0		0.0		(3.3	)		0.0			0.0			(3.3	)
Unrecognized pension net loss		0.0		0.0		0.0		0.8			0.0			0.0			0.8
Repurchases of common stock		0.0		0.0		0.0		0.0			(3.3	)		0.0			(3.3	)
Balance at September 30, 2021	$	0.2	$	246.2	$	278.8	$	(33.0	)	$	(95.8	)	$	4.1		$	400.5

(Dollars in millions)	Common Stock		Additional Paid-In Capital		Retained Earnings		Accumulated Other Comprehensive Loss			Treasury Stock			Noncontrolling Interests			Total Equity
Balance at December 31, 2019	$	0.2	$	221.9	$	93.8	$	(77.7	)	$	(90.9	)	$	11.4		$	158.7
Net income (loss)		0.0		0.0		103.4		0.0			0.0			(1.0	)		102.4
Issuance of common stock		0.0		0.8		0.0		0.0			0.0			0.0			0.8
Employee stock plans		0.0		8.5		0.0		0.0			0.0			0.0			8.5
Sale of discontinued operations		0.0		0.0		0.0		0.0			0.0			(7.2	)		(7.2	)
Other comprehensive income (loss)
Currency translation adjustment		0.0		0.0		0.0		2.1			0.0			1.0			3.1
Unrealized gain on cash flow hedges		0.0		0.0		0.0		19.8			0.0			0.0			19.8
Unrecognized pension net loss		0.0		0.0		0.0		0.6			0.0			0.0			0.6
Repurchases of common stock		0.0		0.0		0.0		0.0			(1.2	)		0.0			(1.2	)
Balance at September 30, 2020	$	0.2	$	231.2	$	197.2	$	(55.1	)	$	(92.1	)	$	4.1		$	285.5

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Earnings per Common Share

9 Months Ended

Sep. 30, 2021

Earnings Per Share [Abstract]

Earnings per Common Share

7. Earnings per Common Share

The computation of basic earnings per common share for the periods presented is based upon the weighted average number of common shares outstanding during the periods. The computation of diluted earnings per common share includes the effect of non-vested nonqualified stock options and restricted stock units assuming such options and stock units were outstanding common shares at the beginning of the period. The effect of antidilutive securities is excluded from the computation of diluted loss per common share, if any.

The following table sets forth the computation of basic and diluted earnings per common share:

	Three Months Ended September 30,					Nine Months Ended September 30,
	2021			2020		2021		2020
(Dollars in millions, except share amounts, in thousands)
Net income attributable to Koppers	$	10.2		$	75.6	$	63.0	$	103.4
Less: Income (loss) from discontinued operations, net of tax		0.0			0.6		0.1		(3.8	)
(Loss) gain on sale of discontinued operations		(0.5	)		35.8		0.0		35.8
Noncontrolling interest related to discontinued operations		0.0			0.1		0.0		0.8
Income from continuing operations attributable to Koppers	$	10.7		$	39.1	$	62.9	$	70.6
Weighted average common shares outstanding:
Basic		21,322			21,047		21,253		20,968
Effect of dilutive securities		625			333		696		259
Diluted		21,947			21,380		21,949		21,227
Earnings per common share – continuing operations:
Basic earnings per common share	$	0.50		$	1.86	$	2.96	$	3.37
Diluted earnings per common share		0.49			1.83		2.87		3.33
Other data:
Antidilutive securities excluded from computation of diluted earnings per common share		563			876		437		862

Stock-based Compensation

9 Months Ended

Sep. 30, 2021

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

8. Stock-based Compensation

We have outstanding stock-based compensation awards that were granted under the amended and restated 2005 Long-Term Incentive Plan (the “2005 LTIP”), the 2018 Long-Term Incentive Plan (the “2018 LTIP”) and the 2020 Long-Term Incentive Plan, as amended (the “2020 LTIP”). The 2005 LTIP, the 2018 LTIP and the 2020 LTIP are collectively referred to as the “LTIP”. The LTIP provides for the grant to eligible persons of stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares, performance awards, dividend equivalents and other stock-based awards, which are collectively referred to as the “awards.” On May 6, 2021, the shareholders approved an amendment to our 2020 LTIP and an Amended and Restated Employee Stock Purchase Plan to increase the number of shares available for grant by 1,500,000 and 300,000, respectively.

Restricted Stock Units and Performance Stock Units

Under the LTIP, the board of directors grants restricted stock units and performance stock units to certain employee participants (collectively, the “stock units”). Compensation expense for non-vested stock units is recorded over the vesting period based on the fair value at the date of grant. The fair value of restricted stock units is the market price of the underlying common stock on the date of grant and the fair value of performance stock units is determined using a Monte Carlo valuation model. For grants to most employees, the restricted stock units vest in four equal annual installments. Restricted stock units that have one-year vesting periods are also issued under the LTIP to members of the board of directors in connection with annual director compensation and, from time to time, are issued to employees in connection with employee compensation with vesting periods of typically two years or less.

Performance stock units have vesting based upon a market condition. These performance stock units have multi-year performance objectives and a three-year period for vesting (if the applicable performance objectives are achieved). The applicable performance objective is based on our total shareholder return relative to the Standard & Poor’s SmallCap 600 Materials Index. The number of performance stock units granted represents the target award and participants have the ability to earn between zero and 200 percent of the target award based upon actual performance. If minimum performance criteria are not achieved, no performance stock units will vest. We have the discretion to settle the award in cash rather than shares, although we currently expect that all awards will be settled by the issuance of shares.

We calculated the fair value of the performance stock unit awards on the date of grant using the assumptions listed below:

	January 2021 Grant			March 2020 Grant			March 2019 Grant
Grant date price per share of performance award	$	29.84		$	19.63		$	26.63
Expected dividend yield per share		0.00	%		0.00	%		0.00	%
Expected volatility		68.70	%		45.60	%		39.00	%
Risk-free interest rate		0.16	%		0.72	%		2.50	%
Look-back period in years		3.00			2.83			2.82
Grant date fair value per share	$	41.50		$	11.56		$	40.30

Dividends declared, if any, on our common stock during the period prior to vesting of the stock units are credited at equivalent value as additional stock units and become payable as additional common shares upon vesting. In the event of termination of employment, other than retirement, death or disability, any non-vested stock units are forfeited, including additional stock units credited from dividends. In the event of termination of employment due to retirement, death or disability, pro-rata vesting of the stock units over the service period will result. There are special vesting provisions for the stock units related to a change in control.

The following table shows a summary of the performance stock units as of September 30, 2021:

Performance Period	Minimum Shares		Target Shares		Maximum Shares
2019 – 2021		0		234,597		281,330
2020 – 2022		0		154,099		308,198
2021 – 2023		0		149,874		299,748

The following table shows a summary of the status and activity of non-vested stock units for the nine months ended September 30, 2021:

	Restricted Stock Units			Performance Stock Units			Total Stock Units			Weighted Average Grant Date Fair Value per Unit
Non-vested at December 31, 2020		509,509			391,744			901,253		$	25.48
Granted		222,299			149,874			372,173		$	34.91
Performance share adjustment		0			(1,296	)		(1,296	)	$	44.29
Vested		(199,837	)		(1,821	)		(201,658	)	$	22.52
Forfeited		(690	)		(103	)		(793	)	$	30.94
Non-vested at September 30, 2021		531,281			538,398			1,069,679		$	29.23

Stock Options

Stock options to executive officers vest and become exercisable in four equal annual installments. The stock options have a term of ten years. In the event of termination of employment, other than retirement, death or disability, any non-vested options are forfeited. In the event of termination of employment due to retirement, death or disability, pro-rata vesting of the options over the service period will result. There are special vesting provisions for the stock options related to a change in control.

Compensation expense for non-vested stock options is recorded over the vesting period based on the fair value at the date of grant. We calculated the fair value of stock options on the date of grant using the Black-Scholes-Merton model and the assumptions listed below:

	January 2021 Grant			March 2020 Grant			March 2019 Grant			March 2018 Grant
Grant date price per share of stock option award	$	29.84		$	19.63		$	26.63		$	41.60
Expected dividend yield per share		0.00	%		0.00	%		0.00	%		0.00	%
Expected life in years		6.64			6.40			6.14			5.73
Expected volatility		54.80	%		42.85	%		39.44	%		37.05	%
Risk-free interest rate		0.59	%		0.87	%		2.53	%		2.67	%
Grant date fair value per share of option awards	$	15.79		$	8.42		$	11.29		$	16.38

We do not expect to declare any dividends for the foreseeable future. The expected life in years is based on historical exercise data of options previously granted by us. Expected volatility is based on the historical volatility of our common stock and the historical volatility of certain other similar public companies. The risk-free interest rate is based on U.S. Treasury bill rates for the expected life of the option.

The following table shows a summary of the status and activity of stock options for the nine months ended September 30, 2021:

	Options			Weighted Average Exercise Price per Option		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in millions)
Outstanding at December 31, 2020		1,120,254		$	26.89
Granted		90,879		$	29.84
Exercised		(71,785	)	$	17.74
Expired		(25,402	)	$	40.26
Outstanding at September 30, 2021		1,113,946		$	27.41		5.21	$	7.8
Exercisable at September 30, 2021		797,911		$	28.16		4.08	$	5.7

Stock Compensation Expense

Total stock-based compensation expense recognized under our LTIP and employee stock purchase plan for the three and nine months ended September 30, 2021 and 2020 is as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
(Dollars in millions)
Stock-based compensation expense recognized:
Selling, general and administrative expenses	$	3.2	$	2.9	$	9.8	$	8.5
Less related income tax benefit		0.8		0.7		2.6		2.2
Decrease in net income attributable to Koppers	$	2.4	$	2.2	$	7.2	$	6.3
Intrinsic value of exercised stock options	$	0.0	$	0.0	$	2.2	$	0.0
Cash received from the exercise of stock options	$	0.0	$	0.0	$	2.3	$	0.0

As of September 30, 2021, total future compensation expense related to non-vested stock-based compensation arrangements is expected to total $19.5 million and the weighted-average period over which this expense is expected to be recognized is approximately 27 months.

Segment Information

9 Months Ended

Sep. 30, 2021

Segment Reporting [Abstract]

Segment Information

9. Segment Information

We have three reportable segments: Railroad and Utility Products and Services, Performance Chemicals (“PC”) and Carbon Materials and Chemicals. Our reportable segments contain multiple aggregated business units since management believes the long-term financial performance of these business units is affected by similar economic conditions. The reportable segments are each managed separately because they manufacture and distribute distinct products with different production processes.

Our RUPS segment sells treated and untreated wood products, manufactured products and services primarily to the railroad and public utility markets. Railroad products and services include procuring and treating items such as crossties, switch ties and various types of lumber used for railroad bridges and crossings and the manufacture of rail joint bars. Utility products include transmission and distribution poles and pilings. The segment also operates a railroad services business that conducts engineering, design, repair and inspection services for railroad bridges, a business related to the recovery of used crossties and a business related to the inspection of utility poles.

Our PC segment develops, manufactures, and markets wood preservation chemicals and wood treatment technologies and services a diverse range of end-markets including infrastructure, residential and commercial construction, and agriculture.

Our CMC segment is primarily a manufacturer of creosote, carbon pitch, naphthalene, phthalic anhydride and carbon black feedstock. Creosote is used in the treatment of wood and carbon black feedstock is used in the production of carbon black. Carbon pitch is used in the production of aluminum and steel in electric arc furnaces. Naphthalene is used for the production of phthalic anhydride and as a surfactant in the production of concrete. Phthalic anhydride is used in the production of plasticizers, polyester resins and alkyd paints.

We evaluate performance and determine resource allocations based on a number of factors, including earnings before interest, taxes, depreciation and amortization (“EBITDA”) and operating profit or loss from operations. Operating profit or loss does not include other income or loss, interest expense, income taxes or operating costs of Koppers Holdings Inc.

Contract Balances

The timing of revenue recognition results in both billed accounts receivable and unbilled receivables, both classified as accounts receivable, net of allowance within the condensed consolidated balance sheet. Contract assets of $13.3 million and $5.8 million are recorded within accounts receivable, net of allowance within the condensed consolidated balance sheet as of September 30, 2021 and December 31, 2020, respectively.

The following table sets forth certain sales and operating data, net of all intersegment transactions, for our segments for the periods indicated:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
(Dollars in millions)
Revenues from external customers:
Railroad and Utility Products and Services	$	186.9		$	191.0		$	574.3		$	590.9
Performance Chemicals		115.2			147.9			384.4			396.4
Carbon Materials and Chemicals^(a)		122.7			98.6			314.6			288.7
Total	$	424.8		$	437.5		$	1,273.3		$	1,276.0
Intersegment revenues:
Performance Chemicals	$	3.9		$	3.4		$	11.2		$	10.3
Carbon Materials and Chemicals		18.9			20.1			55.2			59.5
Total	$	22.8		$	23.5		$	66.4		$	69.8
Depreciation and amortization expense:
Railroad and Utility Products and Services	$	5.3		$	4.9		$	16.9		$	14.8
Performance Chemicals		4.1			4.3			13.6			13.2
Carbon Materials and Chemicals^(b)		4.0			3.7			12.9			11.7
Total	$	13.4		$	12.9		$	43.4		$	39.7
Operating profit (loss):
Railroad and Utility Products and Services	$	(0.7	)	$	15.0		$	12.3		$	40.4
Performance Chemicals		11.6			30.4			65.1			67.1
Carbon Materials and Chemicals^(c)		14.9			13.7			39.1			15.9
Corporate		(1.2	)		(0.5	)		(3.7	)		(1.5	)
Total	$	24.6		$	58.6		$	112.8		$	121.9

(a)

Revenue excludes KJCC revenue of $8.8 million and $31.6 million for the three and nine months ended September 30, 2020, respectively.

(b)

Depreciation and amortization expense excludes KJCC expenses of $0.1 million and $0.6 million for the three and nine months ended September 30, 2020, respectively.

(c)

Operating profit (loss) excludes KJCC amounts of $0.3 million and $(5.0) million for the three and nine months ended September 30, 2020, respectively.

The following table sets forth revenues for significant product lines, net of all intersegment transactions, for our segments for the periods indicated:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
(Dollars in millions)
Railroad and Utility Products and Services:
Railroad treated products	$	107.1	$	108.4	$	327.6	$	345.5
Utility poles		59.6		63.6		186.2		191.4
Railroad infrastructure services		8.8		9.2		25.3		23.3
Rail joints		5.7		4.5		17.9		16.9
Other products		5.7		5.3		17.3		13.8
Total		186.9		191.0		574.3		590.9
Performance Chemicals:
Wood preservative products		110.1		140.5		366.4		375.7
Other products		5.1		7.4		18.0		20.7
Total		115.2		147.9		384.4		396.4
Carbon Materials and Chemicals:
Pitch and related products		70.4		62.9		177.7		171.4
Phthalic anhydride and other chemicals		19.7		14.5		52.2		49.8
Creosote and distillates		19.0		10.3		48.6		33.8
Naphthalene		6.2		3.8		15.5		12.7
Other products		7.4		7.2		20.6		21.0
Total		122.7		98.6		314.6		288.7
Total	$	424.8	$	437.5	$	1,273.3	$	1,276.0

The following table sets forth assets and goodwill allocated to each of our segments as of the dates indicated:

	September 30, 2021		December 31, 2020
(Dollars in millions)
Segment assets:
Railroad and Utility Products and Services	$	590.9	$	583.1
Performance Chemicals		586.6		536.1
Carbon Materials and Chemicals		445.2		424.2
All other		29.5		55.2
Total	$	1,652.2	$	1,598.6
Goodwill:
Railroad and Utility Products and Services	$	120.8	$	121.1
Performance Chemicals		175.3		176.7
Total	$	296.1	$	297.8

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

+ References

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+ Details

Income Taxes

9 Months Ended

Sep. 30, 2021

Income Tax Disclosure [Abstract]

Income Taxes

10. Income Taxes

Effective Tax Rate

The income tax provision for interim periods is comprised of an estimated annual effective income tax rate applied to current year ordinary income and tax associated with discrete items. These discrete items generally relate to excess stock compensation deductions, changes in tax laws, adjustments to unrecognized tax benefits and changes of estimated tax liability to the actual liability determined upon filing income tax returns. To determine the annual effective tax rate, management is required to make estimates of annual pretax income in each domestic and foreign jurisdiction in which we conduct business. Entities that have historical pre-tax losses and current year estimated pre-tax losses that are not projected to generate a future benefit are excluded from the estimated annual effective income tax rate.

The estimated annual effective income tax rate, excluding discrete items discussed above, was 26.8 percent and 25.8 percent for nine months ended September 30, 2021 and 2020, respectively. The estimated annual effective income tax rate differs from the U.S. federal statutory tax rate due to:

	September 30,
	2021			2020
Federal income tax rate		21.0	%		21.0	%
Foreign earnings taxed at different rates		3.6			2.7
Nondeductible expenses		1.3			1.3
State income taxes, net of federal tax benefit		0.8			0.5
Change in tax contingency reserves		0.1			0.1
GILTI inclusion, net of foreign tax credits		0.0			0.2
Estimated annual effective income tax rate		26.8	%		25.8	%

Income taxes as a percentage of pretax income were 31.4 percent for the three months ended September 30, 2021. This is higher than the estimated annual effective income tax rate primarily due to an increase in the estimated annual effective income tax rate when compared to the previous quarter’s estimate.

Income taxes as a percentage of pretax income were 18.0 percent for the three months ended September 30, 2020. This was lower than the estimated annual effective income tax rate due to discrete items, which were a net benefit of $3.1 million for the three months ended September 30, 2020. Discrete items were primarily related to the legislative changes and finalized regulations regarding the allowable business interest expense deduction that is discussed below and a benefit due to an amended tax return.

Income taxes as a percentage of pretax income were 26.4 percent for the nine months ended September 30, 2021. This is lower than the estimated annual effective income tax rate due to several discrete items, none of which are material.

Income taxes as a percentage of pretax income were 17.4 percent for the nine months ended September 30, 2020. This was lower than the estimated annual effective income tax rate due to discrete items, which were a net benefit of $7.3 million for the nine months ended September 30, 2020. Discrete items were primarily related to the legislative changes and finalized regulations regarding the limitation on the interest expense deduction that is discussed below and a benefit due to an amended tax return. These discrete items were offset by a tax deduction reduction for vested stock awards.

During 2020, two events occurred which enabled us to adjust our interest expense limitations on our 2018 and 2019 U.S. tax returns. In March 2020, the Coronavirus Aid, Relief, and Economic Security Act (the “CARES Act”) was enacted and a provision of the CARES Act increased the allowable business interest expense deduction to 50 percent of adjusted taxable income retroactively to January 1, 2019. In July 2020, the Internal Revenue Service released regulations that were retroactive to January 1, 2018 and favorably impacted our calculation of adjusted taxable income. After application of these new regulations, the limitation of our interest expense deduction was significantly reduced when compared to the same calculations under the previous regulations. Due to these changes, in the three and nine months ended September 30, 2020 we recorded an income tax expense of $2.0 million and an income tax benefit of $2.4 million, respectively, to adjust a previously recorded valuation allowance for disallowed interest expense deductions that are eligible for carry-forward. After review and application of these changes, we determined that we would be able to fully utilize these disallowed interest expense deductions. Effective January 1, 2021, the limitation on the deduction was restored to 30 percent.

During the year, management regularly updates estimates of pre-tax income and income tax expense based on changes in pre-tax income projections by taxable jurisdiction, repatriation of foreign earnings, unrecognized tax benefits and other tax matters. To the extent that actual results vary from these estimates, the actual annual effective income tax rate at the end of the year could be materially different from the estimated annual effective income tax rate for the three and nine months ended September 30, 2021.

Unrecognized Tax Benefits

We file income tax returns in the U.S. federal jurisdiction, individual U.S. state jurisdictions and non-U.S. jurisdictions. With few exceptions, we are no longer subject to U.S. federal, U.S. state, or non-U.S. income tax examinations by tax authorities for years prior to 2016.

Unrecognized tax benefits totaled $2.3 million and $2.5 million as of September 30, 2021 and December 31, 2020. The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate, was approximately $2.3 million and $2.5 million as of September 30, 2021 and December 31, 2020. We recognize interest expense and any related penalties from unrecognized tax benefits in income tax expense. As of September 30, 2021 and December 31, 2020, we had accrued approximately $0.8 million and $0.8 million for interest and penalties, respectively.

We do not anticipate material changes to the amount of unrecognized tax benefits within the next twelve months.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References

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+ Details

Inventories

9 Months Ended

Sep. 30, 2021

Inventory Disclosure [Abstract]

Inventories

11. Inventories

Net inventories as of September 30, 2021 and December 31, 2020 are summarized in the table below:

	September 30, 2021		December 31, 2020
(Dollars in millions)
Raw materials	$	244.6	$	233.7
Work in process		13.3		12.4
Finished goods		102.7		99.3
	$	360.6	$	345.4
Less revaluation to LIFO		65.5		49.6
Net	$	295.1	$	295.8

Property, Plant and Equipment

9 Months Ended

Sep. 30, 2021

Property Plant And Equipment [Abstract]

Property, Plant and Equipment

12. Property, Plant and Equipment

Property, plant and equipment as of September 30, 2021 and December 31, 2020 are summarized in the table below:

	September 30, 2021		December 31, 2020
(Dollars in millions)
Land	$	15.5	$	16.7
Buildings		71.6		75.0
Machinery and equipment		807.2		812.1
	$	894.3	$	903.8
Less accumulated depreciation		433.4		494.7
Net	$	460.9	$	409.1

Pensions and Post-Retirement Benefit Plans

9 Months Ended

Sep. 30, 2021

Compensation And Retirement Disclosure [Abstract]

Pensions and Post-Retirement Benefit Plans

13. Pensions and Post-Retirement Benefit Plans

We maintain a number of defined benefit and defined contribution plans to provide retirement benefits for employees in the United States, as well as employees outside the United States. These plans are maintained and contributions are made in accordance with the Employee Retirement Income Security Act of 1974 (“ERISA”), local statutory law or as determined by the board of directors. The defined benefit pension plans generally provide benefits based upon years of service and compensation. Pension plans are funded except for three domestic non-qualified defined benefit pension plans for certain key executives.

In the United States, all qualified and two of the non-qualified defined benefit pension plans for salaried and hourly employees have been frozen and are closed to new participants. Accordingly, these pension plans no longer accrue additional years of service or recognize future increases in compensation for benefit purposes.

The defined contribution plans generally provide retirement assets to employee participants based upon employer and employee contributions to the participant’s individual investment account. We also provide retiree medical insurance coverage to certain U.S. employees and a life insurance benefit to most U.S. employees. For salaried employees, the retiree medical and retiree insurance plans have been closed to new participants.

The following table provides the components of net periodic benefit cost for the pension plans for the three and nine months ended September 30, 2021 and 2020:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
(Dollars in millions)
Service cost	$	0.4		$	0.3		$	1.1		$	1.0
Interest cost		1.1			1.6			3.8			4.7
Expected return on plan assets		(1.8	)		(2.0	)		(5.5	)		(5.8	)
Amortization of net loss		0.4			0.4			1.1			1.1
Net periodic benefit cost	$	0.1		$	0.3		$	0.5		$	1.0
Defined contribution plan expense	$	1.9		$	1.6		$	6.5		$	5.6

- Definition

The entire disclosure for retirement benefits.

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+ Details

Debt

9 Months Ended

Sep. 30, 2021

Debt Disclosure [Abstract]

Debt

14. Debt

Debt as of September 30, 2021 and December 31, 2020 was as follows:

	Weighted Average Interest Rate			Maturity	September 30, 2021		December 31, 2020
(Dollars in millions)
Term Loan		2.56	%	2024	$	4.6	$	12.2
Revolving Credit Facility		2.56	%	2024		308.9		272.0
Senior Notes due 2025		6.00	%	2025		500.0		500.0
Debt						813.5		784.2
Less short-term debt and current maturities of long-term debt						4.6		10.1
Less unamortized debt issuance costs						6.3		8.3
Long-term debt					$	802.6	$	765.8

Credit Facility

We maintain a $600.0 million senior secured revolving credit facility and a $100.0 million secured term loan facility (collectively, the “Credit Facility”), as amended. The secured term loan has a quarterly amortization of $2.5 million and the interest rate on the Credit Facility is variable and is based on LIBOR.

Borrowings under the Credit Facility are secured by a first priority lien on substantially all of the assets of Koppers Inc., Koppers Holdings Inc. and their material domestic subsidiaries. The Credit Facility contains certain covenants for Koppers Inc. and its restricted subsidiaries that limit capital expenditures, additional indebtedness, liens, dividends, investments or acquisitions. In addition, such covenants give rise to events of default upon the failure by Koppers Inc. and its restricted subsidiaries to meet certain financial ratios.

As of September 30, 2021, we had $283.4 million of unused revolving credit availability for working capital purposes after restrictions from certain letter of credit commitments and other covenants. As of September 30, 2021, $7.7 million of commitments were utilized by outstanding letters of credit.

Senior Notes due 2025

The 2025 Notes are senior obligations of Koppers Inc., are unsecured and are guaranteed by Koppers Holdings Inc. and certain of Koppers Inc.’s domestic subsidiaries. The 2025 Notes pay interest semi-annually in arrears on February 15 and August 15 and will mature on February 15, 2025 unless earlier redeemed or repurchased. We are entitled to redeem all or a portion of the 2025 Senior Notes at a redemption price of 104.5 percent of principal value, declining to a redemption price of 101.5 percent on or after February 15, 2022 until the redemption price is equivalent to the principal value on April 15, 2023.

The indenture governing the 2025 Senior Notes includes customary covenants that restrict, among other things, the ability of Koppers Inc. and its restricted subsidiaries to incur additional debt, pay dividends or make certain other restricted payments, incur liens, merge or sell all or substantially all of the assets of Koppers Inc. or its subsidiaries or enter into various transactions with affiliates.

- Definition

The entire disclosure for information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, own-share lending arrangements and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.

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+ Details

Asset Retirement Obligations

9 Months Ended

Sep. 30, 2021

Asset Retirement Obligation Disclosure [Abstract]

Asset Retirement Obligations

15. Asset Retirement Obligations

We recognize asset retirement obligations for the removal and disposal of residues; dismantling of certain tanks required by governmental authorities; cleaning and dismantling costs for owned railcars; cleaning costs for leased rail-cars and barges; and site demolition, when required by governmental authorities or by contract. The following table reflects changes in the carrying values of asset retirement obligations:

	September 30, 2021			December 31, 2020
(Dollars in millions)
Asset retirement obligation at beginning of year	$	19.8		$	20.7
Accretion expense		0.7			1.1
Revision in estimated cash flows		(2.8	)		4.6
Cash expenditures		(6.7	)		(6.6	)
Balance at end of period	$	11.0		$	19.8

Leases

9 Months Ended

Sep. 30, 2021

Leases [Abstract]

Leases

16. Leases

We recognize lease obligations and associated right-of-use assets for existing non-cancelable leases. We have non-cancelable operating leases primarily associated with railcars, office and manufacturing facilities, storage tanks, ships, production equipment and vehicles. Many of our leases include both lease (e.g., fixed rent) and non-lease components (e.g., maintenance and services). For certain asset classes such as railcars, storage tanks and ships, we have separated the lease and non-lease components based on the estimated stand-alone price for each component. For the remaining asset classes, we have elected to account for these components as a single lease component. In addition, we exclude leases expiring within twelve months from balance sheet recognition.

Many of our leases include one or more options to renew. We evaluate renewal options at the lease commencement date and regularly thereafter to determine if we are reasonably certain to exercise the option, in which case we include the renewal period in our lease term. As most of our leases do not provide an implicit rate, we use our incremental borrowing rate based on information available to determine the present value of the lease payments.

Lease expense for operating leases is recognized on a straight-line basis over the lease term. Variable lease expense is recognized in the period in which the obligation for those payments is incurred. Operating lease costs were $7.5 million and $22.8 million during the three and nine months ended September 30, 2021, respectively, and $8.1 million and $23.2 million during the three and nine months ended September 30, 2020, respectively. Variable lease costs were $0.8 million and $2.4 million during the three and nine months ended September 30, 2021, respectively, and $0.8 million and $2.7 million during the three and nine months ended September 30, 2020, respectively.

The following table presents information about the amount and timing of cash flows arising from our operating leases as of September 30, 2021:

(Dollars in millions)
2021	$	7.3
2022		27.3
2023		21.3
2024		17.3
2025		14.0
Thereafter		34.8
Total lease payments	$	122.0
Less: Interest		(26.6	)
Present value of lease liabilities	$	95.4

Supplemental condensed consolidated balance sheet information related to leases is as follows:

	September 30,			December 31,
	2021			2020
(Dollars in millions)
Operating leases:
Operating lease right-of-use assets	$	95.2		$	102.5
Current operating lease liabilities	$	21.6		$	21.2
Operating lease liabilities		73.8			81.3
Total operating lease liabilities	$	95.4		$	102.5
Weighted average remaining lease term, in years		5.7			6.4
Weighted average discount rate		7.2	%		7.5	%

Derivative Financial Instruments

9 Months Ended

Sep. 30, 2021

Derivative Instruments And Hedging Activities Disclosure [Abstract]

Derivative Financial Instruments

17. Derivative Financial Instruments

We utilize derivative instruments to manage exposures to risks that have been identified, measured and are capable of being mitigated. The primary risks that we manage by using derivative instruments are commodity price risk associated with copper and foreign currency exchange risk associated with a number of currencies, principally the U.S. dollar, the Euro and British pounds. Swap contracts on copper are used to manage the price risk associated with forecasted purchases of materials used in our manufacturing processes. Generally, we will not hedge cash flow exposures for durations longer than 36 months and we have hedged certain volumes of copper through the end of 2022. We enter into foreign currency forward contracts to manage foreign currency risk associated with our receivable and payable balances in addition to foreign-denominated sales. Generally, we enter into master netting arrangements with the counterparties and offset net derivative positions with the same counterparties. Currently, our agreements do not require cash collateral.

ASC Topic 815-10, “Derivatives and Hedging,” requires companies to recognize all derivative instruments as either assets or liabilities at fair value in the balance sheet. Derivative instruments’ fair value is determined using significant other observable inputs, or Level 2 in the fair value hierarchy. In accordance with ASC Topic 815-10, we designate certain of our commodity swaps as cash flow hedges of forecasted purchases of commodities. For derivative instruments that are designated and qualify as cash flow hedges, the effective portion of the gain or loss on the derivative is reported as a component of other comprehensive income and is reclassified into earnings in the same period or periods during which the hedged transaction affects earnings. Gains and losses on the derivative instruments representing hedge ineffectiveness are recognized in current earnings.

For those commodity swaps where hedge accounting is not elected, the fair value of the commodity swap is recognized as an asset or liability in the consolidated balance sheet and the related gain or loss on the derivative is reported in current earnings. These amounts are classified in cost of sales in the condensed consolidated statement of operations.

As of September 30, 2021 and December 31, 2020, we had outstanding copper swap contracts of the following amounts:

	Units Outstanding (in Pounds)				Net Fair Value - Asset
	September 30, 2021		December 31, 2020		September 30, 2021		December 31, 2020
(Amounts in millions)
Cash flow hedges		36.1		62.3	$	54.0	$	58.3
Contracts where hedge accounting was not elected		5.1		11.5	$	8.1		10.9
Total		41.2		73.8	$	62.1	$	69.2

As of September 30, 2021 and December 31, 2020, the fair value of the outstanding copper swap contracts is recorded in the balance sheet as follows:

	September 30, 2021		December 31, 2020
(Dollars in millions)
Derivative contracts	$	51.7	$	37.3
Non-current derivative contracts		10.4		31.9
Asset on balance sheet	$	62.1	$	69.2
Accumulated other comprehensive gain, net of tax	$	41.3	$	44.4

In the next twelve months, we estimate that $34.4 million of unrealized gains, net of tax, related to commodity price hedging will be reclassified from other comprehensive (loss) income into earnings.

See Note 6 – “Comprehensive (Loss) Income and Equity”, for amounts recorded in other comprehensive (loss) income and for amounts reclassified from accumulated other comprehensive loss into net income for the periods specified below.

For the three and nine months ended September 30, 2021 and 2020, the unrealized (loss) gain from contracts where hedge accounting was not elected is as follows:

	Three Months Ended September 30,					Nine Months Ended September 30,
	2021			2020		2021			2020
(Dollars in millions)
(Loss) gain from contracts where hedge accounting was not elected		(4.4	)		3.9		(2.7	)		4.2

The fair value associated with forward contracts related to foreign currency that are not designated as hedges are immediately charged to earnings. These amounts are classified in cost of sales in the condensed consolidated statement of operations and comprehensive (loss) income.

As of September 30, 2021 and December 31, 2020, the fair value of outstanding foreign currency forward contracts is recorded in the balance sheet as follows:

	September 30, 2021			December 31, 2020
(Dollars in millions)
Derivative contracts	$	0.1		$	1.2
Accrued liabilities		(0.3	)		(0.5	)
Net (liability) asset on balance sheet	$	(0.2	)	$	0.7

As of September 30, 2021 and December 31, 2020, the net currency units outstanding for these contracts were:

	September 30, 2021		December 31, 2020
(In millions)
British Pounds		GBP 0.0		GBP 2.0
United States Dollars		USD 5.2		USD 7.6
Euro		EUR 0.4		EUR 0.0

Commitments and Contingent Liabilities

9 Months Ended

Sep. 30, 2021

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingent Liabilities

18. Commitments and Contingent Liabilities

We are involved in litigation and various proceedings relating to environmental laws and regulations, product liability and other matters. Certain of these matters are discussed below. The ultimate resolution of these contingencies is subject to significant uncertainty and should we fail to prevail in any of these legal matters or should several of these legal matters be resolved against us in the same reporting period, these legal matters could, individually or in the aggregate, be material to the consolidated financial statements.

Legal Proceedings

Coal Tar Pitch Cases. Koppers Inc. is one of several defendants in lawsuits filed in two states in which the plaintiffs claim they suffered a variety of illnesses (including cancer) as a result of exposure to coal tar pitch sold by the defendants. There were 61 plaintiffs in 32 cases pending as of September 30, 2021, compared to 64 plaintiffs in 34 cases pending as of December 31, 2020. As of September 30, 2021, there were 31 cases pending in the Court of Common Pleas of Allegheny County, Pennsylvania, and one case pending in the Circuit Court of Knox County, Tennessee.

The plaintiffs in all 32 pending cases seek to recover compensatory damages. Plaintiffs in 27 of those cases also seek to recover punitive damages. The plaintiffs in the 31 cases filed in Pennsylvania seek unspecified damages in excess of the court’s minimum jurisdictional limit. The plaintiff in the Tennessee state court case seeks damages of $15.0 million. The other defendants in these lawsuits vary from case to case and include companies such as Beazer East, Inc. (“Beazer East”), Honeywell International Inc., Graftech International Holdings, Dow Chemical Company, UCAR Carbon Company, Inc., and SGL Carbon Corporation. Discovery is proceeding in these cases. No trial dates have been set in any of these cases.

We have not provided a reserve for the coal tar pitch lawsuits because, at this time, we cannot reasonably determine the probability of a loss, and the amount of loss, if any, cannot be reasonably estimated. The timing of resolution of these cases cannot be reasonably determined. Although Koppers Inc. is vigorously defending these cases, an unfavorable resolution of these matters may have a material adverse effect on our business, financial condition, cash flows and results of operations.

Environmental and Other Litigation Matters

We are subject to federal, state, local and foreign laws and regulations and potential liabilities relating to the protection of the environment and human health and safety including, among other things, the cleanup of contaminated sites, the treatment, storage and disposal of wastes, the discharge of effluent into waterways, the emission of substances into the air and various health and safety matters. We expect to incur substantial costs for ongoing compliance with such laws and regulations. We may also face governmental or third-party claims, or otherwise incur costs, relating to cleanup of, or for injuries resulting from, contamination at sites associated with past and present operations. We accrue for environmental liabilities when a determination can be made that a liability is probable and reasonably estimable.

Environmental and Other Liabilities Retained or Assumed by Others. We have agreements with former owners of certain of our operating locations under which the former owners retained, assumed and/or agreed to indemnify us against certain environmental and other liabilities. The most significant of these agreements was entered into at Koppers Inc.’s formation on December 29, 1988 (the “Acquisition”). Under the related asset purchase agreement between Koppers Inc. and Beazer East, subject to certain limitations, Beazer East retained the responsibility for and agreed to indemnify Koppers Inc. against certain liabilities, damages, losses and costs, including, with certain limited exceptions, liabilities under and costs to comply with environmental laws to the extent attributable to acts or omissions occurring prior to the Acquisition and liabilities related to products sold by Beazer East prior to the Acquisition (the “Indemnity”). Beazer Limited, the parent company of Beazer East, unconditionally guaranteed Beazer East’s performance of the Indemnity pursuant to a guarantee (the “Guarantee”).

The Indemnity provides different mechanisms, subject to certain limitations, by which Beazer East is obligated to indemnify Koppers Inc. with regard to certain environmental, product and other liabilities and imposes certain conditions on Koppers Inc. before receiving such indemnification, including, in some cases, certain limitations regarding the time period as to which claims for indemnification can be brought. In July 2004, Koppers Inc. and Beazer East agreed to amend the environmental indemnification provisions of the December 29, 1988 asset purchase agreement to extend the indemnification period for pre-closing environmental liabilities, subject to the following paragraph, and agreed to share toxic tort litigation defense arising from any sites acquired from Beazer East.

Qualified expenditures under the Indemnity are not subject to a monetary limit. Qualified expenditures under the Indemnity include (i) environmental cleanup liabilities required by third parties, such as investigation, remediation and closure costs, relating to pre-December 29, 1988 (“Pre-Closing”) acts or omissions of Beazer East or its predecessors; (ii) environmental claims by third parties for personal injuries, property damages and natural resources damages relating to Pre-Closing acts or omissions of Beazer East or its predecessors; (iii) punitive damages for the acts or omissions of Beazer East and its predecessors without regard to the date of the alleged conduct and (iv) product liability claims for products sold by Beazer East or its predecessors without regard to the date of the alleged conduct. The indemnification period ended July 14, 2019 (the “Claim Deadline”) and Beazer East may now tender certain third-party claims described in sections (i) and (ii) above to Koppers Inc. However, to the extent the third-party claims described in sections (i) and (ii) above were tendered to Beazer East by the Claim Deadline, Beazer East will continue to be required to pay the costs arising from such claims under the Indemnity. Furthermore, the Claim Deadline did not change the provisions of the Indemnity with respect to indemnification for non-environmental claims, such as product liability claims, which claims may continue to be tendered by Koppers Inc. to Beazer East.

The Indemnity provides for the resolution of issues between Koppers Inc. and Beazer East by an arbitrator on an expedited basis upon the request of either party. The arbitrator could be asked, among other things, to make a determination regarding the allocation of environmental responsibilities between Koppers Inc. and Beazer East. Arbitration decisions under the Indemnity are final and binding on the parties.

Contamination has been identified at most manufacturing and other sites of our subsidiaries. One site currently owned and operated by Koppers Inc. in the United States is listed on the National Priorities List promulgated under the Comprehensive Environmental Response, Compensation, and Liability Act of 1980, as amended (“CERCLA”). Currently, at the properties acquired from Beazer East (which includes the National Priorities List site and all but one of the sites permitted under the Resource Conservation and Recovery Act (“RCRA”)), a significant portion of all investigative, cleanup and closure activities are being conducted and paid for by Beazer East pursuant to the terms of the Indemnity. In addition, other of Koppers Inc.’s sites are or have been operated under RCRA and various other environmental permits, and remedial and closure activities are being conducted at some of these sites.

To date, the parties that retained, assumed and/or agreed to indemnify us against the liabilities referred to above, including Beazer East, have performed their obligations in all material respects. We believe that, for the last three years ended December 31, 2020, amounts paid by Beazer East as a result of its environmental remediation obligations under the Indemnity have averaged, in total, approximately $6.4 million per year. Periodically, issues have arisen between Koppers Inc. and Beazer East and/or other indemnitors that have been resolved without arbitration. Koppers Inc. and Beazer East engage in discussions from time to time that involve, among other things, the allocation of environmental costs related to certain operating and closed facilities.

If for any reason (including disputed coverage or financial incapability) one or more of such parties fail to perform their obligations and we are held liable for or otherwise required to pay all or part of such liabilities without reimbursement, the imposition of such liabilities on us could have a material adverse effect on our business, financial condition, cash flows and results of operations. Furthermore, we could be required to record a contingent liability on our balance sheet with respect to such matters, which could result in a negative impact to our business, financial condition, cash flows and results of operations.

Domestic Environmental Matters. Koppers Inc. has been named as one of the potentially responsible parties (“PRPs”) at the Portland Harbor CERCLA site located on the Willamette River in Oregon. Koppers Inc. operated a coal tar pitch terminal near the site. Koppers Inc. has responded to an EPA information request and has executed a PRP agreement which outlines a private process to develop an allocation of past and future costs among more than 80 parties to the site. Koppers Inc. believes it is a de minimis contributor at the site.

The EPA issued its Record of Decision (“ROD”) in January 2017 for the Portland Harbor CERCLA site. The selected remedy includes a combination of sediment removal, capping, enhanced and monitored natural recovery and riverbank improvements. The ROD does not determine who is responsible for remediation costs. The net present value and undiscounted costs of the selected remedy as estimated in the ROD are approximately $1.1 billion and $1.7 billion, respectively. These costs may increase given the remedy will not be implemented for several years. Responsibility for implementing and funding that work will be decided in the separate private allocation process which is ongoing.

Additionally, Koppers Inc. is involved in two separate matters involving natural resource damages at the Portland Harbor site. One matter involves claims by the trustees to recover damages based upon an assessment of damages to natural resources caused by the releases of hazardous substances to the Willamette River. The assessment serves as the foundation to estimate liabilities for settlements of natural resource damages claims or litigation to recover from those who do not settle with the trustee groups. Koppers Inc. has been engaged in a process to resolve its natural resource damage liabilities for the assessment area. A second matter involves a lawsuit filed in January 2017 by the Yakama Nation in Oregon federal court. Yakama Nation seeks recovery for response costs and the costs of assessing injury to natural resources to waterways beyond the current assessment area. Following the most recent court rulings, the Yakama Nation case has been stayed pending completion of the private allocation process for the Portland Harbor CERCLA site.

In September 2009, Koppers Inc. received a general notice letter notifying it that it may be a PRP at the Newark Bay CERCLA site. In January 2010, Koppers Inc. submitted a response to the general notice letter asserting that Koppers Inc. is a de minimis party at this site.

We have accrued the estimated costs of participating in the PRP group at the Portland Harbor and Newark Bay CERCLA sites as a de minimis contributor and estimated such settlement amounts at the sites totaling $3.4 million as of September 30, 2021. The actual cost could be materially higher as there has not been a determination of how those costs will be allocated among the PRPs at the sites. Accordingly, an unfavorable resolution of these matters may have a material adverse effect on our business, financial condition, cash flows and results of operations.

There are two plant sites related to the Performance Chemicals business and one plant site related to the Utility and Industrial Products business in the United States where we have recorded environmental remediation liabilities for soil and groundwater contamination which occurred prior to our acquisition of the businesses. As of September 30, 2021, our estimated environmental remediation liability for these acquired sites totals $4.2 million.

Foreign Environmental Matters. There is one plant site related to the Performance Chemicals business located in Australia where we have recorded an environmental remediation liability for soil and groundwater contamination which occurred prior to the acquisition of the site. As of September 30, 2021, our estimated environmental remediation liability for the site totals $1.4 million.

Environmental Reserves Rollforward. The following table reflects changes in the accrual for environmental remediation. A total of $2.8 million and $2.9 million are classified as current liabilities as of September 30, 2021 and December 31, 2020:

	Period ended
	September 30, 2021			December 31, 2020
(Dollars in millions)
Balance at beginning of year	$	11.0		$	9.5
Expense		0.2			1.8
Reversal of reserves		(0.1	)		0.0
Cash expenditures		(0.3	)		(0.4	)
Currency translation		(0.2	)		0.1
Balance at end of period	$	10.6		$	11.0

New Accounting Pronouncements (Policies)	9 Months Ended
New Accounting Pronouncements (Policies)	Sep. 30, 2021
Accounting Changes And Error Corrections [Abstract]
New Accounting Pronouncements	In March 2020, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2020-04: Reference Rate Reform (Topic 848) Facilitation of the Effects of Reference Rate Reform on Financial Reporting. This update provides temporary optional expedients and exceptions to U.S. GAAP on contract modifications, hedging relationships, and other transactions affected by reference rate reform to ease entities' financial reporting burdens as the market transitions from the London Interbank Offered Rate (LIBOR) and other interbank offered rates to alternative reference rates. The guidance was effective upon issuance and may be applied prospectively to contract modifications made, hedging relationships entered into, and other transactions affected by reference rate reform, evaluated on or before December 31, 2022, beginning during the reporting period in which the guidance has been elected. Our debt agreements include the use of alternate rates when LIBOR is not available and we do not maintain hedging relationships applicable to this ASU. We do not expect the application of this update to have a material impact on our financial statements and, to the extent we enter into modifications of agreements that are impacted by the LIBOR phase-out, we will apply such guidance to those contract modifications.

Plant Closures and Divestitures (Tables)

9 Months Ended

Sep. 30, 2021

Discontinued Operations And Disposal Groups [Abstract]

Summary of Restructuring Activities and Related Reserves

Details of the restructuring activities and related reserves are as follows:

	Severance and employee benefits			Asset Retirement			Other			Total
(Dollars in millions)
Reserve at December 31, 2019	$	0.9		$	0.7		$	2.4		$	4.0
Accrual		0.5			2.9			3.4			6.8
Cost charged against assets		0.0			0.0			(3.4	)		(3.4	)
Reversal of accrued charges		(0.3	)		0.0			0.0			(0.3	)
Cash paid		(0.2	)		(0.8	)		(0.3	)		(1.3	)
Currency translation		0.0			0.0			0.2			0.2
Reserve at December 31, 2020	$	0.9		$	2.8		$	2.3		$	6.0
Accrual		0.0			0.0			2.4			2.4
Cost charged against assets		0.0			0.0			(2.4	)		(2.4	)
Reversal of accrued charges		(0.2	)		(0.7	)		0.0			(0.9	)
Cash paid		(0.7	)		(1.9	)		0.0			(2.6	)
Sale of subsidiary		0.0			(0.2	)		(2.3	)		(2.5	)
Reserve at September 30, 2021	$	0.0		$	0.0		$	0.0		$	0.0

Discontinued Operations (Tables)

9 Months Ended

Sep. 30, 2021

Discontinued Operations And Disposal Groups [Abstract]

Schedule of Net Sales and Operating (Loss) Profit from Discontinued Operations

Net sales and operating loss from discontinued operations for the three and nine months ended September 30, 2020 consisted of the following amounts:

	Three Months Ended September 30,		Nine Months Ended September 30,
	2020		2020
(Dollars in millions)
Net sales	$	8.8	$	31.6
Operating income (loss)		0.3		(5.0	)

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2021

Fair Value Disclosures [Abstract]

Schedule of Company's Financial Instruments

Carrying amounts and the related estimated fair values of our financial instruments as of September 30, 2021 and December 31, 2020 are as follows:

	September 30, 2021				December 31, 2020
	Fair Value		Carrying Value		Fair Value		Carrying Value
(Dollars in millions)
Financial assets:
Investments and other assets	$	1.2	$	1.2	$	1.2	$	1.2
Financial liabilities:
Long-term debt (including current portion)	$	828.5	$	813.5	$	799.2	$	784.2

Comprehensive (Loss) Income and Equity (Tables)

9 Months Ended

Sep. 30, 2021

Equity [Abstract]

Schedule of Comprehensive (Loss) Income

Total comprehensive (loss) income for the three and nine months ended September 30, 2021 and 2020 is summarized in the table below:

	Three Months Ended September 30,					Nine Months Ended September 30,
	2021			2020		2021			2020
(Dollars in millions)
Net income	$	10.0		$	75.5	$	62.7		$	102.4
Changes in other comprehensive (loss) income:
Currency translation adjustment		(11.1	)		12.3		(14.5	)		3.1
Unrealized (loss) gain on cash flow hedges, net of tax benefit (expense) of $6.1, $(5.3), $1.2 and $(6.9)		(16.6	)		15.3		(3.3	)		19.8
Unrecognized pension net loss, net of tax expense of $0.1, $0.1, $0.3 and $0.2		0.3			0.2		0.8			0.6
Total comprehensive (loss) income		(17.4	)		103.3		45.7			125.9
Comprehensive (loss) income attributable to noncontrolling interests		(0.2	)		0.9		(0.2	)		(0.1	)
Comprehensive (loss) income attributable to Koppers	$	(17.2	)	$	102.4	$	45.9		$	126.0

Schedule of Change in Equity

The following tables present the change in equity for the three months ended September 30, 2021 and 2020, respectively:

(Dollars in millions)	Common Stock		Additional Paid-In Capital		Retained Earnings			Accumulated Other Comprehensive Loss			Treasury Stock			Noncontrolling Interests			Total Equity
Balance at June 30, 2021	$	0.2	$	242.7	$	268.8		$	(5.5	)	$	(94.4	)	$	4.3		$	416.1
Net income (loss)		0.0		0.0		10.2			0.0			0.0			(0.2	)		10.0
Issuance of common stock		0.0		0.3		0.0			0.0			0.0			0.0			0.3
Employee stock plans		0.0		3.3		0.0			0.0			0.0			0.0			3.3
Other comprehensive income (loss)
Currency translation adjustment		0.0		0.0		(0.3	)		(11.1	)		0.0			0.0			(11.4	)
Unrealized loss on cash flow hedges		0.0		0.0		0.0			(16.6	)		0.0			0.0			(16.6	)
Unrecognized pension net loss		0.0		0.0		0.0			0.3			0.0			0.0			0.3
Repurchases of common stock		0.0		0.0		0.0			0.0			(1.4	)		0.0			(1.4	)
Balance at September 30, 2021	$	0.2	$	246.2	$	278.8		$	(33.0	)	$	(95.8	)	$	4.1		$	400.5

(Dollars in millions)	Common Stock		Additional Paid-In Capital		Retained Earnings		Accumulated Other Comprehensive Loss			Treasury Stock			Noncontrolling Interests			Total Equity
Balance at June 30, 2020	$	0.2	$	228.0	$	121.6	$	(81.8	)	$	(92.1	)	$	10.4		$	186.3
Net income (loss)		0.0		0.0		75.6		0.0			0.0			(0.1	)		75.5
Issuance of common stock		0.0		0.3		0.0		0.0			0.0			0.0			0.3
Employee stock plans		0.0		2.9		0.0		0.0			0.0			0.0			2.9
Sale of discontinued operations		0.0		0.0		0.0		0.0			0.0			(7.2	)		(7.2	)
Other comprehensive income
Currency translation adjustment		0.0		0.0		0.0		11.3			0.0			1.0			12.3
Unrealized gain on cash flow hedges		0.0		0.0		0.0		15.3			0.0			0.0			15.3
Unrecognized pension net loss		0.0		0.0		0.0		0.2			0.0			0.0			0.2
Balance at September 30, 2020	$	0.2	$	231.2	$	197.2	$	(55.1	)	$	(92.1	)	$	4.1		$	285.5

The following tables present the change in equity for the nine months ended September 30, 2021 and 2020, respectively:

(Dollars in millions)	Common Stock		Additional Paid-In Capital		Retained Earnings		Accumulated Other Comprehensive Loss			Treasury Stock			Noncontrolling Interests			Total Equity
Balance at December 31, 2020	$	0.2	$	234.1	$	215.8	$	(15.9	)	$	(92.5	)	$	4.3		$	346.0
Net income (loss)		0.0		0.0		63.0		0.0			0.0			(0.3	)		62.7
Issuance of common stock		0.0		2.2		0.0		0.0			0.0			0.0			2.2
Employee stock plans		0.0		9.9		0.0		0.0			0.0			0.0			9.9
Other comprehensive income (loss)
Currency translation adjustment		0.0		0.0		0.0		(10.2	)		0.0			0.1			(10.1	)
Cumulative translation adjustment loss on sale of subsidiary		0.0		0.0		0.0		(4.4	)		0.0			0.0			(4.4	)
Unrealized loss on cash flow hedges		0.0		0.0		0.0		(3.3	)		0.0			0.0			(3.3	)
Unrecognized pension net loss		0.0		0.0		0.0		0.8			0.0			0.0			0.8
Repurchases of common stock		0.0		0.0		0.0		0.0			(3.3	)		0.0			(3.3	)
Balance at September 30, 2021	$	0.2	$	246.2	$	278.8	$	(33.0	)	$	(95.8	)	$	4.1		$	400.5

(Dollars in millions)	Common Stock		Additional Paid-In Capital		Retained Earnings		Accumulated Other Comprehensive Loss			Treasury Stock			Noncontrolling Interests			Total Equity
Balance at December 31, 2019	$	0.2	$	221.9	$	93.8	$	(77.7	)	$	(90.9	)	$	11.4		$	158.7
Net income (loss)		0.0		0.0		103.4		0.0			0.0			(1.0	)		102.4
Issuance of common stock		0.0		0.8		0.0		0.0			0.0			0.0			0.8
Employee stock plans		0.0		8.5		0.0		0.0			0.0			0.0			8.5
Sale of discontinued operations		0.0		0.0		0.0		0.0			0.0			(7.2	)		(7.2	)
Other comprehensive income (loss)
Currency translation adjustment		0.0		0.0		0.0		2.1			0.0			1.0			3.1
Unrealized gain on cash flow hedges		0.0		0.0		0.0		19.8			0.0			0.0			19.8
Unrecognized pension net loss		0.0		0.0		0.0		0.6			0.0			0.0			0.6
Repurchases of common stock		0.0		0.0		0.0		0.0			(1.2	)		0.0			(1.2	)
Balance at September 30, 2020	$	0.2	$	231.2	$	197.2	$	(55.1	)	$	(92.1	)	$	4.1		$	285.5

Earnings per Common Share (Tables)

9 Months Ended

Sep. 30, 2021

Earnings Per Share [Abstract]

Schedule of Computation of Basic and Diluted Earnings per Common Share

	Three Months Ended September 30,					Nine Months Ended September 30,
	2021			2020		2021		2020
(Dollars in millions, except share amounts, in thousands)
Net income attributable to Koppers	$	10.2		$	75.6	$	63.0	$	103.4
Less: Income (loss) from discontinued operations, net of tax		0.0			0.6		0.1		(3.8	)
(Loss) gain on sale of discontinued operations		(0.5	)		35.8		0.0		35.8
Noncontrolling interest related to discontinued operations		0.0			0.1		0.0		0.8
Income from continuing operations attributable to Koppers	$	10.7		$	39.1	$	62.9	$	70.6
Weighted average common shares outstanding:
Basic		21,322			21,047		21,253		20,968
Effect of dilutive securities		625			333		696		259
Diluted		21,947			21,380		21,949		21,227
Earnings per common share – continuing operations:
Basic earnings per common share	$	0.50		$	1.86	$	2.96	$	3.37
Diluted earnings per common share		0.49			1.83		2.87		3.33
Other data:
Antidilutive securities excluded from computation of diluted earnings per common share		563			876		437		862

Stock-based Compensation (Tables)

9 Months Ended

Sep. 30, 2021

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Stock Options And Performance Award Fair Value Assumptions

We calculated the fair value of stock options on the date of grant using the Black-Scholes-Merton model and the assumptions listed below:

	January 2021 Grant			March 2020 Grant			March 2019 Grant			March 2018 Grant
Grant date price per share of stock option award	$	29.84		$	19.63		$	26.63		$	41.60
Expected dividend yield per share		0.00	%		0.00	%		0.00	%		0.00	%
Expected life in years		6.64			6.40			6.14			5.73
Expected volatility		54.80	%		42.85	%		39.44	%		37.05	%
Risk-free interest rate		0.59	%		0.87	%		2.53	%		2.67	%
Grant date fair value per share of option awards	$	15.79		$	8.42		$	11.29		$	16.38

Summary of Performance Stock Units

The following table shows a summary of the performance stock units as of September 30, 2021:

Performance Period	Minimum Shares		Target Shares		Maximum Shares
2019 – 2021		0		234,597		281,330
2020 – 2022		0		154,099		308,198
2021 – 2023		0		149,874		299,748

Summary of Status and Activity of Non-Vested Stock Awards

The following table shows a summary of the status and activity of non-vested stock units for the nine months ended September 30, 2021:

	Restricted Stock Units			Performance Stock Units			Total Stock Units			Weighted Average Grant Date Fair Value per Unit
Non-vested at December 31, 2020		509,509			391,744			901,253		$	25.48
Granted		222,299			149,874			372,173		$	34.91
Performance share adjustment		0			(1,296	)		(1,296	)	$	44.29
Vested		(199,837	)		(1,821	)		(201,658	)	$	22.52
Forfeited		(690	)		(103	)		(793	)	$	30.94
Non-vested at September 30, 2021		531,281			538,398			1,069,679		$	29.23

Summary of Status and Activity of Stock Options

The following table shows a summary of the status and activity of stock options for the nine months ended September 30, 2021:

	Options			Weighted Average Exercise Price per Option		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in millions)
Outstanding at December 31, 2020		1,120,254		$	26.89
Granted		90,879		$	29.84
Exercised		(71,785	)	$	17.74
Expired		(25,402	)	$	40.26
Outstanding at September 30, 2021		1,113,946		$	27.41		5.21	$	7.8
Exercisable at September 30, 2021		797,911		$	28.16		4.08	$	5.7

Schedule of Stock-based Compensation Expense Recognized

Total stock-based compensation expense recognized under our LTIP and employee stock purchase plan for the three and nine months ended September 30, 2021 and 2020 is as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
(Dollars in millions)
Stock-based compensation expense recognized:
Selling, general and administrative expenses	$	3.2	$	2.9	$	9.8	$	8.5
Less related income tax benefit		0.8		0.7		2.6		2.2
Decrease in net income attributable to Koppers	$	2.4	$	2.2	$	7.2	$	6.3
Intrinsic value of exercised stock options	$	0.0	$	0.0	$	2.2	$	0.0
Cash received from the exercise of stock options	$	0.0	$	0.0	$	2.3	$	0.0

Performance Stock Units [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Stock Options And Performance Award Fair Value Assumptions

We calculated the fair value of the performance stock unit awards on the date of grant using the assumptions listed below:

	January 2021 Grant			March 2020 Grant			March 2019 Grant
Grant date price per share of performance award	$	29.84		$	19.63		$	26.63
Expected dividend yield per share		0.00	%		0.00	%		0.00	%
Expected volatility		68.70	%		45.60	%		39.00	%
Risk-free interest rate		0.16	%		0.72	%		2.50	%
Look-back period in years		3.00			2.83			2.82
Grant date fair value per share	$	41.50		$	11.56		$	40.30

Segment Information (Tables)

9 Months Ended

Sep. 30, 2021

Segment Reporting [Abstract]

Summary of Results of Segment Operations

The following table sets forth certain sales and operating data, net of all intersegment transactions, for our segments for the periods indicated:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
(Dollars in millions)
Revenues from external customers:
Railroad and Utility Products and Services	$	186.9		$	191.0		$	574.3		$	590.9
Performance Chemicals		115.2			147.9			384.4			396.4
Carbon Materials and Chemicals^(a)		122.7			98.6			314.6			288.7
Total	$	424.8		$	437.5		$	1,273.3		$	1,276.0
Intersegment revenues:
Performance Chemicals	$	3.9		$	3.4		$	11.2		$	10.3
Carbon Materials and Chemicals		18.9			20.1			55.2			59.5
Total	$	22.8		$	23.5		$	66.4		$	69.8
Depreciation and amortization expense:
Railroad and Utility Products and Services	$	5.3		$	4.9		$	16.9		$	14.8
Performance Chemicals		4.1			4.3			13.6			13.2
Carbon Materials and Chemicals^(b)		4.0			3.7			12.9			11.7
Total	$	13.4		$	12.9		$	43.4		$	39.7
Operating profit (loss):
Railroad and Utility Products and Services	$	(0.7	)	$	15.0		$	12.3		$	40.4
Performance Chemicals		11.6			30.4			65.1			67.1
Carbon Materials and Chemicals^(c)		14.9			13.7			39.1			15.9
Corporate		(1.2	)		(0.5	)		(3.7	)		(1.5	)
Total	$	24.6		$	58.6		$	112.8		$	121.9

(a)

Revenue excludes KJCC revenue of $8.8 million and $31.6 million for the three and nine months ended September 30, 2020, respectively.

(b)

Depreciation and amortization expense excludes KJCC expenses of $0.1 million and $0.6 million for the three and nine months ended September 30, 2020, respectively.

(c)

Operating profit (loss) excludes KJCC amounts of $0.3 million and $(5.0) million for the three and nine months ended September 30, 2020, respectively.

Schedule of Segment Revenues for Significant Product Lines

The following table sets forth revenues for significant product lines, net of all intersegment transactions, for our segments for the periods indicated:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
(Dollars in millions)
Railroad and Utility Products and Services:
Railroad treated products	$	107.1	$	108.4	$	327.6	$	345.5
Utility poles		59.6		63.6		186.2		191.4
Railroad infrastructure services		8.8		9.2		25.3		23.3
Rail joints		5.7		4.5		17.9		16.9
Other products		5.7		5.3		17.3		13.8
Total		186.9		191.0		574.3		590.9
Performance Chemicals:
Wood preservative products		110.1		140.5		366.4		375.7
Other products		5.1		7.4		18.0		20.7
Total		115.2		147.9		384.4		396.4
Carbon Materials and Chemicals:
Pitch and related products		70.4		62.9		177.7		171.4
Phthalic anhydride and other chemicals		19.7		14.5		52.2		49.8
Creosote and distillates		19.0		10.3		48.6		33.8
Naphthalene		6.2		3.8		15.5		12.7
Other products		7.4		7.2		20.6		21.0
Total		122.7		98.6		314.6		288.7
Total	$	424.8	$	437.5	$	1,273.3	$	1,276.0

Summary of Tangible and Intangible Assets by Segments

The following table sets forth assets and goodwill allocated to each of our segments as of the dates indicated:

	September 30, 2021		December 31, 2020
(Dollars in millions)
Segment assets:
Railroad and Utility Products and Services	$	590.9	$	583.1
Performance Chemicals		586.6		536.1
Carbon Materials and Chemicals		445.2		424.2
All other		29.5		55.2
Total	$	1,652.2	$	1,598.6
Goodwill:
Railroad and Utility Products and Services	$	120.8	$	121.1
Performance Chemicals		175.3		176.7
Total	$	296.1	$	297.8

Income Taxes (Tables)

9 Months Ended

Sep. 30, 2021

Income Tax Disclosure [Abstract]

Summary of Income Taxes Reconciled with Federal Statutory Rate

	September 30,
	2021			2020
Federal income tax rate		21.0	%		21.0	%
Foreign earnings taxed at different rates		3.6			2.7
Nondeductible expenses		1.3			1.3
State income taxes, net of federal tax benefit		0.8			0.5
Change in tax contingency reserves		0.1			0.1
GILTI inclusion, net of foreign tax credits		0.0			0.2
Estimated annual effective income tax rate		26.8	%		25.8	%

Inventories (Tables)

9 Months Ended

Sep. 30, 2021

Inventory Disclosure [Abstract]

Components of Net Inventories

Net inventories as of September 30, 2021 and December 31, 2020 are summarized in the table below:

	September 30, 2021		December 31, 2020
(Dollars in millions)
Raw materials	$	244.6	$	233.7
Work in process		13.3		12.4
Finished goods		102.7		99.3
	$	360.6	$	345.4
Less revaluation to LIFO		65.5		49.6
Net	$	295.1	$	295.8

Property, Plant and Equipment (Tables)

9 Months Ended

Sep. 30, 2021

Property Plant And Equipment [Abstract]

Property, Plant and Equipment

Property, plant and equipment as of September 30, 2021 and December 31, 2020 are summarized in the table below:

	September 30, 2021		December 31, 2020
(Dollars in millions)
Land	$	15.5	$	16.7
Buildings		71.6		75.0
Machinery and equipment		807.2		812.1
	$	894.3	$	903.8
Less accumulated depreciation		433.4		494.7
Net	$	460.9	$	409.1

Pensions and Post-Retirement Benefit Plans (Tables)

9 Months Ended

Sep. 30, 2021

Compensation And Retirement Disclosure [Abstract]

Components of Net Periodic Benefit Cost for Pension Plans

The following table provides the components of net periodic benefit cost for the pension plans for the three and nine months ended September 30, 2021 and 2020:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
(Dollars in millions)
Service cost	$	0.4		$	0.3		$	1.1		$	1.0
Interest cost		1.1			1.6			3.8			4.7
Expected return on plan assets		(1.8	)		(2.0	)		(5.5	)		(5.8	)
Amortization of net loss		0.4			0.4			1.1			1.1
Net periodic benefit cost	$	0.1		$	0.3		$	0.5		$	1.0
Defined contribution plan expense	$	1.9		$	1.6		$	6.5		$	5.6

Debt (Tables)

9 Months Ended

Sep. 30, 2021

Debt Disclosure [Abstract]

Schedule of Long-Term Debt Instruments

Debt as of September 30, 2021 and December 31, 2020 was as follows:

	Weighted Average Interest Rate			Maturity	September 30, 2021		December 31, 2020
(Dollars in millions)
Term Loan		2.56	%	2024	$	4.6	$	12.2
Revolving Credit Facility		2.56	%	2024		308.9		272.0
Senior Notes due 2025		6.00	%	2025		500.0		500.0
Debt						813.5		784.2
Less short-term debt and current maturities of long-term debt						4.6		10.1
Less unamortized debt issuance costs						6.3		8.3
Long-term debt					$	802.6	$	765.8

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

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+ Details

Asset Retirement Obligations (Tables)

9 Months Ended

Sep. 30, 2021

Asset Retirement Obligation Disclosure [Abstract]

Schedule of Changes in Carrying Values of Asset Retirement Obligations

The following table reflects changes in the carrying values of asset retirement obligations:

	September 30, 2021			December 31, 2020
(Dollars in millions)
Asset retirement obligation at beginning of year	$	19.8		$	20.7
Accretion expense		0.7			1.1
Revision in estimated cash flows		(2.8	)		4.6
Cash expenditures		(6.7	)		(6.6	)
Balance at end of period	$	11.0		$	19.8

Leases (Tables)

9 Months Ended

Sep. 30, 2021

Leases [Abstract]

Summary of Amount and Timing of Cash Flows From Operating Leases

The following table presents information about the amount and timing of cash flows arising from our operating leases as of September 30, 2021:

(Dollars in millions)
2021	$	7.3
2022		27.3
2023		21.3
2024		17.3
2025		14.0
Thereafter		34.8
Total lease payments	$	122.0
Less: Interest		(26.6	)
Present value of lease liabilities	$	95.4

Schedule of Supplemental Condensed Consolidated Balance Sheet Information Related to Leases

Supplemental condensed consolidated balance sheet information related to leases is as follows:

	September 30,			December 31,
	2021			2020
(Dollars in millions)
Operating leases:
Operating lease right-of-use assets	$	95.2		$	102.5
Current operating lease liabilities	$	21.6		$	21.2
Operating lease liabilities		73.8			81.3
Total operating lease liabilities	$	95.4		$	102.5
Weighted average remaining lease term, in years		5.7			6.4
Weighted average discount rate		7.2	%		7.5	%

Derivative Financial Instruments (Tables)

9 Months Ended

Sep. 30, 2021

Derivative Instruments And Hedging Activities Disclosure [Abstract]

Schedule of Outstanding Copper Swap Contracts

As of September 30, 2021 and December 31, 2020, we had outstanding copper swap contracts of the following amounts:

	Units Outstanding (in Pounds)				Net Fair Value - Asset
	September 30, 2021		December 31, 2020		September 30, 2021		December 31, 2020
(Amounts in millions)
Cash flow hedges		36.1		62.3	$	54.0	$	58.3
Contracts where hedge accounting was not elected		5.1		11.5	$	8.1		10.9
Total		41.2		73.8	$	62.1	$	69.2

Schedule of Fair Value of Outstanding Copper Swap Contracts Recorded in Balance Sheet

As of September 30, 2021 and December 31, 2020, the fair value of the outstanding copper swap contracts is recorded in the balance sheet as follows:

	September 30, 2021		December 31, 2020
(Dollars in millions)
Derivative contracts	$	51.7	$	37.3
Non-current derivative contracts		10.4		31.9
Asset on balance sheet	$	62.1	$	69.2
Accumulated other comprehensive gain, net of tax	$	41.3	$	44.4

Schedule of Unrealized Gain (Loss) from Contract Where Hedge Accounting Was Not Elected

For the three and nine months ended September 30, 2021 and 2020, the unrealized (loss) gain from contracts where hedge accounting was not elected is as follows:

	Three Months Ended September 30,					Nine Months Ended September 30,
	2021			2020		2021			2020
(Dollars in millions)
(Loss) gain from contracts where hedge accounting was not elected		(4.4	)		3.9		(2.7	)		4.2

Schedule of Fair Value of Outstanding Foreign Currency Forward Contracts

As of September 30, 2021 and December 31, 2020, the fair value of outstanding foreign currency forward contracts is recorded in the balance sheet as follows:

	September 30, 2021			December 31, 2020
(Dollars in millions)
Derivative contracts	$	0.1		$	1.2
Accrued liabilities		(0.3	)		(0.5	)
Net (liability) asset on balance sheet	$	(0.2	)	$	0.7

Summary of Net Currency Units Outstanding

As of September 30, 2021 and December 31, 2020, the net currency units outstanding for these contracts were:

	September 30, 2021		December 31, 2020
(In millions)
British Pounds		GBP 0.0		GBP 2.0
United States Dollars		USD 5.2		USD 7.6
Euro		EUR 0.4		EUR 0.0

Commitments and Contingent Liabilities (Tables)

9 Months Ended

Sep. 30, 2021

Commitments And Contingencies Disclosure [Abstract]

Changes in Accrued Liability for Environmental Matters

The following table reflects changes in the accrual for environmental remediation. A total of $2.8 million and $2.9 million are classified as current liabilities as of September 30, 2021 and December 31, 2020:

	Period ended
	September 30, 2021			December 31, 2020
(Dollars in millions)
Balance at beginning of year	$	11.0		$	9.5
Expense		0.2			1.8
Reversal of reserves		(0.1	)		0.0
Cash expenditures		(0.3	)		(0.4	)
Currency translation		(0.2	)		0.1
Balance at end of period	$	10.6		$	11.0

Plant Closure and Divestitures - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended			3 Months Ended			9 Months Ended
	Oct. 31, 2021	Jun. 30, 2021	Feb. 28, 2021	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Restructuring Cost and Reserve [Line Items]
Gain on sale				$ 0.0		$ 0.0	$ 7.8	$ 0.0
Railroad And Utility Products And Services [Member] \| Facility [Member] \| Subsequent Event [Member]
Restructuring Cost and Reserve [Line Items]
Gain on sale	$ 23.0
Netherlands [Member]
Restructuring Cost and Reserve [Line Items]
Gain on sale		$ 0.3
West Virginia [Member] \| Coal Tar Distillation [Member]
Restructuring Cost and Reserve [Line Items]
Gain on sale			$ 5.7
Cash proceed from sale			$ 2.6
Pennsylvania [Member] \| Coal Tar Distillation [Member]
Restructuring Cost and Reserve [Line Items]
Gain on sale					$ 1.8

Plant Closures and Divestitures - Summary of Restructuring Activities and Related Reserves (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Restructuring Cost and Reserve [Line Items]
Reserve, Beginning Balance	$ 6.0	$ 4.0
Accrual	2.4	6.8
Cost charged against assets	(2.4)	(3.4)
Reversal of accrued charges	(0.9)	(0.3)
Cash paid	(2.6)	(1.3)
Currency translation		0.2
Sale of subsidiary	(2.5)
Reserve, Ending Balance	0.0	6.0
Severance and Employee Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Reserve, Beginning Balance	0.9	0.9
Accrual	0.0	0.5
Cost charged against assets	(0.0)	(0.0)
Reversal of accrued charges	(0.2)	(0.3)
Cash paid	(0.7)	(0.2)
Currency translation		0.0
Sale of subsidiary	0.0
Reserve, Ending Balance	0.0	0.9
Asset Retirement [Member]
Restructuring Cost and Reserve [Line Items]
Reserve, Beginning Balance	2.8	0.7
Accrual	0.0	2.9
Cost charged against assets	(0.0)	(0.0)
Reversal of accrued charges	(0.7)	(0.0)
Cash paid	(1.9)	(0.8)
Currency translation		0.0
Sale of subsidiary	(0.2)
Reserve, Ending Balance	0.0	2.8
Other [Member]
Restructuring Cost and Reserve [Line Items]
Reserve, Beginning Balance	2.3	2.4
Accrual	2.4	3.4
Cost charged against assets	(2.4)	(3.4)
Reversal of accrued charges	(0.0)	(0.0)
Cash paid	(0.0)	(0.3)
Currency translation		0.2
Sale of subsidiary	(2.3)
Reserve, Ending Balance	$ 0.0	$ 2.3

Discontinued Operations - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Schedule Of Equity Method Investments [Line Items]
Restricted cash held with escrow	$ 2.3
Period of time from acquisition date for indemnity claims	6 months
Accrued liabilities	$ (29.4)	$ 6.7
Koppers (Jiangsu) Carbon Chemical Company Limited [Member]
Schedule Of Equity Method Investments [Line Items]
Accrued liabilities	$ 6.1		$ 5.6
Koppers (Jiangsu) Carbon Chemical Company Limited [Member] \| Carbon Materials and Chemicals [Member]
Schedule Of Equity Method Investments [Line Items]
Ownership percentage	75.00%

Discontinued Operations - Schedule of Net Sales and Operating Loss Profit from Discontinued Operations (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020
Discontinued Operations And Disposal Groups [Abstract]
Net sales	$ 8.8	$ 31.6
Operating income (loss)	$ 0.3	$ (5.0)

Fair Value Measurements - Schedule of Company's Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value [Member]
Financial assets:
Investments and other assets	$ 1.2	$ 1.2
Financial liabilities:
Long-term debt (including current portion)	828.5	799.2
Carrying Value [Member]
Financial assets:
Investments and other assets	1.2	1.2
Financial liabilities:
Long-term debt (including current portion)	$ 813.5	$ 784.2

Comprehensive (Loss) Income and Equity - Schedule of Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Equity [Abstract]
Net income	$ 10.0	$ 75.5	$ 62.7	$ 102.4
Changes in other comprehensive (loss) income:
Currency translation adjustment	(11.1)	12.3	(14.5)	3.1
Unrealized (loss) gain on cash flow hedges, net of tax benefit (expense) of $6.1, $(5.3), $1.2 and $(6.9)	(16.6)	15.3	(3.3)	19.8
Unrecognized pension net loss, net of tax expense of $0.1, $0.1, $0.3 and $0.2	0.3	0.2	0.8	0.6
Total comprehensive (loss) income	(17.4)	103.3	45.7	125.9
Comprehensive (loss) income attributable to noncontrolling interests	(0.2)	0.9	(0.2)	(0.1)
Comprehensive (loss) income attributable to Koppers	$ (17.2)	$ 102.4	$ 45.9	$ 126.0

- Definition

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income (loss) and other comprehensive income (loss), attributable to noncontrolling interests. Excludes changes in equity resulting from investments by owners and distributions to owners.

+ References + Details

- Definition

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income. Excludes changes in equity resulting from investments by owners and distributions to owners.

+ References + Details

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

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-Name Accounting Standards Codification
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-SubTopic 10
-Section 55
-Paragraph 4K
-URI http://asc.fasb.org/extlink&oid=120409616&loc=SL4591552-111686

Reference 33: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=124431687&loc=d3e22658-107794

+ Details

Comprehensive (Loss) Income and Equity - Schedule of Comprehensive (Loss) Income (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Equity [Abstract]
Tax benefit (expense), unrealized (loss) gains on cash flow hedges	$ 6.1	$ (5.3)	$ 1.2	$ (6.9)
Tax expense, unrecognized pension net loss	$ 0.1	$ 0.1	$ 0.3	$ 0.2

Comprehensive (Loss) Income and Equity - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Equity [Abstract]
Loss related to derivative instrument	$ 11.1	$ 1.3	$ 31.0	$ 2.1

Comprehensive (Loss) Income and Equity - Schedule of Change in Equity (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 416.1	$ 186.3	$ 346.0	$ 158.7
Net income	10.0	75.5	62.7	102.4
Issuance of common stock	0.3	0.3	2.2	0.8
Employee stock plans	3.3	2.9	9.9	8.5
Sale of discontinued operations		(7.2)		(7.2)
Changes in other comprehensive (loss) income:
Currency translation adjustment	(11.4)	12.3	(10.1)	3.1
Cumulative translation adjustment loss on sale of subsidiary			(4.4)
Unrealized gain (loss) on cash flow hedges	(16.6)	15.3	(3.3)	19.8
Unrecognized pension net loss	0.3	0.2	0.8	0.6
Repurchases of common stock	(1.4)		(3.3)	(1.2)
Balance	400.5	285.5	400.5	285.5
Common Stock [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	0.2	0.2	0.2	0.2
Net income	0.0	0.0	0.0	0.0
Issuance of common stock	0.0	0.0	0.0	0.0
Employee stock plans	0.0	0.0	0.0	0.0
Sale of discontinued operations		0.0		0.0
Changes in other comprehensive (loss) income:
Currency translation adjustment	0.0	0.0	0.0	0.0
Cumulative translation adjustment loss on sale of subsidiary			0.0
Unrealized gain (loss) on cash flow hedges	0.0	0.0	0.0	0.0
Unrecognized pension net loss	0.0	0.0	0.0	0.0
Repurchases of common stock	(0.0)		(0.0)	(0.0)
Balance	0.2	0.2	0.2	0.2
Additional Paid-In Capital [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	242.7	228.0	234.1	221.9
Net income	0.0	0.0	0.0	0.0
Issuance of common stock	0.3	0.3	2.2	0.8
Employee stock plans	3.3	2.9	9.9	8.5
Sale of discontinued operations		0.0		0.0
Changes in other comprehensive (loss) income:
Currency translation adjustment	0.0	0.0	0.0	0.0
Cumulative translation adjustment loss on sale of subsidiary			0.0
Unrealized gain (loss) on cash flow hedges	0.0	0.0	0.0	0.0
Unrecognized pension net loss	0.0	0.0	0.0	0.0
Repurchases of common stock	(0.0)		(0.0)	(0.0)
Balance	246.2	231.2	246.2	231.2
Retained Earnings [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	268.8	121.6	215.8	93.8
Net income	10.2	75.6	63.0	103.4
Issuance of common stock	0.0	0.0	0.0	0.0
Employee stock plans	0.0	0.0	0.0	0.0
Sale of discontinued operations		0.0		0.0
Changes in other comprehensive (loss) income:
Currency translation adjustment	(0.3)	0.0	0.0	0.0
Cumulative translation adjustment loss on sale of subsidiary			0.0
Unrealized gain (loss) on cash flow hedges	0.0	0.0	0.0	0.0
Unrecognized pension net loss	0.0	0.0	0.0	0.0
Repurchases of common stock	(0.0)		(0.0)	(0.0)
Balance	278.8	197.2	278.8	197.2
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(5.5)	(81.8)	(15.9)	(77.7)
Net income	0.0	0.0	0.0	0.0
Issuance of common stock	0.0	0.0	0.0	0.0
Employee stock plans	0.0	0.0	0.0	0.0
Sale of discontinued operations		0.0		0.0
Changes in other comprehensive (loss) income:
Currency translation adjustment	(11.1)	11.3	(10.2)	2.1
Cumulative translation adjustment loss on sale of subsidiary			(4.4)
Unrealized gain (loss) on cash flow hedges	(16.6)	15.3	(3.3)	19.8
Unrecognized pension net loss	0.3	0.2	0.8	0.6
Repurchases of common stock	(0.0)		(0.0)	(0.0)
Balance	(33.0)	(55.1)	(33.0)	(55.1)
Treasury Stock [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(94.4)	(92.1)	(92.5)	(90.9)
Net income	0.0	0.0	0.0	0.0
Issuance of common stock	0.0	0.0	0.0	0.0
Employee stock plans	0.0	0.0	0.0	0.0
Sale of discontinued operations		0.0		0.0
Changes in other comprehensive (loss) income:
Currency translation adjustment	0.0	0.0	0.0	0.0
Cumulative translation adjustment loss on sale of subsidiary			0.0
Unrealized gain (loss) on cash flow hedges	0.0	0.0	0.0	0.0
Unrecognized pension net loss	0.0	0.0	0.0	0.0
Repurchases of common stock	(1.4)		(3.3)	(1.2)
Balance	(95.8)	(92.1)	(95.8)	(92.1)
Noncontrolling Interests [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	4.3	10.4	4.3	11.4
Net income	(0.2)	(0.1)	(0.3)	(1.0)
Issuance of common stock	0.0	0.0	0.0	0.0
Employee stock plans	0.0	0.0	0.0	0.0
Sale of discontinued operations		(7.2)		(7.2)
Changes in other comprehensive (loss) income:
Currency translation adjustment	0.0	1.0	0.1	1.0
Cumulative translation adjustment loss on sale of subsidiary			0.0
Unrealized gain (loss) on cash flow hedges	0.0	0.0	0.0	0.0
Unrecognized pension net loss	0.0	0.0	0.0	0.0
Repurchases of common stock	(0.0)		(0.0)	(0.0)
Balance	$ 4.1	$ 4.1	$ 4.1	$ 4.1

Earnings per Common Share - Schedule of Computation of Basic and Diluted Earnings per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income attributable to Koppers	$ 10.2	$ 75.6	$ 63.0	$ 103.4
Less: Income (loss) from discontinued operations, net of tax	0.0	0.6	0.1	(3.8)
(Loss) gain on sale of discontinued operations	(0.5)	35.8	0.0	35.8
Noncontrolling interest related to discontinued operations	0.0	0.1	0.0	0.8
Income from continuing operations attributable to Koppers	$ 10.7	$ 39.1	$ 62.9	$ 70.6
Weighted average common shares outstanding:
Basic	21,322	21,047	21,253	20,968
Effect of dilutive securities	625	333	696	259
Diluted	21,947	21,380	21,949	21,227
Earnings per common share – continuing operations:
Basic earnings per common share	$ 0.50	$ 1.86	$ 2.96	$ 3.37
Diluted earnings per common share	$ 0.49	$ 1.83	$ 2.87	$ 3.33
Other data:
Antidilutive securities excluded from computation of diluted earnings per common share	563	876	437	862

Stock-based Compensation - Additional Information (Detail) - USD ($) $ in Millions		9 Months Ended
	May 06, 2021	Sep. 30, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Future compensation expense related to non-vested stock-based compensation arrangements		$ 19.5
Future compensation expense, weighted-average expected period of recognition in months		27 months
Restricted Stock Units (RSUs) [Member] \| Board of Directors [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based awards, vesting period		1 year
Restricted Stock Units (RSUs) [Member] \| Employee [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based awards, vesting period		4 years
Restricted Stock Units (RSUs) [Member] \| Employee [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based awards, vesting period		2 years
Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting performance stock units if minimum performance criteria are not achieved		0
Stock options, term in years		3 years
Performance Stock Units [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of target award earned by participants		200.00%
Performance Stock Units [Member] \| Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of target award earned by participants		0.00%
Employee Stock Option [Member] \| Grants in March 2015 and Thereafter [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options, term in years		10 years
Employee Stock Option [Member] \| Executive Officer [Member] \| Grants in March 2015 and Thereafter [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based awards, vesting period		4 years
2020 LTIP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in number of awards available for grant	1,500,000
Employee Share Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in number of awards available for grant	300,000

Stock-based Compensation - Performance Award Fair Value Assumptions (Detail)	9 Months Ended
	Sep. 30, 2021 $ / shares
January 2021 Grant [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant date price per share of performance award	$ 29.84
Expected dividend yield per share	0.00%
Expected volatility	54.80%
Risk-free interest rate	0.59%
Grant date fair value per share	$ 15.79
January 2021 Grant [Member] \| Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant date price per share of performance award	$ 29.84
Expected dividend yield per share	0.00%
Expected volatility	68.70%
Risk-free interest rate	0.16%
Look-back period in years	3 years
Grant date fair value per share	$ 41.50
March 2020 Grant [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant date price per share of performance award	$ 19.63
Expected dividend yield per share	0.00%
Expected volatility	42.85%
Risk-free interest rate	0.87%
Grant date fair value per share	$ 8.42
March 2020 Grant [Member] \| Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant date price per share of performance award	$ 19.63
Expected dividend yield per share	0.00%
Expected volatility	45.60%
Risk-free interest rate	0.72%
Look-back period in years	2 years 9 months 29 days
Grant date fair value per share	$ 11.56
March 2019 Grant [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant date price per share of performance award	$ 26.63
Expected dividend yield per share	0.00%
Expected volatility	39.44%
Risk-free interest rate	2.53%
Grant date fair value per share	$ 11.29
March 2019 Grant [Member] \| Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant date price per share of performance award	$ 26.63
Expected dividend yield per share	0.00%
Expected volatility	39.00%
Risk-free interest rate	2.50%
Look-back period in years	2 years 9 months 25 days
Grant date fair value per share	$ 40.30

Stock-based Compensation - Summary of Performance Stock Units (Detail) - shares	Sep. 30, 2021	Dec. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares outstanding	1,069,679	901,253
Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares outstanding	538,398	391,744
2019 - 2021 [Member] \| Minimum [Member] \| Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares outstanding	0
2019 - 2021 [Member] \| Target Shares [Member] \| Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares outstanding	234,597
2019 - 2021 [Member] \| Maximum [Member] \| Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares outstanding	281,330
2020 - 2022 [Member] \| Minimum [Member] \| Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares outstanding	0
2020 - 2022 [Member] \| Target Shares [Member] \| Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares outstanding	154,099
2020 - 2022 [Member] \| Maximum [Member] \| Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares outstanding	308,198
2021 - 2023 [Member] \| Minimum [Member] \| Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares outstanding	0
2021 - 2023 [Member] \| Target Shares [Member] \| Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares outstanding	149,874
2021 - 2023 [Member] \| Maximum [Member] \| Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares outstanding	299,748

Stock-based Compensation - Summary of Status and Activity of Non-Vested Stock Units (Detail)	9 Months Ended
	Sep. 30, 2021 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Non-vested, Beginning Balance	901,253
Granted	372,173
Performance share adjustment	(1,296)
Vested	(201,658)
Forfeited	(793)
Non-vested, Ending Balance	1,069,679
Beginning Balance, Non-vested, Weighted Average Grant Date Fair Value per Unit \| $ / shares	$ 25.48
Granted, Weighted Average Grant Date Fair Value per Unit \| $ / shares	34.91
Performance share adjustment, Weighted Average Grant Date Fair Value per Unit \| $ / shares	44.29
Vested, Weighted Average Grant Date Fair Value per Unit \| $ / shares	22.52
Forfeited, Weighted Average Grant Date Fair Value per Unit \| $ / shares	30.94
Ending Balance, Non-vested, Weighted Average Grant Date Fair Value per Unit \| $ / shares	$ 29.23
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Non-vested, Beginning Balance	509,509
Granted	222,299
Performance share adjustment	0
Vested	(199,837)
Forfeited	(690)
Non-vested, Ending Balance	531,281
Performance Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Non-vested, Beginning Balance	391,744
Granted	149,874
Performance share adjustment	(1,296)
Vested	(1,821)
Forfeited	(103)
Non-vested, Ending Balance	538,398

Stock-based Compensation - Stock Options Fair Value Assumptions (Detail)	9 Months Ended
	Sep. 30, 2021 $ / shares
January 2021 Grant [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant date price per share of stock option award	$ 29.84
Expected dividend yield per share	0.00%
Expected life in years	6 years 7 months 20 days
Expected volatility	54.80%
Risk-free interest rate	0.59%
Grant date fair value per share of option awards	$ 15.79
March 2020 Grant [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant date price per share of stock option award	$ 19.63
Expected dividend yield per share	0.00%
Expected life in years	6 years 4 months 24 days
Expected volatility	42.85%
Risk-free interest rate	0.87%
Grant date fair value per share of option awards	$ 8.42
March 2019 Grant [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant date price per share of stock option award	$ 26.63
Expected dividend yield per share	0.00%
Expected life in years	6 years 1 month 20 days
Expected volatility	39.44%
Risk-free interest rate	2.53%
Grant date fair value per share of option awards	$ 11.29
March 2018 Grant [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant date price per share of stock option award	$ 41.60
Expected dividend yield per share	0.00%
Expected life in years	5 years 8 months 23 days
Expected volatility	37.05%
Risk-free interest rate	2.67%
Grant date fair value per share of option awards	$ 16.38

Stock-based Compensation - Summary of Status and Activity of Stock Options (Detail) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options, Outstanding at December 31, 2020 \| shares	1,120,254
Options, Granted \| shares	90,879
Options, Exercised \| shares	(71,785)
Options, Expired \| shares	(25,402)
Options, Outstanding at September 30, 2021 \| shares	1,113,946
Options, Exercisable at September 30, 2021 \| shares	797,911
Weighted Average Exercise Price per Option, Outstanding at December 31, 2020 \| $ / shares	$ 26.89
Weighted Average Exercise Price per Option, Granted \| $ / shares	29.84
Weighted Average Exercise Price per Option, Exercised \| $ / shares	17.74
Weighted Average Exercise Price per Option, Expired \| $ / shares	40.26
Weighted Average Exercise Price per Option, Outstanding at September 30, 2021 \| $ / shares	27.41
Weighted Average Exercise Price per Option, Exercisable at September 30, 2021 \| $ / shares	$ 28.16
Weighted Average Remaining Contractual Term, Outstanding at September 30, 2021	5 years 2 months 15 days
Weighted Average Remaining Contractual Term, Exercisable at September 30, 2021	4 years 29 days
Aggregate Intrinsic Value, Outstanding at September 30, 2021 \| $	$ 7.8
Aggregate Intrinsic Value, Exercisable at September 30, 2021 \| $	$ 5.7

Stock-based Compensation - Schedule of Stock-Based Compensation Expense Recognized (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Stock-based compensation expense recognized:
Less related income tax benefit	$ 0.8	$ 0.7	$ 2.6	$ 2.2
Decrease in net income attributable to Koppers	2.4	2.2	7.2	6.3
Intrinsic value of exercised stock options	0.0	0.0	2.2	0.0
Cash received from the exercise of stock options	0.0	0.0	2.3	0.0
Selling, General and Administrative Expenses [Member]
Stock-based compensation expense recognized:
Stock-based compensation expense	$ 3.2	$ 2.9	$ 9.8	$ 8.5

Segment Information - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($) Segment	Dec. 31, 2020 USD ($)
Segment Reporting [Abstract]
Number of reportable segments \| Segment	3
Contract with customer, asset, net, current \| $	$ 13.3	$ 5.8

Segment Information - Summary of Results of Segment Operations (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues from external customers
Net sales	$ 424.8	$ 437.5	$ 1,273.3	$ 1,276.0
Depreciation and amortization expense	13.4	12.9	43.4	39.7
Operating profit (loss)	24.6	58.6	112.8	121.9
Railroad And Utility Products And Services [Member]
Revenues from external customers
Net sales	186.9	191.0	574.3	590.9
Performance Chemicals [Member]
Revenues from external customers
Net sales	115.2	147.9	384.4	396.4
Carbon Materials and Chemicals [Member]
Revenues from external customers
Net sales	122.7	98.6	314.6	288.7
Operating Segments [Member] \| Railroad And Utility Products And Services [Member]
Revenues from external customers
Net sales	186.9	191.0	574.3	590.9
Depreciation and amortization expense	5.3	4.9	16.9	14.8
Operating profit (loss)	(0.7)	15.0	12.3	40.4
Operating Segments [Member] \| Performance Chemicals [Member]
Revenues from external customers
Net sales	115.2	147.9	384.4	396.4
Depreciation and amortization expense	4.1	4.3	13.6	13.2
Operating profit (loss)	11.6	30.4	65.1	67.1
Operating Segments [Member] \| Carbon Materials and Chemicals [Member]
Revenues from external customers
Net sales	122.7	98.6	314.6	288.7
Depreciation and amortization expense	4.0	3.7	12.9	11.7
Operating profit (loss)	14.9	13.7	39.1	15.9
Intersegment [Member]
Revenues from external customers
Net sales	22.8	23.5	66.4	69.8
Intersegment [Member] \| Performance Chemicals [Member]
Revenues from external customers
Net sales	3.9	3.4	11.2	10.3
Intersegment [Member] \| Carbon Materials and Chemicals [Member]
Revenues from external customers
Net sales	18.9	20.1	55.2	59.5
Corporate, Non-Segment
Revenues from external customers
Operating profit (loss)	$ (1.2)	$ (0.5)	$ (3.7)	$ (1.5)

Segment Information - Summary of Results of Segment Operations (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020
Segment Reporting [Abstract]
Net sales	$ 8.8	$ 31.6
Depreciation and amortization expense	0.1	0.6
Operating (loss) profit	$ 0.3	$ (5.0)

Segment Information - Schedule of Segment Revenues for Significant Product Lines (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Net sales	$ 424.8	$ 437.5	$ 1,273.3	$ 1,276.0
Railroad And Utility Products And Services [Member]
Segment Reporting Information [Line Items]
Net sales	186.9	191.0	574.3	590.9
Railroad And Utility Products And Services [Member] \| Railroad Treated Products [Member]
Segment Reporting Information [Line Items]
Net sales	107.1	108.4	327.6	345.5
Railroad And Utility Products And Services [Member] \| Utility Poles [Member]
Segment Reporting Information [Line Items]
Net sales	59.6	63.6	186.2	191.4
Railroad And Utility Products And Services [Member] \| Railroad Infrastructure Services [Member]
Segment Reporting Information [Line Items]
Net sales	8.8	9.2	25.3	23.3
Railroad And Utility Products And Services [Member] \| Rail Joints [Member]
Segment Reporting Information [Line Items]
Net sales	5.7	4.5	17.9	16.9
Railroad And Utility Products And Services [Member] \| Other Products [Member]
Segment Reporting Information [Line Items]
Net sales	5.7	5.3	17.3	13.8
Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales	115.2	147.9	384.4	396.4
Performance Chemicals [Member] \| Other Products [Member]
Segment Reporting Information [Line Items]
Net sales	5.1	7.4	18.0	20.7
Performance Chemicals [Member] \| Wood Preservative Products [Member]
Segment Reporting Information [Line Items]
Net sales	110.1	140.5	366.4	375.7
Carbon Materials and Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales	122.7	98.6	314.6	288.7
Carbon Materials and Chemicals [Member] \| Other Products [Member]
Segment Reporting Information [Line Items]
Net sales	7.4	7.2	20.6	21.0
Carbon Materials and Chemicals [Member] \| Pitch and Related Products [Member]
Segment Reporting Information [Line Items]
Net sales	70.4	62.9	177.7	171.4
Carbon Materials and Chemicals [Member] \| Phthalic Anhydride and Other Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales	19.7	14.5	52.2	49.8
Carbon Materials and Chemicals [Member] \| Creosote and Distillates [Member]
Segment Reporting Information [Line Items]
Net sales	19.0	10.3	48.6	33.8
Carbon Materials and Chemicals [Member] \| Naphthalene [Member]
Segment Reporting Information [Line Items]
Net sales	$ 6.2	$ 3.8	$ 15.5	$ 12.7

Segment Information - Summary of Tangible and Intangible Assets by Segments (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Segment Reporting Asset Reconciling Item [Line Items]
Assets	$ 1,652.2	$ 1,598.6
Goodwill	296.1	297.8
Railroad And Utility Products And Services [Member] \| Operating Segments [Member]
Segment Reporting Asset Reconciling Item [Line Items]
Assets	590.9	583.1
Goodwill	120.8	121.1
Performance Chemicals [Member] \| Operating Segments [Member]
Segment Reporting Asset Reconciling Item [Line Items]
Assets	586.6	536.1
Goodwill	175.3	176.7
Carbon Materials and Chemicals [Member] \| Operating Segments [Member]
Segment Reporting Asset Reconciling Item [Line Items]
Assets	445.2	424.2
All Other [Member] \| Operating Segments [Member]
Segment Reporting Asset Reconciling Item [Line Items]
Assets	$ 29.5	$ 55.2

- Definition

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(11))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56426-112766

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section S99
-Paragraph 1B
-Subparagraph (SX 210.13-02(a)(4)(iii)(A))
-URI http://asc.fasb.org/extlink&oid=124359900&loc=SL124442552-122756

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section S99
-Paragraph 1A
-Subparagraph (SX 210.13-01(a)(4)(iv))
-URI http://asc.fasb.org/extlink&oid=124359900&loc=SL124442526-122756

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(12))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
-Paragraph 1B
-Subparagraph (SX 210.13-02(a)(4)(i))
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Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
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-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=124433192&loc=SL2890621-112765

Reference 9: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section S99
-Paragraph 1A
-Subparagraph (SX 210.13-01(a)(4)(ii))
-URI http://asc.fasb.org/extlink&oid=124359900&loc=SL124442526-122756

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
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-Section S99
-Paragraph 1A
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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
-Paragraph 1A
-Subparagraph (SX 210.13-01(a)(4)(iii)(A))
-URI http://asc.fasb.org/extlink&oid=124359900&loc=SL124442526-122756

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(ii))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
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Reference 14: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
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Reference 15: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=123596393&loc=d3e14064-108612

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section S99
-Paragraph 1B
-Subparagraph (SX 210.13-02(a)(4)(iii)(B))
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Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 18: http://www.xbrl.org/2003/role/disclosureRef
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Reference 19: http://www.xbrl.org/2003/role/disclosureRef
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Reference 20: http://www.xbrl.org/2003/role/disclosureRef
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Reference 21: http://www.xbrl.org/2003/role/disclosureRef
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Reference 22: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 23: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123359005&loc=d3e8906-108599

+ Details

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Income Taxes [Line Items]
Estimated annual effective income tax rate			26.80%	25.80%
Income taxes as a percentage of pretax income	31.40%	18.00%	26.40%	17.40%
Net tax benefit included in income taxes, discrete items		$ 3,100,000	$ 0	$ 7,300,000
Percentage of allowable business interest expense deduction from adjusted taxable income				50.00%
Income tax expense (benefit)	$ 4,800,000	8,600,000	22,400,000	$ 14,800,000
Unrecognized tax benefits	2,300,000		2,300,000		$ 2,500,000
Unrecognized tax benefits with impact on the effective tax rate	2,300,000		2,300,000		2,500,000
Accrued interest expense and penalties	$ 800,000		$ 800,000		$ 800,000
CARES Act [Member]
Income Taxes [Line Items]
Percentage of allowable business interest expense deduction from adjusted taxable income				30.00%
Reverse Valuation Allowance Carryforward [Member]
Income Taxes [Line Items]
Income tax expense (benefit)		$ 2,000,000.0		$ (2,400,000)

Income Taxes - Summary of Income Taxes Reconciled with Federal Statutory Rate (Detail)	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Federal income tax rate	21.00%	21.00%
Foreign earnings taxed at different rates	3.60%	2.70%
Nondeductible expenses	1.30%	1.30%
State income taxes, net of federal tax benefit	0.80%	0.50%
Change in tax contingency reserves	0.10%	0.10%
GILTI inclusion, net of foreign tax credits	0.00%	0.20%
Estimated annual effective income tax rate	26.80%	25.80%

Inventories - Components of Net Inventories (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Inventory Disclosure [Abstract]
Raw materials	$ 244.6	$ 233.7
Work in process	13.3	12.4
Finished goods	102.7	99.3
Inventories, gross	360.6	345.4
Less revaluation to LIFO	65.5	49.6
Net	$ 295.1	$ 295.8

Property, Plant and Equipment - Property, Plant and Equipment (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Property, plant and equipment, gross	$ 894.3	$ 903.8
Less accumulated depreciation	433.4	494.7
Net	460.9	409.1
Land [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, gross	15.5	16.7
Buildings [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, gross	71.6	75.0
Machinery and Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, gross	$ 807.2	$ 812.1

Pensions and Post-Retirement Benefit Plans - Additional Information (Detail)	Sep. 30, 2021 Plan
Defined Benefit Plan Disclosure [Line Items]
Number of domestic non-qualified defined benefit plans	3
Salaried and Hourly Employees [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of domestic non-qualified defined benefit plans	2

Pensions and Post-Retirement Benefit Plans - Components of Net Periodic Benefit Cost for Pension Plans (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Postemployment Benefits [Abstract]
Service cost	$ 0.4	$ 0.3	$ 1.1	$ 1.0
Interest cost	1.1	1.6	3.8	4.7
Expected return on plan assets	(1.8)	(2.0)	(5.5)	(5.8)
Amortization of net loss	0.4	0.4	1.1	1.1
Net periodic benefit cost	0.1	0.3	0.5	1.0
Defined contribution plan expense	$ 1.9	$ 1.6	$ 6.5	$ 5.6

Debt - Schedule of Long-Term Debt Instruments (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Debt	$ 813.5	$ 784.2
Less short-term debt and current maturities of long-term debt	4.6	10.1
Less unamortized debt issuance costs	6.3	8.3
Long-term debt	$ 802.6	765.8
Term Loan [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.56%
Debt, Maturity	2024
Debt	$ 4.6	12.2
Senior Secured Credit Facilities [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.56%
Debt, Maturity	2024
Debt	$ 308.9	272.0
6.00 percent Senior Notes due 2025 [Member] \| Senior Notes [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.00%
Debt, Maturity	2025
Debt	$ 500.0	$ 500.0

Debt - Additional Information (Detail) - USD ($)		9 Months Ended
Debt - Additional Information (Detail) - USD ($)	Feb. 15, 2022	Sep. 30, 2021
Senior Secured Credit Facilities [Member]
Debt Instrument [Line Items]
Letters of credit, amount outstanding		$ 7,700,000
6.00 percent Senior Notes due 2025 [Member] \| Senior Notes [Member]
Debt Instrument [Line Items]
Senior notes, payment terms		The 2025 Notes pay interest semi-annually in arrears on February 15 and August 15 and will mature on February 15, 2025 unless earlier redeemed or repurchased.
Debt instrument, redemption price percentage		104.50%
Debt maturity date		Feb. 15, 2025
6.00 percent Senior Notes due 2025 [Member] \| Senior Notes [Member] \| Scenario, Forecast [Member]
Debt Instrument [Line Items]
Debt instrument, redemption price percentage	101.50%
Senior Secured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Revolving credit facility		$ 600,000,000.0
Revolving credit facility, variable interest rate basis		LIBOR
Secured Term Loan Facility [Member]
Debt Instrument [Line Items]
Secured term loan		$ 100,000,000.0
Quarterly amortization		2,500,000
Revolving Credit Facility [Member] \| Senior Secured Credit Facilities [Member]
Debt Instrument [Line Items]
Debt instrument, unused borrowing capacity		$ 283,400,000

Asset Retirement Obligations - Schedule of Changes in Carrying Values of Asset Retirement Obligations (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Asset Retirement Obligation Disclosure [Abstract]
Asset retirement obligation at beginning of year	$ 19.8	$ 20.7
Accretion expense	0.7	1.1
Revision in estimated cash flows	(2.8)	4.6
Cash expenditures	(6.7)	(6.6)
Balance at end of period	$ 11.0	$ 19.8

Leases - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Leases [Abstract]
Operating lease costs	$ 7.5	$ 8.1	$ 22.8	$ 23.2
Variable lease costs	$ 0.8	$ 0.8	$ 2.4	$ 2.7

Leases - Summary of Amount and Timing of Cash Flows From Operating Leases (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Leases [Abstract]
2021	$ 7.3
2022	27.3
2023	21.3
2024	17.3
2025	14.0
Thereafter	34.8
Total lease payments	122.0
Less: Interest	(26.6)
Present value of lease liabilities	$ 95.4	$ 102.5

Leases - Schedule of Supplemental Condensed Consolidated Balance Sheet Information Related to Leases (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Operating leases:
Operating lease right-of-use assets	$ 95.2	$ 102.5
Current operating lease liabilities	21.6	21.2
Operating lease liabilities	73.8	81.3
Total operating lease liabilities	$ 95.4	$ 102.5
Weighted average remaining lease term, in years	5 years 8 months 12 days	6 years 4 months 24 days
Weighted average discount rate	7.20%	7.50%

Derivative Financial Instruments - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Maximum period hedged in cash flow hedge	36 months
Unrealized gains, reclassification period	12 months
Unrealized gains, net of tax, expected to be reclassified from other comprehensive (loss) income into earnings	$ 34.4

Derivative Financial Instruments - Schedule of Outstanding Copper Swap Contracts (Detail) - Copper Swap Contracts [Member] £ in Millions, $ in Millions	Sep. 30, 2021 USD ($)	Sep. 30, 2021 GBP (£)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)
Derivatives Fair Value [Line Items]
Copper Swap Contracts Units Outstanding \| £		£ 41.2		£ 73.8
Net Fair Value - Asset \| $	$ 62.1		$ 69.2
Designated as Hedging Instrument [Member] \| Cash Flow Hedging [Member]
Derivatives Fair Value [Line Items]
Copper Swap Contracts Units Outstanding \| £		36.1		62.3
Net Fair Value - Asset \| $	54.0		58.3
Contracts Where Hedge Accounting Was not Elected [Member]
Derivatives Fair Value [Line Items]
Copper Swap Contracts Units Outstanding \| £		£ 5.1		£ 11.5
Net Fair Value - Asset \| $	$ 8.1		$ 10.9

Derivative Financial Instruments - Schedule of Fair Value of Outstanding Copper Swap Contracts Recorded in Balance Sheet (Detail) - Copper Swap Contracts [Member] - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Derivative [Line Items]
Net Fair Value - Asset	$ 62.1	$ 69.2
Accumulated other comprehensive gain, net of tax	41.3	44.4
Derivative Contracts [Member]
Derivative [Line Items]
Net Fair Value - Asset	51.7	37.3
Non Current Derivative Contracts [Member]
Derivative [Line Items]
Net Fair Value - Asset	$ 10.4	$ 31.9

Derivative Financial Instruments - Schedule of Unrealized Gain (Loss) from Contract Where Hedge Accounting Was Not Elected (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Hedges \| Contracts Where Hedge Accounting Was not Elected [Member]
Derivative [Line Items]
(Loss) gain from contracts where hedge accounting was not elected	$ (4.4)	$ 3.9	$ (2.7)	$ 4.2

Derivative Financial Instruments - Schedule of Fair Value of Outstanding Foreign Currency Forward Contracts (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivative [Line Items]
Not designated gross derivative liability	$ (0.2)	$ 0.7
Forward Contracts [Member] \| Accrued Liabilities [Member]
Derivative [Line Items]
Not designated gross derivative liability	(0.3)	(0.5)
Forward Contracts [Member] \| Derivative Contracts [Member]
Derivative [Line Items]
Not designated gross derivative liability	$ 0.1	$ 1.2

Derivative Financial Instruments - Summary of Net Currency Units Outstanding (Detail)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 GBP (£)	Sep. 30, 2021 EUR (€)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)	Dec. 31, 2020 EUR (€)
Offsetting [Abstract]
Net currency units outstanding	$ 5,200,000	£ 0	€ 400,000	$ 7,600,000	£ 2,000,000	€ 0

Commitments and Contingent Liabilities - Additional Information (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) State Plaintiff Case site Party Plant	Dec. 31, 2020 USD ($) Plaintiff Case	Dec. 31, 2019 USD ($)
Loss Contingencies [Line Items]
Number of states with new claims filed \| State	2
Number of plaintiffs \| Plaintiff	61	64
Environmental remediation and regulation liability	$ 10.6	$ 11.0	$ 9.5
Accrued liability for environmental matters, current	2.8	2.9
Portland Harbor and Newark Bay CERCLA sites [Member]
Loss Contingencies [Line Items]
Costs of participating in PRP group	$ 3.4
Beazer East [Member]
Loss Contingencies [Line Items]
Sites listed on National Priorities List \| site	1
Compensatory Damages [Member]
Loss Contingencies [Line Items]
Number of cases \| Case	32
Punitive Damages [Member]
Loss Contingencies [Line Items]
Number of cases \| Case	27
Indemnification Agreement [Member] \| Beazer East [Member]
Loss Contingencies [Line Items]
Environmental remediation costs paid by others, per year		$ 6.4
Coal Tar Pitch Cases [Member]
Loss Contingencies [Line Items]
Number of cases \| Case	32	34
Pennsylvania [Member]
Loss Contingencies [Line Items]
Number of cases \| Case	31
Pennsylvania [Member] \| Coal Tar Pitch Cases [Member]
Loss Contingencies [Line Items]
Number of cases \| Case	31
Tennessee [Member] \| Coal Tar Pitch Cases [Member]
Loss Contingencies [Line Items]
Number of cases \| Case	1
Compensatory damages	$ 15.0
Oregon [Member]
Loss Contingencies [Line Items]
Number of potential responsible parties \| Party	80
Net present value of selected remedy estimation	$ 1,100.0
Undiscounted cost of selected remedy estimation	1,700.0
United States [Member]
Loss Contingencies [Line Items]
Environmental remediation and regulation liability	$ 4.2
United States [Member] \| Performance Chemicals [Member]
Loss Contingencies [Line Items]
Number of plant sites \| Plant	2
United States [Member] \| Utility and Industrial Products [Member]
Loss Contingencies [Line Items]
Number of plant sites \| Plant	1
Australia [Member] \| Performance Chemicals [Member]
Loss Contingencies [Line Items]
Environmental remediation and regulation liability	$ 1.4

Commitments and Contingent Liabilities - Changes in Accrued Liability for Environmental Matters (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Balance at beginning of year	$ 11.0	$ 9.5
Expense	0.2	1.8
Reversal of reserves	(0.1)	0.0
Cash expenditures	(0.3)	(0.4)
Currency translation	(0.2)	0.1
Balance at end of period	$ 10.6	$ 11.0