Koppers (KOP) 10-Q 8 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 31, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Trading Symbol	KOP
Entity Registrant Name	Koppers Holdings Inc.
Entity Central Index Key	1,315,257
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		20,508,315

Condensed Consolidated Statemen

Condensed Consolidated Statement of Operations and Comprehensive Income (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Net sales	$ 442.7	$ 384.8	$ 1,284.8	$ 1,109.4
Cost of sales (excluding items below)	359.7	293.5	1,023.9	863
Depreciation and amortization	13	12.1	38.5	35
Impairment and restructuring charges	0.9	2.2	3.8	5.8
Loss on pension settlement	0	8.8	0	8.8
Selling, general and administrative expenses	37.9	33.2	121.8	95.6
Operating profit	31.2	35	96.8	101.2
Other (loss) income	(0.6)	0.3	(1.1)	2
Interest expense	15.1	10.5	40.1	31.9
Loss on extinguishment of debt	0	0	0	13.3
Income before income taxes	15.5	24.8	55.6	58
Income tax provision	8.6	4.8	24.4	12.4
Income from continuing operations	6.9	20	31.2	45.6
Income (loss) from discontinued operations, net of tax (expense) benefit of $0.0, $0.0, $(0.3) and $0.4	0	(0.1)	0.4	(1.3)
Net income	6.9	19.9	31.6	44.3
Net (loss) income attributable to noncontrolling interests	(0.7)	0.1	5.6	0.4
Net income attributable to Koppers	$ 7.6	$ 19.8	$ 26	$ 43.9
Basic -
Continuing operations	$ 0.36	$ 0.96	$ 1.22	$ 2.17
Discontinued operations	0	0	0.02	(0.06)
Earnings per basic common share	0.36	0.96	1.24	2.11
Diluted -
Continuing operations	0.35	0.91	1.17	2.06
Discontinued operations	0	0	0.02	(0.06)
Earnings per diluted common share	$ 0.35	$ 0.91	$ 1.19	$ 2
Comprehensive (loss) income	$ (2)	$ 36.8	$ (6.5)	$ 73.7
Comprehensive (loss) income attributable to noncontrolling interests	(1.1)	0.2	4.7	0.6
Comprehensive (loss) income attributable to Koppers	$ (0.9)	$ 36.6	$ (11.2)	$ 73.1
Weighted average shares outstanding (in thousands):
Basic	20,946	20,746	20,992	20,750
Diluted	21,700	21,911	21,892	21,927

Condensed Consolidated Statem_2

Condensed Consolidated Statement of Operations and Comprehensive Income (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
(Loss) income from discontinued operations, tax benefit (expense)	$ 0	$ 0	$ (0.3)	$ 0.4

Condensed Consolidated Balance

Condensed Consolidated Balance Sheet - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets
Cash and cash equivalents	$ 62.5	$ 60.3
Accounts receivable, net of allowance of $2.2 and $2.5	220.8	159.2
Income tax receivable	1.5	1.7
Inventories, net	289.3	236.9
Other current assets	31.5	48.6
Total current assets	605.6	506.7
Property, plant and equipment, net	398.4	328
Goodwill	296.5	188.2
Intangible assets, net	193.5	129.6
Deferred tax assets	18.4	18.4
Other assets	23.2	29.3
Total assets	1,535.6	1,200.2
Liabilities
Accounts payable	166.2	141.9
Accrued liabilities	101.7	127.9
Current maturities of long-term debt	15.1	11.4
Total current liabilities	283	281.2
Long-term debt	1,029.8	665.6
Accrued postretirement benefits	46.6	46.3
Deferred tax liabilities	6.8	7.3
Other long-term liabilities	90	94
Total liabilities	1,456.2	1,094.4
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
Common Stock, $0.01 par value per share; 80,000,000 shares authorized; 22,979,575 and 22,384,476 shares issued	0.2	0.2
Additional paid-in capital	202.3	190.6
Retained earnings	29.8	7.4
Accumulated other comprehensive loss	(73.7)	(40.1)
Treasury stock, at cost, 2,472,299 and 1,606,028 shares	(89.8)	(58.2)
Total Koppers shareholders’ equity	68.8	99.9
Noncontrolling interests	10.6	5.9
Total equity	79.4	105.8
Total liabilities and equity	$ 1,535.6	$ 1,200.2

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Accounts receivable, allowance	$ 2.2	$ 2.5
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	22,979,575	22,384,476
Treasury stock, shares	2,472,299	1,606,028

Condensed Consolidated Statem_3

Condensed Consolidated Statement of Cash Flows (Unaudited) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash provided by (used in) operating activities:
Net income	$ 31.6	$ 44.3
Adjustments to reconcile net cash provided by (used in) operating activities:
Depreciation and amortization	38.5	35
Loss on extinguishment of debt	0	13.3
Loss (gain) on disposal of assets and investment	2.2	(1.4)
Gain on insurance proceeds	(1.5)	0
Deferred income taxes	5	0.8
Change in other liabilities	(6.2)	(18.6)
Non-cash interest expense	1.8	1.5
Stock-based compensation	9.3	7.8
Loss on pension settlement	0	8.8
Other - net	6.5	2
Changes in working capital:
Accounts receivable	(36.3)	(37.6)
Inventories	(20.3)	10.5
Accounts payable	17.7	(14.4)
Accrued liabilities	(38.6)	(1.1)
Other working capital	(1.7)	(2.4)
Net cash provided by operating activities	8	48.5
Cash (used in) provided by investing activities:
Capital expenditures	(81.4)	(48.6)
Acquisitions, net of cash acquired	(264)	0
Insurance proceeds received	1.5	0
Repayments received on loan	0	9.5
Net cash provided by divestitures and asset sales	2.3	1.1
Net cash used in investing activities	(341.6)	(38)
Cash provided by (used in) financing activities:
Net increase in revolving credit facility borrowings	282.8	74.1
Borrowings of long-term debt	100	500
Repayments of long-term debt	(12.9)	(541.4)
Issuances of Common Stock	2.5	1.9
Repurchases of Common Stock	(31.7)	(5.1)
Payment of debt issuance costs	(2.9)	(11)
Net cash (used in) provided by financing activities	337.8	18.5
Effect of exchange rate changes on cash	(2)	0.4
Net increase in cash and cash equivalents	2.2	29.4
Cash and cash equivalents at beginning of period	60.3	20.8
Cash and cash equivalents at end of period	$ 62.5	$ 50.2

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2018
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” or the “Company”) 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 the Company’s 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 for December 31, 2017 has been summarized from the audited balance sheet contained in the Annual Report on Form 10-K for the year ended December 31, 2017. Certain prior period amounts in the notes to the consolidated financial statements have been reclassified to conform to the current period’s presentation. The financial information included herein should be read in conjunction with the Company’s audited consolidated financial statements and related notes included in its Annual Report on Form 10-K for the year ended December 31, 2017.

New Accounting Pronouncements

New Accounting Pronouncements	9 Months Ended
	Sep. 30, 2018
Accounting Changes And Error Corrections [Abstract]
New Accounting Pronouncements	2. New Accounting Pronouncements In August 2018, the Securities and Exchange Commission (“SEC”) issued SEC Release No. 33-10532, “Disclosure Update and Simplification”, which In August 2017, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2017-12, “Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities.” This ASU amends and simplifies existing guidance in order to allow companies to more accurately present the economic effects of risk management activities in the financial statements. The Company adopted this ASU effective January 1, 2018 and the Company reclassified a $3.6 million unrealized gain, net of tax, from retained earnings to accumulated other comprehensive loss upon adoption. In March 2017, the FASB issued ASU No. 2017-07, “Compensation – Retirement Benefits (Topic 715)”, in order to improve the presentation of net periodic pension and postretirement costs. The amendment requires that an employer report the service cost component in the same line item or items as other compensation costs arising from services rendered by the pertinent employees during the period. The other components of net benefit cost are required to be presented in the income statement separately from the service cost component and outside a subtotal of income from operations. The amendments in this update related to income statement activity are applied. As a practical expedient, the Company has used the amounts disclosed in its pension and post-retirement benefits footnote as the estimation basis for applying the retrospective presentation requirements. The Company adopted this ASU effective January 1, 2018, and for retrospective presentation, reclassified $0.1 and $0.6 million In January 2017, the FASB issued ASU No. 2017-04, “Simplifying the Test for Goodwill Impairment (Topic 350).” The update is intended to simplify how an entity is required to test goodwill for impairment by eliminating Step 2 from the goodwill impairment test. Step 2 measures a goodwill impairment loss by comparing the implied fair value of a reporting unit's goodwill with the carrying amount of that goodwill. The amendments in this update are effective for periods beginning after December 15, 2019. Entities are required to apply the amendments in this update prospectively from the date of adoption, with early adoption permitted. The Company adopted this ASU effective January 1, 2018. In August 2016, the FASB The Company adopted this ASU effective January 1, 2018 In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842).” ASU 2016-02 requires an entity to recognize a right-of-use asset and lease liability for all leases with terms of more than one year. The standard is effective January 1, 2019 and recognition, measurement and presentation of expenses will depend on classification as a finance or operating lease. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements, which provides clarifications and improvements to ASU 2016-02 including allowing entities to elect an additional transition method with which to adopt ASU 2016-02. The approved transition method enables entities to apply the transition requirements in this ASU at the effective date of ASU 2016-02 (rather than at the beginning of the earliest comparative period presented) with the effect of initially applying ASU 2016-02 recognized as a cumulative-effect adjustment to retained earnings in the period of adoption. Consequently, an entity’s reporting for the comparative periods presented in the year of adoption would continue to be in accordance with ASC 840, Leases (Topic 840) (“ASC 840”), including the disclosure requirements of ASC 840. We plan to adopt ASU 2016-02 effective at the beginning of 2019 using the modified retrospective approach with no restatement of comparative periods presented. Practical expedients are available for election as a package and if applied consistently to all leases. The Company has a project team analyzing all of its leases and we have selected our leasing software solution and are in the process of identifying changes to our business processes, systems and controls to support adoption of the new standard in 2019. The Company will elect the practical expedient to retain its current classification of leases, and as a result, anticipates that the initial impact of adopting this new standard on its consolidated statement of income and consolidated statement of cash flows will not be material. The Company is still in the process of determining the impact of this adoption on the balance sheet and expects this analysis will be completed during the fourth quarter of 2018. While we are unable to quantify the impact at this time we expect that the adoption of the new standard to result in a material effect on total assts and total liabilities. In May 2014, the FASB issued ASU No. 2014-09, “Revenue from Contracts with Customers (Topic 606).” ASU 2014-09 requires an entity to recognize revenue in a manner that depicts the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Subsequent to the issuance of ASU 2014-09, the FASB issued multiple ASUs which either amended or clarified ASU 2014-09. Collectively, the revenue recognition ASUs were effective for annual reporting periods beginning after December 15, 2017. The Company elected to use the modified retrospective method for transition in which the cumulative effect was recognized at its January 1, 2018 date of adoption with no restatement of comparative periods presented. The implementation of the guidance had no material impact on the measurement of recognition of revenue of prior periods; however, additional disclosures have been added in accordance with the ASU.

Plant Closures and Divestitures

Plant Closures and Divestitures	9 Months Ended
	Sep. 30, 2018
Discontinued Operations And Disposal Groups [Abstract]
Plant Closures and Divestitures	3. Plant Closures and Divestitures Over the past four years, the Company has been restructuring its Carbon Materials and Chemicals (“CMC”) business unit in order to concentrate its facilities in regions where the Company believes it holds key competitive advantages to better serve its global customers. These closure activities include: • The anticipated cessation of naphthalene refining activities at the Company’s Follansbee, West Virginia coal tar distillation facility in the fourth quarter of 2018 subsequent to commissioning of a new naphthalene refining plant in Stickney, Illinois. • In September 2018, the Company sold its UK-based specialty chemicals business. • In November 2016, the Company sold its 30-percent interest in Tangshan Kailuan Koppers Carbon Chemical Company Limited (“TKK”) located in the Hebei Province in China. • In July 2016, the Company discontinued coal tar distillation activities at its CMC plant located in Clairton, Pennsylvania. In October 2018, the Company sold the facility and will recognize a loss on the transaction of $6.1 million during the three months ended December 31, 2018. As part of the transaction, the Company transferred cash to the buyer and the buyer assumed decommissioning, demolition and site restoration responsibilities. • In March 2016, the Company discontinued production at its 60-percent owned CMC plant located in Tangshan, China. • In February 2016, the Company ceased coal tar distillation and specialty pitch operations at both of its United Kingdom CMC facilities. In July 2016, the Company sold substantially all of its CMC tar distillation properties and assets in the United Kingdom. In exchange, the Company transferred cash to the buyer and the buyer assumed historical environmental and asset retirement obligations. • In April 2014, the Company ceased its coal tar distillation activities at its CMC facility located in Uithoorn, the Netherlands. Other closure and divestiture activity relates to the Company’s Railroad Utility Products and Services (“RUPS”) business unit. These actions include: • In October 2016, the Company agreed to a long-term lease of its wood treatment facility in Houston, Texas to a third party. • In August 2015, the Company closed its RUPS plant located in Green Spring, West Virginia. • In July 2015, the Company sold the assets of its 50-percent interest in KSA Limited Partnership, a concrete crosstie manufacturer. • In January 2015, the Company sold its RUPS North American utility pole business. In addition, in 2011, the Company ceased carbon black production at its CMC facility located in Kurnell, Australia. Costs associated with this closure are included in “Income (Loss) from discontinued operations” on the Condensed Consolidated Statement of Operations and Comprehensive (Loss) Income. Details of the restructuring activities and related reserves are as follows: Severance employee Environmental remediation Site demolition Other Total (Dollars in millions) Reserve at December 31, 2016 $ 1.4 $ 1.5 $ 10.0 $ 3.2 $ 16.1 Accrual 0.9 2.1 4.7 6.9 14.6 Cost charged against assets 0.0 0.0 0.0 (6.3 ) (6.3 ) Reversal of accrued charges (0.3 ) 0.0 (1.8 ) 0.0 (2.1 ) Cash paid (0.3 ) (1.1 ) (2.4 ) (1.0 ) (4.8 ) Currency translation 0.0 0.2 0.1 0.5 0.8 Reserve at December 31, 2017 $ 1.7 $ 2.7 $ 10.6 $ 3.3 $ 18.3 Accrual 0.0 0.9 1.6 2.2 4.7 Cost charged against assets 0.0 0.0 0.0 (2.1 ) (2.1 ) Cash paid 0.0 (3.4 ) (6.3 ) (0.1 ) (9.8 ) Currency translation 0.0 (0.2 ) 0.0 0.0 (0.2 ) Reserve at September 30, 2018 $ 1.7 $ 0.0 $ 5.9 $ 3.3 $ 10.9

Acquisitions

Acquisitions	9 Months Ended
	Sep. 30, 2018
Business Combinations [Abstract]
Acquisitions	4. Acquisitions On April 10, 2018, Koppers Inc. acquired Cox Industries, Inc. (“Cox”) for net cash consideration of $201.2 million. The transaction was funded by borrowings on Koppers Inc.’s revolving credit facility discussed in “Note 14 - Debt.” Cox was renamed Koppers Utility and Industrial Products Inc. (“UIP”) subsequent to the acquisition. UIP is a manufacturer of treated wood transmission and distribution poles for utility companies and cooperative utility companies. It is also a manufacturer of treated wood pilings used for construction and marine applications. UIP manufactures and sells its treated wood poles and pilings through a network of eight manufacturing facilities and 19 distribution yards located throughout the United States. UIP treats its products with a variety of wood protection chemicals, including copper chromium arsenate and creosote, which are produced by the Company’s Performance Chemicals and CMC segments, respectively. UIP’s revenues were $147 million for the year ended December 31, 2017. On February 28, 2018, Koppers Inc. acquired M.A. Energy Resources, LLC (“MAER”) for net cash consideration of $62.8 million. The purchase price was funded by revolving this recovered material into alternative fuels, such as crosstie-derived fuel or biomass-derived fuel, that is used as a substitute for conventional higher-cost carbon-based fuel. MAER currently operates two processing facilities, each of which is located to serve its Class I railroad customer base. MAER’s revenues were $30 million for the year ended December 31, 2017. The Company has completed a preliminary valuation analysis to determine the fair values of UIP’s and MAER’s acquired assets and liabilities. Accordingly, the unaudited condensed consolidated financial statements include a preliminary fair value determination based on assumptions and estimates that, while considered reasonable, are subject to changes, which may be material. Upon completion of detailed valuation analyses, there may be additional increases or decreases to the recorded fair values of the acquired assets and liabilities. This includes, but is not limited to customer contracts, other intangible assets; property, plant and equipment that could give rise to future amounts of depreciation and amortization expense; and changes in related deferred taxes that are not reflected in the information contained in this unaudited condensed consolidated information. Accordingly, once the necessary valuation analyses have been performed and the final fair value determination has been completed, actual results may differ materially from the information presented in the unaudited condensed consolidated financial statements. The preliminary valuation of identifiable assets acquired and liabilities assumed upon the acquisition of UIP and MAER are shown in the table below. UIP MAER (Dollars in millions) Cash and cash equivalents $ 1.5 $ 3.2 Accounts receivable 20.2 4.9 Inventory 43.1 0.0 Other current assets 1.2 0.3 Property, plant and equipment 19.8 6.6 Intangible assets, net 59.4 19.3 Goodwill 76.9 34.0 Other non-current assets 0.2 0.0 Total assets acquired 222.3 68.3 Accounts payable & accrued expenses 18.7 2.3 Other non-current liabilities 0.8 0.0 Net assets acquired $ 202.8 $ 66.0 Goodwill for both acquisitions has been allocated to the Company’s Railroad and Utility Products and Services segment. UIP MAER (Dollars in millions) Amount Remaining Life Amount Remaining Life Customer contracts $ 58.2 15 years $ 18.7 15 years Non-compete agreements 1.2 5 years Trademarks 0.6 2 years Total $ 59.4 $ 19.3 Combined costs related to these two acquisitions were $0.6 million and $6.2 million for the three and nine months ended September 30, 2018, respectively, and are charged to selling, general and administrative expenses. Net sales of $50.5 million and $97.4 million and income (loss) before taxes of $0.5 million and $(3.8) million, including $0.5 million and $6.0 million of inventory fair value purchase price accounting adjustments, was attributable to UIP and included in the Condensed Consolidated Statement of Operations and Comprehensive (Loss) Income for the three and nine months ended September 30, 2018, respectively. The following unaudited pro forma information presents a summary of the Company’s revenues and income from continuing operations as if the UIP acquisition occurred on January 1, 2017 (the first day of the most recently completed fiscal year). The unaudited pro forma information is not necessarily indicative of operating results that would have been achieved had the acquisition been completed as of January 1, 2017 and is not intended to project the future financial results of the Company after the acquisition. The unaudited pro forma information is based on certain assumptions, which management believes are reasonable, and does not reflect the cost of any integration activities or the benefits from the acquisition and synergies that may be derived from any integration activities. Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) Pro forma revenue $ 442.7 $ 422.7 $ 1,335.5 $ 1,221.6 Pro forma income from continuing operations attributable to Koppers 7.9 19.9 34.5 42.6 Pro forma income per share - continuing operations: Basic - $ 0.38 $ 0.96 $ 1.64 $ 2.05 Diluted - $ 0.36 $ 0.91 $ 1.58 $ 1.94

Revenue Recognition

Revenue Recognition	9 Months Ended
	Sep. 30, 2018
Revenue From Contract With Customer [Abstract]
Revenue Recognition	5. Revenue Recognition On January 1, 2018, the Company adopted Topic 606, the accounting standard for recognized revenue from contracts with customers, using the modified retrospective method. The Company recognized the cumulative effect of applying the new revenue standard as an adjustment to the opening balance of retained earnings. The comparative information has not been restated and continues to be reported under the accounting standards in effect for those periods. The Company has identified certain contracts with customers where revenue has been accelerated upon adoption of Topic 606 as the related performance obligations under the contract have been satisfied and control of the goods or services have been transferred to the customer. The Company calculated the cumulative effect to the opening balance of retained earnings recognized at January 1, 2018 to be an increase of $0.3 million, including $5.3 million in revenue and $5.0 million in cost of goods sold not previously recognized during the year ended December 31, 2017. Revenue recognized during the nine months ended September 30, 2018 from the adoption of Topic 606 was $3.2 million and, as such, the net impact of adopting Topic 606 for the three and nine months ending September 30, 2018 was a decrease in sales and cost of sales as follows: Three Months Ended Nine Months Ended September 30, September 30, 2018 2018 (Dollars in millions) Sales $ (0.1 ) $ (2.1 ) Cost of sales (0.1 ) (2.0 ) Topic 606 impacted the timing of revenue recognized related to certain services to untreated cross-ties within the Company’s RUPS segment where those specific performance obligations were fulfilled prior to shipment and were historically not recognized as revenue until shipped. Refer to “Note 9 – Segment Information” for relevant disclosures regarding the disaggregation of revenue. Revenue is recognized upon the completion of performance obligations under the Company’s contracts with customers and when control of a good or service is transferred to the customer. Substantially all of the Company’s contracts with its customers are ship and invoice arrangements where revenue is recognized when the Company transfers control to the customer which is at the time of shipment or delivery. Contract Balances The timing of revenue recognition in accordance with Topic 606 and subsequent billings related to such revenue recognized results in both billed accounts receivable and unbilled receivables (contract assets), both classified as accounts receivable, net of allowance within the condensed consolidated balance sheet. Certain contracts with customers within the Company’s RUPS and CMC segments have agreed-upon contractual terms for the timing of billing. In some instances, amounts are not billed at the time specific performance obligations are fulfilled. For these contracts with customers, revenue is recognized prior to billings, resulting in contract assets. Contract assets recorded within accounts receivable, net of allowance within the condensed consolidated balance sheet as of September 30, 2018 and January 1, 2018 due to performance obligations being fulfilled prior to billing was $11.4 million and $5.3 million, respectively.

Comprehensive (Loss) Income and

Comprehensive (Loss) Income and Equity	9 Months Ended
	Sep. 30, 2018
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, 2018 and 2017 is summarized in the table below: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) Net income $ 6.9 $ 19.9 $ 31.6 $ 44.3 Other comprehensive (loss) income: Change in currency translation adjustment (2.9 ) 6.3 (18.9 ) 17.2 Unrealized (loss) gain on cash flow hedges, net of tax (benefit) expense of $(3.7), $1.1, $(12.0) and $1.4 (6.3 ) 4.1 (20.3 ) 5.0 Change in accounting standard 0.0 0.0 0.3 0.0 Unrecognized pension net loss, net of tax expense of $0.1, $0.2, $0.3 and $0.4 0.3 6.5 0.8 7.2 Total comprehensive (loss) income (2.0 ) 36.8 (6.5 ) 73.7 Less: Comprehensive (loss) income attributable to noncontrolling interests (1.1 ) 0.2 4.7 0.6 Comprehensive (loss) income attributable to Koppers $ (0.9 ) $ 36.6 $ (11.2 ) $ 73.1 Amounts reclassified from accumulated other comprehensive loss to net income consist of amounts shown for changes in 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 Postretirement Benefit Plans”. Other amounts reclassified from accumulated other comprehensive loss include income related to derivative financial instruments, net of tax, of $1.1 million and $6.5 million The following tables present the change in equity for the nine months ended September 30, 2018 and 2017, respectively: (Dollars in millions) Total Koppers Shareholders’ Equity Noncontrolling Interests Total Equity Balance at December 31, 2017 $ 99.9 $ 5.9 $ 105.8 Net income 26.0 5.6 31.6 Issuance of common stock 2.5 0.0 2.5 Employee stock plans 9.3 0.0 9.3 Other comprehensive loss (37.2 ) (0.9 ) (38.1 ) Repurchases of common stock (31.7 ) 0.0 (31.7 ) Balance at September 30, 2018 $ 68.8 $ 10.6 $ 79.4 (Dollars in millions) Total Koppers Shareholders’ Equity Noncontrolling Interests Total Equity Balance at December 31, 2016 $ 30.4 $ 4.2 $ 34.6 Net income 43.9 0.4 44.3 Issuance of common stock 1.9 0.0 1.9 Employee stock plans 8.0 0.0 8.0 Other comprehensive income 29.2 0.2 29.4 Repurchases of common stock (5.1 ) 0.0 (5.1 ) Balance at September 30, 2017 $ 108.3 $ 4.8 $ 113.1

Earnings per Common Share

Earnings per Common Share	9 Months Ended
	Sep. 30, 2018
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, 2018 2017 2018 2017 (Dollars in millions, except share amounts, in thousands, and per share amounts) Net income attributable to Koppers $ 7.6 $ 19.8 $ 26.0 $ 43.9 Less: Income (loss) from discontinued operations 0.0 (0.1 ) 0.4 (1.3 ) Income from continuing operations attributable to Koppers $ 7.6 $ 19.9 $ 25.6 $ 45.2 Weighted average common shares outstanding: Basic 20,946 20,746 20,992 20,750 Effect of dilutive securities 754 1,165 900 1,177 Diluted 21,700 21,911 21,892 21,927 Income per common share – continuing operations: Basic income per common share $ 0.36 $ 0.96 $ 1.22 $ 2.17 Diluted income per common share 0.35 0.91 1.17 2.06 Other data: Antidilutive securities excluded from computation of diluted earnings per common share 571 543 214 482

Stock-based Compensation

Stock-based Compensation	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation	8. Stock-based Compensation The Company has outstanding stock-based compensation awards that were granted under the amended and restated 2005 Long-Term Incentive Plan (the “2005 LTIP”) and the 2018 Long-Term Incentive Plan (the “2018 LTIP”). Both the 2005 LTIP and the 2018 LTIP are collectively referred to as the ”LTIP”. On May 3, 2018, the 2018 LTIP was approved by our shareholders and the 2005 LTIP was frozen. Similar to the 2005 LTIP, the 2018 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”. 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”). 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 two years or less. 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 and performance stock units with a performance condition is the market price of the underlying common stock on the date of grant. Performance stock units granted prior to 2016 have vesting based upon a performance condition. These performance stock units generally have three-year performance objectives and all performance stock units have a three-year period for vesting (if the applicable performance objective is achieved). For awards granted prior to 2016, the applicable performance objective is based upon a multi-year cumulative value creation calculation that considers the Company’s financial performance commencing on the first day of each grant year. The number of performance stock units granted represents the target award and participants have the ability to earn between zero and 200 percent (depending on the grant date) of the target award based upon actual performance. If minimum performance criteria are not achieved, no performance stock units will vest. Performance stock units granted in 2015 exceeded the maximum value creation and vested at 200 percent in March 2018. Performance stock units granted in 2016 and thereafter have vesting based upon a market condition. These performance stock units have a three-year performance objective and a three-year period for vesting (if the applicable performance objective is achieved). The applicable performance objective is based on the Company’s 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. The Company has the discretion to settle the award in cash rather than shares, although the Company currently expects that all awards will be settled by the issuance of shares. Compensation expense for non-vested performance stock units with a market condition is recorded over the vesting period based on the fair value at the date of grant. The Company calculated the fair value of the awards on the date of grant using the Monte Carlo valuation model and the assumptions listed below: May March March March 2016 Grant Grant date price per share $ 39.10 $ 41.60 $ 44.10 $ 18.11 Expected dividend yield per share 0.00 % 0.00 % 0.00 % 0.00 % Expected volatility 39.40 % 39.40 % 43.50 % 40.86 % Risk-free interest rate 2.35 % 2.35 % 1.54 % 0.96 % Look-back period in years 2.84 2.84 2.83 2.84 Grant date fair value per share $ 44.29 $ 47.12 $ 64.02 $ 23.70 Dividends declared, if any, on the Company’s 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, 2018: Performance Period Minimum Shares Target Shares Maximum Shares 2016 – 2018 0 255,172 510,344 2017 – 2019 0 113,675 227,350 2018 – 2020 0 130,782 261,564 The following table shows a summary of the status and activity of non-vested stock units for the nine months ended September 30, 2018: Restricted Stock Units Performance Stock Units Total Stock Units Weighted Grant Date Fair Value per Unit Non-vested at December 31, 2017 238,140 581,551 819,691 $ 29.14 Granted 118,297 134,351 252,648 $ 43.91 Performance share adjustment 0 204,866 204,866 $ 17.57 Vested (94,589 ) (409,732 ) (504,321 ) $ 19.41 Forfeited (7,094 ) (11,407 ) (18,501 ) $ 38.77 Non-vested at September 30, 2018 254,754 499,629 754,383 $ 37.20 Stock Options Stock options to most 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. The Company calculated the fair value of stock options on the date of grant using the Black-Scholes-Merton model and the assumptions listed below: May March March March 2016 Grant March 2015 Grant Grant date price per share $ 39.10 $ 41.60 $ 44.10 $ 18.11 $ 17.57 Expected dividend yield per share 0.00 % 0.00 % 0.00 % 0.00 % 3.40 % Expected life in years 5.73 5.73 5.77 5.96 5.75 Expected volatility 37.05 % 37.05 % 39.70 % 40.86 % 42.27 % Risk-free interest rate 2.82 % 2.67 % 2.13 % 1.45 % 1.73 % Grant date fair value per share $ 15.48 $ 16.38 $ 17.90 $ 7.41 $ 5.20 The Company suspended its dividend in February 2015 and does 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 the Company. Expected volatility is based on the historical volatility of the Company’s 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, 2018: Options Weighted Exercise Price per Option Weighted Average Remaining Contractual Term (in years) Aggregate Value (in millions) Outstanding at December 31, 2017 942,537 $ 27.59 Granted 144,825 $ 41.50 Exercised (70,307 ) $ 27.75 Forfeited (12,329 ) $ 30.49 Outstanding at September 30, 2018 1,004,726 $ 29.54 6.14 $ 6.6 Exercisable at September 30, 2018 617,361 $ 28.49 4.89 $ 4.2 Stock Compensation Expense Total stock-based compensation expense recognized under the Company’s LTIP and employee stock purchase plan for three and nine months ended September 30, 2018 and 2017 is as follows: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) Stock-based compensation expense recognized: Selling, general and administrative expenses $ 3.2 $ 2.7 $ 9.3 $ 7.8 Less related income tax benefit 1.2 0.6 3.5 1.7 Decrease in net income attributable to Koppers $ 2.0 $ 2.1 $ 5.8 $ 6.1 Intrinsic value of exercised stock options $ 0.1 $ 0.0 $ 1.1 $ 1.1 Cash received from the exercise of stock options $ 0.3 $ 0.0 $ 2.5 $ 1.9 As of September 30, 2018, total future gross compensation expense related to non-vested stock-based compensation arrangements, which are expected to vest, totaled $18.7 million and the weighted-average period over which this cost is expected to be recognized is approximately 26 months.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Segment Information	9. Segment Information The Company has three reportable segments: Railroad and Utility Products and Services, Performance Chemicals and Carbon Materials and Chemicals. The Company’s 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. The Company’s Railroad and Utility Products and Services 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. The segment also operates a railroad services business that conducts engineering, design, repair and inspection services for railroad bridges and a business related to the recovery of used crossties. Utility products include the treating of transmission and distribution poles and pilings. In April 2018, the Company acquired UIP, a manufacturer of treated wood utility transmission and distribution poles for utility companies and cooperative utility companies. It is also a manufacturer of treated wood pilings used for construction and marine applications. In February 2018, the Company acquired MAER, recovered material into alternative fuels, such as crosstie-derived fuel or biomass-derived fuel, that is used as a substitute for conventional higher-cost carbon-based fuel. The Company’s Performance Chemicals 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. The Company’s Carbon Materials and Chemicals 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 a critical raw material used in the production of aluminum and for the production of 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. The Company evaluates performance and determines resource allocations based on a number of factors, the primary measure being operating profit or loss from operations. Operating profit does not include equity in earnings of affiliates, other (loss) income, interest expense, income taxes or operating costs of Koppers Holdings Inc. The accounting policies of the reportable segments are the same as those described in the summary of significant accounting policies as disclosed in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017. Refer to “Note 5 – Revenue Recognition” for accounting policies specific to revenue recognition. Intersegment transactions are eliminated in consolidation. The following table sets forth certain sales and operating data, net of all intersegment transactions, for the Company’s segments for the periods indicated: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) Revenues from external customers: Railroad and Utility Products and Services $ 185.0 $ 131.7 $ 470.6 $ 403.1 Performance Chemicals 108.2 109.7 320.7 318.2 Carbon Materials and Chemicals 149.5 143.4 493.5 388.1 Total $ 442.7 $ 384.8 $ 1,284.8 $ 1,109.4 Intersegment revenues: Performance Chemicals $ 3.8 $ 1.8 $ 8.0 $ 5.0 Carbon Materials and Chemicals 21.4 22.1 58.8 60.0 Total $ 25.2 $ 23.9 $ 66.8 $ 65.0 Depreciation and amortization expense: Railroad and Utility Products and Services $ 4.9 $ 2.9 $ 12.8 $ 8.8 Performance Chemicals 4.4 4.4 13.3 13.3 Carbon Materials and Chemicals 3.7 4.8 12.4 12.9 Total $ 13.0 $ 12.1 $ 38.5 $ 35.0 Operating profit (loss): Railroad and Utility Products and Services (a) $ 5.8 $ 9.5 $ 5.9 $ 30.1 Performance Chemicals 11.0 18.4 28.2 56.6 Carbon Materials and Chemicals 14.9 16.3 64.6 24.9 Corporate (b) (0.5 ) (9.2 ) (1.9 ) (10.4 ) Total $ 31.2 $ 35.0 $ 96.8 $ 101.2 (a) Operating profit for the three and nine months ended September 30, 2018 includes $0.5 million and $6.0 million of inventory fair value purchase price adjustment related to the (b) Operating loss for Corporate includes primarily general and administrative costs for Koppers Holdings Inc., the parent company of Koppers Inc. The following table sets forth revenues for significant product lines, net of all intersegment transactions, for the Company’s segments for the periods indicated: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) Railroad and Utility Products and Services: Railroad treated products $ 96.7 $ 96.1 $ 259.8 $ 299.8 Utility poles 59.5 12.9 129.9 34.0 Rail joints 8.5 7.1 26.5 22.1 Railroad infrastructure services 10.0 9.0 26.2 27.0 Other products 10.3 6.6 28.2 20.2 185.0 131.7 470.6 403.1 Performance Chemicals: Wood preservative products 101.5 101.7 299.0 295.5 Other products 6.7 8.0 21.7 22.7 108.2 109.7 320.7 318.2 Carbon Materials and Chemicals: Pitch and related products 88.9 78.6 304.1 195.4 Creosote and distillates 19.2 21.6 61.3 61.0 Phthalic anhydride and other chemicals 23.3 24.5 64.0 69.3 Naphthalene 8.8 9.9 31.5 27.8 Other products 9.3 8.8 32.6 34.6 149.5 143.4 493.5 388.1 Total $ 442.7 $ 384.8 $ 1,284.8 $ 1,109.4 The following table sets forth certain tangible and intangible assets allocated to each of the Company’s segments as of the dates indicated: September 30, 2018 December 31, 2017 (Dollars in millions) Segment assets: Railroad and Utility Products and Services (a) $ 556.3 $ 249.7 Performance Chemicals 472.0 494.0 Carbon Materials and Chemicals 469.3 414.2 All other 38.0 42.3 Total $ 1,535.6 $ 1,200.2 Goodwill: Railroad and Utility Products and Services (a) $ 120.8 $ 10.5 Performance Chemicals 175.7 177.7 Total $ 296.5 $ 188.2 (a) The increase in RUPS assets and goodwill as of September 30, 2018 includes $279.3 million and $110.9 million, respectively, attributable to the Company’s acquisitions of UIP and MAER during 2018.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
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 uncertain tax positions and changes of estimated tax to the actual liability determined upon filing 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 the Company conducts 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. On December 22, 2017, the Tax Cut and Jobs Act (“Tax Act”) was signed into law. The Tax Act significantly revised the U.S. corporate income tax system with changes that are effective in 2017 and 2018. Changes in tax rates and tax laws and their impact on deferred taxes are accounted for in the period of legislative enactment. The Company recorded income tax expense of $21.2 million for this one-time transition tax. In its December 31, 2017 income tax provision, the Company provisionally recorded income tax expense of $13.1 million. In both of the three months ended June 30, 2018 and September 30, 2018, the Company revised its original estimate and recorded additional income tax expense as a result of additional guidance issued by the IRS. The increase in the transition tax was partially offset by revisions to the carrying value of its deferred tax assets and liabilities of $3.8 million. Due to the availability of net operating losses, the Company’s cash payments for this one-time transition tax is approximately $5.0 million. The Company elected to pay this amount in pre-defined installments through 2024. If the IRS releases additional guidance regarding the one-time transition tax, the ultimate impact of the Tax Act may differ from the current estimates, possibly materially, due to changes in interpretations and assumptions that the Company has made, future guidance that may be issued, and actions that the Company may take as a result. The Tax Act introduced a new provision that imposes a minimum tax on earnings of a foreign corporation that are deemed to exceed a certain threshold return relative to the underlying business investment. These earnings are referred to as global intangible low-taxed income (“GILTI”) and this GILTI provision is effective for 2018. The Company has elected to treat the effect of this GILTI provision as a period expense in the year incurred and has included an estimate of the impact in its estimated annual effective income tax rate. The Company will continue to analyze this GILTI provision and it is possible that the Company’s The Tax Act introduces other new provisions that are effective for 2018 and changes how certain provisions are calculated beginning in 2018. Other than the GILTI provision, the Company does not expect that any of these new provisions or changes will have a material impact to the Company’s income tax expense. The estimated annual effective income tax rate, excluding discrete items discussed above, was 37.1 percent and 21.7 percent for the nine months ended September 30, 2018 and 2017, respectively. The estimated annual effective income tax rate differs from the U.S. federal statutory tax rate due to: September 30, September 30, 2018 2017 Federal income tax rate 21.0 % 35.0 % State income taxes, net of federal tax benefit (0.8 ) 1.1 Foreign earnings taxed at different rates 1.8 (16.8 ) GILTI inclusion (net of foreign tax credits) 16.7 0.0 Valuation allowance adjustments (11.5 ) 0.0 Interest expense deduction limitation 7.7 0.0 Nondeductible expenses 2.6 1.6 Change in tax contingency reserves 0.2 0.2 Other (0.6 ) 0.6 Estimated annual effective income tax rate 37.1 % 21.7 % The estimated annual effective income tax rate included a benefit related to the release of a valuation allowance adjustment for a subsidiary in China. Management determined that sufficient positive evidence exists to support that this entity’s net operating losses are more likely than not to be realized. Income taxes as a percentage of pretax income were 55.5 percent for the three months ended September 30, 2018. This was 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 due to a higher estimated interest expense deduction limitation. Income taxes as a percentage of pretax income were 19.4 percent for the three months ended September 30, 2017. This was lower than the estimated annual effective income tax rate primarily due to a reduction in the estimated annual effective income tax rate when compared to the previous quarter’s estimate. As a result, income tax expense for the previous quarter was favorably adjusted in the three months ended September 30, 2017. Discrete items included in income taxes for the three months ended September 30, 2017 were not material. Income taxes as a percentage of pretax income were 43.9 percent for the nine months ended September 30, 2018. This is higher than the estimated annual effective income tax rate due to discrete items. Discrete items included in income taxes for the nine months ended September 30, 2018 were a net cost of $3.8 million. Discrete items of $6.1 million include income tax expense related to the impact of the Tax Act and based on the filings of the Company’s 2017 domestic and foreign tax returns. This expense is offset by $2.3 million of excess tax benefits for stock-based compensation and an income tax benefit of $0.5 million for the reversal of uncertain tax positions due to statute expirations. Income taxes as a percentage of pretax income were 21.4 percent for the nine months ended September 30, 2017. This was lower than the estimated annual effective tax rate due to discrete items. Discrete items included in income taxes for the nine months ended September 30, 2017 were a net benefit of $0.2 million. This discrete benefit primarily relates to excess tax benefits for stock-based compensation of $0.9 million, a benefit of $0.5 million for the reversal of uncertain tax positions due to statute expirations partially offset by additional accruals of $0.8 million for uncertain tax positions and $0.4 million for adjustments to deferred tax assets. 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, uncertain tax positions 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 nine months ended September 30, 2018. Uncertain Tax Positions The Company and its subsidiaries file income tax returns in the U.S. federal jurisdiction, individual U.S. state jurisdictions and non-U.S. jurisdictions. With few exceptions, the Company is no longer subject to U.S. federal, state and local, or non-U.S. income tax examinations by tax authorities for years prior to 2014. Unrecognized tax benefits totaled $8.5 million and $8.7 million as of September 30, 2018 and December 31, 2017, respectively. As of September 30, 2018 and December 31, 2017, the total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate, was approximately $4.2 million and $4.4 million, respectively. The Company recognizes interest expense and any related penalties from uncertain tax positions in income tax expense. As of September 30, 2018 and December 31, 2017, the Company had accrued approximately $4.3 million and $3.6 million for interest and penalties, respectively. The Company believes that it is reasonably possible that the amount of unrecognized tax benefits will decrease in the next twelve months by approximately $2 million due to the expirations of certain foreign and state statutes of limitations and potential audit resolutions. The Company does not anticipate significant increases to the amount of unrecognized tax benefits within the next twelve months.

Inventories

Inventories	9 Months Ended
	Sep. 30, 2018
Inventory Disclosure [Abstract]
Inventories	11. Inventories Net inventories as of September 30, 2018 and December 31, 2017 are summarized in the table below: September 30, 2018 December 31, 2017 (Dollars in millions) Raw materials $ 203.4 $ 173.6 Work in process 14.8 11.2 Finished goods 123.9 98.4 $ 342.1 $ 283.2 Less revaluation to LIFO 52.8 46.3 Net $ 289.3 $ 236.9

Property, Plant and Equipment

Property, Plant and Equipment	9 Months Ended
	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Property, Plant and Equipment	12. Property, Plant and Equipment Property, plant and equipment as of September 30, 2018 and December 31, 2017 are summarized in the table below: September 30, 2018 December 31, 2017 (Dollars in millions) Land $ 17.0 $ 17.6 Buildings 61.9 63.4 Machinery and equipment 835.2 756.6 $ 914.1 $ 837.6 Less accumulated depreciation 515.7 509.6 Net $ 398.4 $ 328.0 Impairments – There were no impairment charges incurred for the nine months ended September 30, 2018 and 2017.

Pensions and Post-Retirement Be

Pensions and Post-Retirement Benefit Plans	9 Months Ended
	Sep. 30, 2018
Compensation And Retirement Disclosure [Abstract]
Pensions and Post-Retirement Benefit Plans	13. Pensions and Post-Retirement Benefit Plans The Company and its subsidiaries maintain a number of defined benefit and defined contribution plans to provide retirement benefits for employees in the U.S., as well as employees outside the U.S. 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 U.S., all qualified and two of the non-qualified defined benefit pension plans for salaried and hourly employees have been closed to new participants and have been frozen. 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. The Company also provides 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, 2018 and 2017: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) Service cost $ 0.5 $ 0.5 $ 1.4 $ 1.5 Interest cost 1.8 2.1 5.6 7.0 Expected return on plan assets (2.1 ) (2.4 ) (6.4 ) (7.4 ) Amortization of net loss 0.4 0.5 1.1 1.5 Settlements 0.0 8.8 0.0 8.8 Net periodic benefit cost $ 0.6 $ 9.5 $ 1.7 $ 11.4 Defined contribution plan expense $ 1.3 $ 2.1 $ 5.5 $ 6.1

Debt

Debt	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Debt	14. Debt Debt as of September 30, 2018 and December 31, 2017 was as follows: Weighted Average Interest Rate Maturity September 30, 2018 December 31, 2017 (Dollars in millions) Term Loan 5.16 % 2023 $ 95.0 $ 0.0 Revolving Credit Facility 5.16 % 2023 437.8 155.0 Construction and other loans 4.69 % 2020 24.9 33.7 Senior Notes due 2025 6.00 % 2025 500.0 500.0 Debt 1,057.7 688.7 Less short-term debt and current maturities of long-term debt 15.1 11.4 Less unamortized debt issuance costs 12.8 11.7 Long-term debt $ 1,029.8 $ 665.6 Senior Notes due 2025 The 2025 Notes are senior obligations, 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 beginning on August 15, 2017 and will mature on February 15, 2025 unless earlier redeemed or repurchased. On or after February 15, 2020, the Company is 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. Revolving Credit Facility On April 10, 2018, the Company amended its $600 million Revolving Credit Facility to enter into a new Secured Term Loan Facility. The new Secured Term Loan Facility includes the $600 million Revolving Credit Facility and a secured term loan of $100 million with a quarterly amortization of $2.5 million and a five-year maturity. In addition, the maturity date of the amended Revolving Credit Facility was extended one year to April 2023. The interest rate on the amended Revolving Credit Facility is variable and is based on LIBOR. The initial average borrowing rates under both facilities were expected to be approximately five percent. Terms under the amended Revolving Credit Facility are substantially consistent with the original Revolving Credit Facility. Borrowings under the Revolving Credit Facility are secured by a first priority lien on substantially all of the assets of Koppers Inc., Koppers Holdings and their material domestic subsidiaries. The Revolving 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, 2018, the Company had $131.3 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, 2018, $32.0 million of commitments were utilized by outstanding letters of credit. Loss on Extinguishment of Debt In February 2017, all of the outstanding Koppers Inc. senior notes due 2019 were repurchased at a premium to carrying value and accordingly, the Company realized a loss on extinguishment of debt totaling $10.0 million consisting of $7.3 million for bond premium and bond tender expenses and $2.7 million for the write-off of unamortized debt issuance costs. Also in February 2017, Koppers Inc. repaid its term loan in full and entered into a new Revolving Credit Facility. Accordingly, the Company realized a loss of $3.3 million for the write-off of unamortized debt issuance costs. Construction Loans The Company’s 75-percent owned subsidiary, Koppers (Jiangsu) Carbon Chemical Company Limited (“KJCC”) entered into committed loan facility agreements with a third-party bank and the 25-percent non-controlling shareholder in KJCC. Borrowings under the third-party bank facility, which includes a construction loan and a working capital facility, totaled $22.2 million at September 30, 2018 and are secured by a letter of credit issued by a bank under the Revolving Credit Facility. KJCC will repay the construction loan portion of the third-party commitment in six installments every six months starting in June 2018 with a final repayment on December 21, 2020, the maturity date of the loans.

Asset Retirement Obligations

Asset Retirement Obligations	9 Months Ended
	Sep. 30, 2018
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligations	15. Asset Retirement Obligations The Company recognizes asset retirement obligations for the removal and disposal of residues; dismantling of certain tanks required by governmental authorities; cleaning and dismantling costs for owned rail cars; 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, 2018 December 31, 2017 (Dollars in millions) Asset retirement obligation at beginning of year $ 37.1 $ 36.0 Accretion expense 1.3 2.4 Revision in estimated cash flows 1.4 9.4 Cash expenditures (10.4 ) (10.9 ) Currency translation 0.0 0.2 Balance at end of period $ 29.4 $ 37.1

Deferred Revenue

Deferred Revenue	9 Months Ended
	Sep. 30, 2018
Deferred Revenue Disclosure [Abstract]
Deferred Revenue	16. Deferred Revenue In 2015, KJCC received a cash advance payment of $30 million as part of an amendment to a soft pitch supply agreement with its customer. Also, the Company defers revenues associated with extended product warranty liabilities based on historical loss experience and sales of extended warranties on certain products. The following table reflects changes in the carrying values of deferred revenue: September 30, 2018 December 31, 2017 (Dollars in millions) Balance at beginning of year $ 28.1 $ 27.2 Revenue earned (0.5 ) (0.7 ) Currency translation (1.6 ) 1.6 Balance at end of period $ 26.0 $ 28.1 Deferred revenue classified in other long-term liabilities in the consolidated balance sheet totaled $25.2 million as of September 30, 2018 and $27.2 million as of December 31, 2017 with the remainder classified in accrued liabilities.

Derivative Financial Instrument

Derivative Financial Instruments	9 Months Ended
	Sep. 30, 2018
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Derivative Financial Instruments	17. Derivative Financial Instruments The Company utilizes derivative instruments to manage exposures to risks that have been identified and measured and are capable of being controlled. The primary risks managed by the Company 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 Canadian dollar, the New Zealand 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 the Company’s manufacturing processes. Generally, the Company will not hedge cash flow exposures for durations longer than 36 months and the Company has hedged certain volumes of copper through December 2020. The Company enters into foreign currency forward contracts to manage foreign currency risk associated with the Company’s receivable and payable balances and foreign currency denominated sales. Generally, the Company enters into master netting arrangements with the counterparties and offsets net derivative positions with the same counterparties. Currently, the Company’s 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, the Company designates certain of its commodity swaps as cash flow hedges of forecasted purchases of commodities and certain of its foreign currency swaps as cash flow hedges of forecasted sales. 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 either hedge ineffectiveness or hedge components excluded from the assessment of effectiveness are recognized in current earnings. The amount of hedge ineffectiveness charged to profit and loss is reported in the table below. For those commodity and foreign currency swaps which are not designated as cash flow hedges, the fair value of the 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. As of September 30, 2018 and December 31, 2017, the Company had outstanding copper swap contracts of the following amounts: Units Outstanding (in Pounds) Net Fair Value - (Liability) Asset September 30, 2018 December 31, 2017 September 30, 2018 December 31, 2017 (Amounts in millions) Cash flow hedges 39.6 37.8 $ (0.2 ) $ 25.5 Not designated as hedges 16.9 11.3 (1.0 ) 4.5 Total 56.5 49.1 $ (1.2 ) $ 30.0 As of September 30, 2018 and December 31, 2017, the fair value of the outstanding copper swap contracts is recorded in the balance sheet as follows: September 30, 2018 December 31, 2017 (Dollars in millions) Other current assets $ 1.2 $ 21.8 Other assets 0.5 8.2 Accrued liabilities (2.6 ) 0.0 Other long-term liabilities (0.3 ) 0.0 Net (liability) asset on balance sheet $ (1.2 ) $ 30.0 Accumulated other comprehensive gain (loss), net of tax $ (0.3 ) $ 15.8 Based upon contracts outstanding at September 30, 2018, in the next twelve months the Company estimates that $0.1 million of unrealized losses, net of tax benefit, related to commodity price hedging will be reclassified from other comprehensive loss into earnings. See Note 6 – Comprehensive Income (Loss) and Equity, for amounts recorded in other comprehensive income and for amounts reclassified from accumulated other comprehensive loss to net income for the periods specified below. For the three and nine months ended September 30, 2018 and 2017, the following amounts were recognized in earnings related to copper swap contracts: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) (Loss) gain from ineffectiveness of cash flow hedges $ 0.0 $ (2.5 ) $ 0.0 $ 0.5 (Loss) gain from contracts not designated as hedges (1.0 ) 1.0 (5.5 ) 2.0 Net $ (1.0 ) $ (1.5 ) $ (5.5 ) $ 2.5 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, 2018 and December 31, 2017, the fair value of outstanding foreign currency forward contracts is recorded in the balance sheet as follows: September 30, 2018 December 31, 2017 (Dollars in millions) Other current assets $ 0.7 $ 0.3 Accrued liabilities (1.1 ) (0.2 ) Net (liability) asset on balance sheet $ (0.4 ) $ 0.1 As of September 30, 2018 and December 31, 2017, the net currency units outstanding for these contracts were: September 30, 2018 December 31, 2017 (In millions) British Pounds GBP 5.7 GBP 7.0 New Zealand Dollars NZD 16.0 NZD 15.5 United States Dollars USD 4.6 USD 12.5 Canadian Dollars CAD 2.5 CAD 2.5 Euro EUR 1.3 EUR 0.0

Commitments and Contingent Liab

Commitments and Contingent Liabilities	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingent Liabilities	18. Commitments and Contingent Liabilities The Company and its subsidiaries 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 the Company or its subsidiaries fail to prevail in any of these legal matters or should several of these legal matters be resolved against the Company or its subsidiaries 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 are 87 plaintiffs in 48 cases pending as of September 30, 2018, which is unchanged from December 31, 2017. As of September 30, 2018, there are 47 cases pending in state court in Pennsylvania, and 1 case pending in state court in Tennessee. The plaintiffs in all 48 pending cases seek to recover compensatory damages. Plaintiffs in 43 of those cases also seek to recover punitive damages. The plaintiffs in the 47 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. The Company has not provided a reserve for these lawsuits because, at this time, the Company 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 the Company’s business, financial condition, cash flows and results of operations. Environmental and Other Litigation Matters The Company and its subsidiaries 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. The Company’s subsidiaries expect to incur substantial costs for ongoing compliance with such laws and regulations. The Company’s subsidiaries 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. The Company accrues 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. The Company’s subsidiaries have agreements with former owners of certain of their operating locations under which the former owners retained, assumed and/or agreed to indemnify such subsidiaries 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 through July 2019. As consideration for the amendment, Koppers Inc. paid Beazer East a total of $7.0 million and agreed to share toxic tort litigation defense costs arising from any sites acquired from Beazer East. The July 2004 amendment 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 asserted after July 2019. 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. If the third-party claims described in sections (i) and (ii) above are not made by July 2019, Beazer East will not be required to pay the costs arising from such claims under the Indemnity. However, with respect to any such claims which are made by July 2019, Beazer East will continue to be responsible for such claims under the Indemnity beyond July 2019. 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 the Company’s 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 the Company against the liabilities referred to above, including Beazer East, have performed their obligations in all material respects. The Company believes that, for the last three years ended December 31, 2017, amounts paid by Beazer East as a result of its environmental remediation obligations under the Indemnity have averaged, in total, approximately $10 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 the Company or its subsidiaries are held liable for or otherwise required to pay all or part of such liabilities without reimbursement, the imposition of such liabilities on the Company or its subsidiaries could have a material adverse effect on its business, financial condition, cash flows and results of operations. Furthermore, the Company could be required to record a contingent liability on its balance sheet with respect to such matters, which could result in a negative impact to the Company’s business, financial condition, cash flows and results of operations. Domestic Environmental Matters. On June 4, 2018, Koppers Inc. received a letter from the U.S. Environmental Protection Agency ("EPA") concerning potential violations of the Clean Water Act observed during inspections and review of Spill Prevention, Control and Countermeasure Plans and Facility Response Plans at the Company’s facilities in Follansbee, WV; Green Spring, WV; and Clairton, PA. In addition, the EPA reviewed one facility’s compliance with an earlier consent order regarding above ground storage tank integrity testing. The Company continues to meet and correspond with the EPA to discuss and present relevant information related to the allegations. The Company currently cannot estimate the potential penalties, fines or other expenditures, if any, that may result from any EPA actions relating to the alleged potential violations and, therefore, the Company cannot determine if the ultimate outcome of this matter will have a material impact on the Company's financial position, results of operations or cash flows. 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 Environmental Protection Agency (“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 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. Responsibility for implementing and funding that work will be decided in the separate private allocation process, which is ongoing. Additionally, the Company is involved in two separate natural resource damages assessments at the Portland Harbor site. An assessment is intended to identify damages to natural resources caused by the releases of hazardous substances to the Willamette River and to serve 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. One of the natural resource damage assessments was filed in 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 The Company has accrued the estimated costs of participating in the PRP group at the Portland Harbor and Newark Bay CERCLA sites and estimated de minimis There are two plant sites in the United States related to the Performance Chemicals business where the Company has recorded an environmental remediation liability for soil and groundwater contamination which occurred prior to the Company’s acquisition of the business. As of September 30, 2018, the Company’s estimated environmental remediation liability for these acquired sites totals $4.9 million. There is one plant site in the United States related to the Utility and Industrial Products business where the Company has recorded an environmental remediation liability for soil and groundwater contamination which occurred prior to the Company’s acquisition of the business. As of September 30, 2018, the Company’s estimated environmental remediation liability for the acquired site totals $1.9 million. Foreign Environmental Matters . There are two plant sites related to the Performance Chemicals business located in the United Kingdom and Australia where the Company has recorded an environmental remediation liability for soil and groundwater contamination which occurred prior to the acquisition of the business. As of September 30, 2018, the Company’s estimated environmental remediation liability for these acquired sites totals $2.5 million. In December 2011, the Company ceased manufacturing operations at its Continental Carbon facility located in Kurnell, Australia. As of September 30, 2018, the Company believes it has fully satisfied all site remediation responsibilities resulting from the closure. Environmental Reserves Rollforward. The following table reflects changes in the accrued liability for environmental matters, of which $3.7 million and $5.1 million are classified as current liabilities at September 30, 2018 and December 31, 2017, respectively: Period ended September 30, 2018 December 31, 2017 (Dollars in millions) Balance at beginning of year $ 13.9 $ 12.9 Expense 1.0 3.2 Reversal of reserves (0.1 ) (0.7 ) Cash expenditures (3.8 ) (1.8 ) Acquisition 1.9 0.0 Currency translation (0.2 ) 0.3 Balance at end of period $ 12.7 $ 13.9

Subsidiary Guarantor Informatio

Subsidiary Guarantor Information for Koppers Inc. Shelf Registration	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Subsidiary Guarantor Information for Koppers Inc. Shelf Registration	19. Subsidiary Guarantor Information for Koppers Inc. Shelf Registration Under a registration statement on Form S-3, Koppers Holdings may sell a combination of securities, including common stock, debt securities, preferred stock, depository shares, warrants, purchase contracts and units, from time to time in one or more offerings. In addition, Koppers Inc. may sell debt securities from time to time under the registration statement. Debt securities may be fully and unconditionally guaranteed, on a joint and several basis, by Koppers Holdings, Koppers Inc., Koppers Asia LLC, Koppers World-Wide Ventures Corporation, Koppers Concrete Products, Inc., Concrete Partners, Inc., Koppers Delaware, Inc., Koppers Ventures, Inc., Koppers Performance Chemicals Inc., Koppers-Nevada Limited Liability Company, Koppers NZ LLC, Koppers Railroad Structures Inc., Wood Protection LP and Wood Management LLC. Non-guarantor subsidiaries are owned directly or indirectly by Koppers Inc. or are owned directly or indirectly by Koppers World-Wide Ventures Corporation. The guarantor subsidiaries that issue guarantees, if any, will be determined when a debt offering actually occurs under the registration statement and accordingly, the condensed consolidating financial information for subsidiary guarantors will be revised to identify the subsidiaries that actually provided guarantees. These guarantees will be governed pursuant to a supplemental indenture which the trustee and the issuing company would enter into concurrent with the debt offering. Reliance of Koppers Holdings on Earnings of Koppers Inc. and its Subsidiaries Koppers Holdings depends on the dividends from the earnings of Koppers Inc. and its subsidiaries to generate the funds necessary to meet its financial obligations, including the payment of any declared dividend of Koppers Holdings. The Revolving Credit Facility prohibits Koppers Inc. from making dividend payments to Koppers Holdings unless (1) such dividend payments are permitted by the indenture governing Koppers Inc.’s 2025 Notes, (2) no event of default or potential default has occurred or is continuing under the credit agreement, and (3) the Company is in pro forma compliance with the Company’s fixed charge coverage ratio covenant after giving effect to such dividend. The indenture governing the 2025 Notes restricts Koppers Inc.’s ability to finance the Company’s payment of dividends if (1) a default has occurred or would result from such financing, (2) Koppers Inc., or a restricted subsidiary of Koppers Inc. which is not a guarantor under the applicable indenture is not able to incur additional indebtedness (as defined in the applicable indenture), and (3) the sum of all restricted payments (as defined in the applicable indenture) have exceeded the permitted amount (which the Company refers to as the “basket”) at such point in time. The Koppers Inc. Revolving Credit Facility agreement provides for a revolving credit facility at variable rates. Borrowings under the Revolving Credit Facility are secured by a first priority lien on substantially all of the assets of Koppers Inc. and its material domestic subsidiaries. The revolving credit facility agreement 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, 2018, Koppers Inc.’s assets exceeded its liabilities by $93.0 million. The amount of restricted net assets unavailable for distribution to Koppers Holdings Inc. by Koppers Inc. totals $38.5 million as of September 30, 2018. Cash dividends paid to Koppers Holdings Inc. by its subsidiaries totaled $8.2 million and $3.7 million for the nine months ended September 30, 2018 and 2017, respectively. Condensed Consolidating Statement of Operations For the Three Months Ended September 30, 2018 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) Net sales $ 0.0 $ 158.3 $ 92.1 $ 216.9 $ (24.6 ) $ 442.7 Cost of sales including depreciation and amortization 0.0 145.3 71.5 181.1 (24.3 ) 373.6 Selling, general and administrative 0.5 12.0 9.9 15.5 0.0 37.9 Operating profit (loss) (0.5 ) 1.0 10.7 20.3 (0.3 ) 31.2 Other (loss) income 0.0 (0.4 ) 0.5 (0.7 ) 0.0 (0.6 ) Equity income (loss) of subsidiaries 8.0 15.6 14.8 0.1 (38.5 ) 0.0 Interest expense 0.0 14.4 0.1 0.6 0.0 15.1 Income taxes (0.1 ) (6.2 ) 10.6 4.3 0.0 8.6 Income from continuing operations 7.6 8.0 15.3 14.8 (38.8 ) 6.9 Noncontrolling interests 0.0 0.0 0.0 (0.7 ) 0.0 (0.7 ) Net income attributable to Koppers $ 7.6 $ 8.0 $ 15.3 $ 15.5 $ (38.8 ) $ 7.6 Comprehensive income attributable to Koppers $ (0.9 ) $ (0.3 ) $ 6.5 $ 13.0 $ (19.2 ) $ (0.9 ) Condensed Consolidating Statement of Operations For the Three Months Ended September 30, 2017 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) Net sales $ 0.0 $ 151.1 $ 92.0 $ 162.9 $ (21.2 ) $ 384.8 Cost of sales including depreciation and amortization 0.0 136.2 64.6 128.2 (21.2 ) 307.8 Loss on pension settlement 0.0 8.8 0.0 0.0 0.0 8.8 Selling, general and administrative 0.5 12.1 10.2 10.4 0.0 33.2 Operating profit (loss) (0.5 ) (6.0 ) 17.2 24.3 0.0 35.0 Other income (loss) 0.0 (0.1 ) 0.6 0.1 (0.3 ) 0.3 Equity income (loss) of subsidiaries 19.9 29.4 17.0 0.1 (66.4 ) 0.0 Interest expense (0.1 ) 9.8 0.1 1.1 (0.4 ) 10.5 Income taxes (0.3 ) (6.4 ) 6.5 5.0 0.0 4.8 Income (loss) from continuing operations 19.8 19.9 28.2 18.4 (66.3 ) 20.0 Discontinued operations 0.0 0.0 0.0 (0.1 ) 0.0 (0.1 ) Noncontrolling interests 0.0 0.0 0.0 0.1 0.0 0.1 Net income (loss) attributable to Koppers $ 19.8 $ 19.9 $ 28.2 $ 18.2 $ (66.3 ) $ 19.8 Comprehensive income (loss) attributable to Koppers $ 36.6 $ 36.8 $ 38.7 $ 24.4 $ (99.9 ) $ 36.6 Condensed Consolidating Statement of Operations For the Nine Months Ended September 30, 2018 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) Net sales $ 0.0 $ 429.6 $ 265.5 $ 651.4 $ (61.7 ) $ 1,284.8 Cost of sales including depreciation and amortization 0.0 397.5 205.9 525.4 (62.6 ) 1,066.2 Selling, general and administrative 1.9 39.3 33.8 46.8 0.0 121.8 Operating profit (loss) (1.9 ) (7.2 ) 25.8 79.2 0.9 96.8 Other (loss) income 0.0 (3.2 ) 1.3 0.8 0.0 (1.1 ) Equity income (loss) of subsidiaries 27.5 74.0 64.7 0.0 (166.2 ) 0.0 Interest expense 0.0 38.2 0.1 1.8 0.0 40.1 Income taxes (0.4 ) (2.1 ) 14.2 12.7 0.0 24.4 Income from continuing operations 26.0 27.5 77.5 65.5 (165.3 ) 31.2 Discontinued operations 0.0 0.0 0.0 0.4 0.0 0.4 Noncontrolling interests 0.0 0.0 0.0 5.6 0.0 5.6 Net income attributable to Koppers $ 26.0 $ 27.5 $ 77.5 $ 60.3 $ (165.3 ) $ 26.0 Comprehensive income attributable to Koppers $ (11.2 ) $ (9.7 ) $ 39.2 $ 41.7 $ (71.2 ) $ (11.2 ) Condensed Consolidating Statement of Operations For the Nine Months Ended September 30, 2017 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) Net sales $ 0.0 $ 455.2 $ 267.4 $ 451.1 $ (64.3 ) $ 1,109.4 Cost of sales including depreciation and amortization 0.0 425.7 183.7 357.0 (62.6 ) 903.8 Loss on pension settlement 0.0 8.8 0.0 0.0 0.0 8.8 Selling, general and administrative 1.5 33.8 30.5 29.8 0.0 95.6 Operating profit (loss) (1.5 ) (13.1 ) 53.2 64.3 (1.7 ) 101.2 Other income 0.0 (0.9 ) 1.8 1.9 (0.8 ) 2.0 Equity income of subsidiaries 44.8 81.2 47.3 0.0 (173.3 ) 0.0 Interest expense (0.1 ) 29.5 0.1 3.2 (0.8 ) 31.9 Loss on extinguishment of debt 0.0 13.3 0.0 0.0 0.0 13.3 Income taxes (0.5 ) (20.4 ) 20.2 13.1 0.0 12.4 Income (loss) from continuing operations 43.9 44.8 82.0 49.9 (175.0 ) 45.6 Discontinued operations 0.0 0.0 0.0 (1.3 ) 0.0 (1.3 ) Noncontrolling interests 0.0 0.0 0.0 0.4 0.0 0.4 Net income (loss) attributable to Koppers $ 43.9 $ 44.8 $ 82.0 $ 48.2 $ (175.0 ) $ 43.9 Comprehensive income (loss) attributable to Koppers $ 73.1 $ 74.2 $ 104.5 $ 66.4 $ (245.1 ) $ 73.1 Condensed Consolidating Balance Sheet September 30, 2018 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) ASSETS Cash and cash equivalents $ 0.0 $ 0.0 $ 0.1 $ 62.4 $ 0.0 $ 62.5 Receivables, net 0.0 70.7 31.7 119.9 0.0 222.3 Affiliated receivables (22.7 ) 39.4 17.3 4.8 (38.8 ) 0.0 Inventories, net 0.0 98.3 39.0 153.0 (1.0 ) 289.3 Other current assets 0.0 7.0 5.3 18.9 0.3 31.5 Total current assets (22.7 ) 215.4 93.4 359.0 (39.5 ) 605.6 Equity investments 91.5 1,020.8 322.9 0.1 (1,435.3 ) 0.0 Property, plant and equipment, net 0.0 203.6 47.6 147.2 0.0 398.4 Goodwill 0.0 0.8 153.1 142.6 0.0 296.5 Intangible assets, net 0.0 6.7 89.1 97.7 0.0 193.5 Deferred tax assets 0.0 23.6 (7.1 ) 1.9 0.0 18.4 Affiliated loan receivables 0.0 45.5 159.1 9.5 (214.1 ) 0.0 Other assets 0.0 4.6 6.4 12.2 0.0 23.2 Total assets $ 68.8 $ 1,521.0 $ 864.5 $ 770.2 $ (1,688.9 ) $ 1,535.6 LIABILITIES AND EQUITY Accounts payable 0.0 75.8 35.2 55.2 0.0 166.2 Affiliated payables 0.0 23.7 3.4 11.8 (38.9 ) 0.0 Accrued liabilities 0.0 50.1 14.8 36.8 0.0 101.7 Current maturities of long-term debt 0.0 10.0 0.1 5.0 0.0 15.1 Total current liabilities 0.0 159.6 53.5 108.8 (38.9 ) 283.0 Long-term debt 0.0 1,010.0 0.1 19.7 0.0 1,029.8 Affiliated debt 0.0 168.6 37.7 7.8 (214.1 ) 0.0 Other long-term liabilities 0.0 89.8 14.1 39.5 0.0 143.4 Total liabilities 0.0 1,428.0 105.4 175.8 (253.0 ) 1,456.2 Koppers shareholders’ equity 68.8 93.0 759.1 583.8 (1,435.9 ) 68.8 Noncontrolling interests 0.0 0.0 0.0 10.6 0.0 10.6 Total liabilities and equity $ 68.8 $ 1,521.0 $ 864.5 $ 770.2 $ (1,688.9 ) $ 1,535.6 Condensed Consolidating Balance Sheet December 31, 2017 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) ASSETS Cash and cash equivalents $ 0.0 $ 0.7 $ 0.0 $ 59.6 $ 0.0 $ 60.3 Receivables, net 0.0 48.6 27.7 84.6 0.0 160.9 Affiliated receivables 0.6 19.4 (83.0 ) (12.1 ) 75.1 0.0 Inventories, net 0.0 80.4 40.5 117.9 (1.9 ) 236.9 Deferred tax assets 0.0 0.0 0.0 (0.1 ) 0.1 0.0 Other current assets 0.0 6.6 23.0 18.8 0.2 48.6 Total current assets 0.6 155.7 8.2 268.7 73.5 506.7 Equity investments 99.3 716.3 276.8 (0.1 ) (1,092.3 ) 0.0 Property, plant and equipment, net 0.0 155.2 47.3 125.5 0.0 328.0 Goodwill 0.0 0.8 153.1 34.3 0.0 188.2 Intangible assets, net 0.0 7.2 96.7 25.7 0.0 129.6 Deferred tax assets 0.0 29.4 (13.2 ) 2.2 0.0 18.4 Affiliated loan receivables 0.0 34.9 224.3 21.4 (280.6 ) 0.0 Other assets 0.0 4.4 15.3 9.6 0.0 29.3 Total assets $ 99.9 $ 1,103.9 $ 808.5 $ 487.3 $ (1,299.4 ) $ 1,200.2 LIABILITIES AND EQUITY Accounts payable $ 0.0 $ 65.1 $ 32.4 $ 44.4 $ 0.0 $ 141.9 Affiliated payables 0.0 (90.3 ) 13.8 2.2 74.3 0.0 Accrued liabilities 0.0 59.9 16.4 51.6 0.0 127.9 Current maturities of long-term debt 0.0 0.1 0.0 11.3 0.0 11.4 Total current liabilities 0.0 34.8 62.6 109.5 74.3 281.2 Long-term debt 0.0 643.3 0.0 22.3 0.0 665.6 Affiliated debt 0.0 233.7 19.3 27.6 (280.6 ) 0.0 Other long-term liabilities 0.0 92.0 14.4 41.2 0.0 147.6 Total liabilities 0.0 1,003.8 96.3 200.6 (206.3 ) 1,094.4 Koppers shareholders’ equity 99.9 100.1 712.2 280.8 (1,093.1 ) 99.9 Noncontrolling interests 0.0 0.0 0.0 5.9 0.0 5.9 Total liabilities and equity $ 99.9 $ 1,103.9 $ 808.5 $ 487.3 $ (1,299.4 ) $ 1,200.2 Condensed Consolidating Statement of Cash Flows For the Nine Months Ended September 30, 2018 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) Cash provided by (used in) operating activities $ 29.2 $ (68.3 ) $ 17.1 $ 38.2 $ (8.2 ) $ 8.0 Cash provided by (used in) investing activities: Capital expenditures and acquisitions 0.0 (326.8 ) (3.9 ) (14.7 ) 0.0 (345.4 ) Repayments (loans to) from affiliates 0.0 (4.5 ) (18.1 ) (5.5 ) 28.1 0.0 Insurance proceeds received 0.0 0.0 0.0 1.5 0.0 1.5 Net cash provided by divestitures and asset sales 0.0 (2.0 ) 0.0 4.3 0.0 2.3 Net cash (used in) provided by investing activities 0.0 (333.3 ) (22.0 ) (14.4 ) 28.1 (341.6 ) Cash provided by (used in) financing activities: Borrowings (repayments) of long-term debt 0.0 377.7 (0.1 ) (7.7 ) 0.0 369.9 Borrowings (repayments) of affiliated debt 0.0 34.3 5.1 (11.3 ) (28.1 ) 0.0 Debt issuance costs 0.0 (2.9 ) 0.0 0.0 0.0 (2.9 ) Dividends paid 0.0 (8.2 ) 0.0 0.0 8.2 0.0 Stock repurchased (29.2 ) 0.0 0.0 0.0 0.0 (29.2 ) Net cash provided by (used in) financing activities (29.2 ) 400.9 5.0 (19.0 ) (19.9 ) 337.8 Effect of exchange rates on cash 0.0 0.0 0.0 (2.0 ) 0.0 (2.0 ) Net increase in cash and cash equivalents 0.0 (0.7 ) 0.1 2.8 0.0 2.2 Cash and cash equivalents at beginning of year 0.0 0.7 0.0 59.6 0.0 60.3 Cash and cash equivalents at end of period $ 0.0 $ 0.0 $ 0.1 $ 62.4 $ 0.0 $ 62.5 Condensed Consolidating Statement of Cash Flows For the Nine Months Ended September 30, 2017 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) Cash provided by (used in) operating activities $ 3.2 $ (15.9 ) $ 33.9 $ 31.0 $ (3.7 ) $ 48.5 Cash provided by (used in) investing activities: Capital expenditures and acquisitions 0.0 (32.0 ) (11.3 ) (5.3 ) 0.0 (48.6 ) Repayments (loans to) from affiliates 0.0 2.0 (19.1 ) 0.6 16.5 0.0 Repayment of loan 0.0 0.0 0.0 9.5 0.0 9.5 Net cash proceeds from divestitures and asset sales 0.0 0.0 0.7 0.4 0.0 1.1 Net cash (used in) provided by investing activities 0.0 (30.0 ) (29.7 ) 5.2 16.5 (38.0 ) Cash provided by (used in) financing activities: Repayments of long-term debt 0.0 36.7 0.0 (4.0 ) 0.0 32.7 Borrowings (repayments) of affiliated debt 0.0 23.9 (4.2 ) (3.2 ) (16.5 ) 0.0 Deferred financing cost 0.0 (11.0 ) 0.0 0.0 0.0 (11.0 ) Dividends paid 0.0 (3.7 ) 0.0 0.0 3.7 0.0 Stock repurchased (3.2 ) 0.0 0.0 0.0 0.0 (3.2 ) Net cash provided by (used in) financing activities (3.2 ) 45.9 (4.2 ) (7.2 ) (12.8 ) 18.5 Effect of exchange rates on cash 0.0 0.0 0.0 0.4 0.0 0.4 Net increase in cash and cash equivalents 0.0 0.0 0.0 29.4 0.0 29.4 Cash and cash equivalents at beginning of year 0.0 0.0 0.0 20.8 0.0 20.8 Cash and cash equivalents at end of period $ 0.0 $ 0.0 $ 0.0 $ 50.2 $ 0.0 $ 50.2

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Related Party Transactions	20. Related Party Transactions During 2016, the Company sold its 30 percent interest in TKK. As disclosed in “Note 14 – Debt”, KJCC has an outstanding loan from its 25-percent non-controlling shareholder. The loan totals $2.5 million and matures in November 2018.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	21. Fair Value Measurements Carrying amounts and the related estimated fair values of the Company’s financial instruments as of September 30, 2018 and December 31, 2017 are as follows: September 30, 2018 December 31, 2017 Fair Value Carrying Value Fair Value Carrying Value (Dollars in millions) Financial assets: Cash and cash equivalents, including restricted cash $ 62.5 $ 62.5 $ 60.3 $ 60.3 Investments and other assets (a) 1.1 1.1 1.1 1.1 Financial liabilities: Total debt (including current portion) $ 1,049.1 $ 1,057.7 $ 706.9 $ 688.7 (a) Excludes equity method investments. Cash and cash equivalents – The carrying value approximates fair value because of the short maturity of those instruments. 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 the Company’s long-term debt is estimated based on the market prices for the same or similar issues or on the current rates offered to the Company for debt of the same remaining maturities (Level 2). The fair value of the Company’s revolving credit facility approximates carrying value due to the variable rate nature of this instrument.

New Accounting Pronouncements (

New Accounting Pronouncements (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Changes And Error Corrections [Abstract]
New Accounting Pronouncements	In August 2018, the Securities and Exchange Commission (“SEC”) issued SEC Release No. 33-10532, “Disclosure Update and Simplification”, which In August 2017, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2017-12, “Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities.” This ASU amends and simplifies existing guidance in order to allow companies to more accurately present the economic effects of risk management activities in the financial statements. The Company adopted this ASU effective January 1, 2018 and the Company reclassified a $3.6 million unrealized gain, net of tax, from retained earnings to accumulated other comprehensive loss upon adoption. In March 2017, the FASB issued ASU No. 2017-07, “Compensation – Retirement Benefits (Topic 715)”, in order to improve the presentation of net periodic pension and postretirement costs. The amendment requires that an employer report the service cost component in the same line item or items as other compensation costs arising from services rendered by the pertinent employees during the period. The other components of net benefit cost are required to be presented in the income statement separately from the service cost component and outside a subtotal of income from operations. The amendments in this update related to income statement activity are applied. As a practical expedient, the Company has used the amounts disclosed in its pension and post-retirement benefits footnote as the estimation basis for applying the retrospective presentation requirements. The Company adopted this ASU effective January 1, 2018, and for retrospective presentation, reclassified $0.1 and $0.6 million In January 2017, the FASB issued ASU No. 2017-04, “Simplifying the Test for Goodwill Impairment (Topic 350).” The update is intended to simplify how an entity is required to test goodwill for impairment by eliminating Step 2 from the goodwill impairment test. Step 2 measures a goodwill impairment loss by comparing the implied fair value of a reporting unit's goodwill with the carrying amount of that goodwill. The amendments in this update are effective for periods beginning after December 15, 2019. Entities are required to apply the amendments in this update prospectively from the date of adoption, with early adoption permitted. The Company adopted this ASU effective January 1, 2018. In August 2016, the FASB The Company adopted this ASU effective January 1, 2018 In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842).” ASU 2016-02 requires an entity to recognize a right-of-use asset and lease liability for all leases with terms of more than one year. The standard is effective January 1, 2019 and recognition, measurement and presentation of expenses will depend on classification as a finance or operating lease. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements, which provides clarifications and improvements to ASU 2016-02 including allowing entities to elect an additional transition method with which to adopt ASU 2016-02. The approved transition method enables entities to apply the transition requirements in this ASU at the effective date of ASU 2016-02 (rather than at the beginning of the earliest comparative period presented) with the effect of initially applying ASU 2016-02 recognized as a cumulative-effect adjustment to retained earnings in the period of adoption. Consequently, an entity’s reporting for the comparative periods presented in the year of adoption would continue to be in accordance with ASC 840, Leases (Topic 840) (“ASC 840”), including the disclosure requirements of ASC 840. We plan to adopt ASU 2016-02 effective at the beginning of 2019 using the modified retrospective approach with no restatement of comparative periods presented. Practical expedients are available for election as a package and if applied consistently to all leases. The Company has a project team analyzing all of its leases and we have selected our leasing software solution and are in the process of identifying changes to our business processes, systems and controls to support adoption of the new standard in 2019. The Company will elect the practical expedient to retain its current classification of leases, and as a result, anticipates that the initial impact of adopting this new standard on its consolidated statement of income and consolidated statement of cash flows will not be material. The Company is still in the process of determining the impact of this adoption on the balance sheet and expects this analysis will be completed during the fourth quarter of 2018. While we are unable to quantify the impact at this time we expect that the adoption of the new standard to result in a material effect on total assts and total liabilities. In May 2014, the FASB issued ASU No. 2014-09, “Revenue from Contracts with Customers (Topic 606).” ASU 2014-09 requires an entity to recognize revenue in a manner that depicts the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Subsequent to the issuance of ASU 2014-09, the FASB issued multiple ASUs which either amended or clarified ASU 2014-09. Collectively, the revenue recognition ASUs were effective for annual reporting periods beginning after December 15, 2017. The Company elected to use the modified retrospective method for transition in which the cumulative effect was recognized at its January 1, 2018 date of adoption with no restatement of comparative periods presented. The implementation of the guidance had no material impact on the measurement of recognition of revenue of prior periods; however, additional disclosures have been added in accordance with the ASU.

Plant Closures and Divestitur_2

Plant Closures and Divestitures (Tables)	9 Months Ended
	Sep. 30, 2018
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 employee Environmental remediation Site demolition Other Total (Dollars in millions) Reserve at December 31, 2016 $ 1.4 $ 1.5 $ 10.0 $ 3.2 $ 16.1 Accrual 0.9 2.1 4.7 6.9 14.6 Cost charged against assets 0.0 0.0 0.0 (6.3 ) (6.3 ) Reversal of accrued charges (0.3 ) 0.0 (1.8 ) 0.0 (2.1 ) Cash paid (0.3 ) (1.1 ) (2.4 ) (1.0 ) (4.8 ) Currency translation 0.0 0.2 0.1 0.5 0.8 Reserve at December 31, 2017 $ 1.7 $ 2.7 $ 10.6 $ 3.3 $ 18.3 Accrual 0.0 0.9 1.6 2.2 4.7 Cost charged against assets 0.0 0.0 0.0 (2.1 ) (2.1 ) Cash paid 0.0 (3.4 ) (6.3 ) (0.1 ) (9.8 ) Currency translation 0.0 (0.2 ) 0.0 0.0 (0.2 ) Reserve at September 30, 2018 $ 1.7 $ 0.0 $ 5.9 $ 3.3 $ 10.9

Acquisitions (Tables)

Acquisitions (Tables)	9 Months Ended
	Sep. 30, 2018
Business Combinations [Abstract]
Summary of Preliminary Valuation of Identifiable Assets Acquired and Liabilities Assumed Upon Acquisition	The preliminary valuation of identifiable assets acquired and liabilities assumed upon the acquisition of UIP and MAER are shown in the table below. UIP MAER (Dollars in millions) Cash and cash equivalents $ 1.5 $ 3.2 Accounts receivable 20.2 4.9 Inventory 43.1 0.0 Other current assets 1.2 0.3 Property, plant and equipment 19.8 6.6 Intangible assets, net 59.4 19.3 Goodwill 76.9 34.0 Other non-current assets 0.2 0.0 Total assets acquired 222.3 68.3 Accounts payable & accrued expenses 18.7 2.3 Other non-current liabilities 0.8 0.0 Net assets acquired $ 202.8 $ 66.0
Schedule of Identifiable Intangible Assets	Net assets acquired include identifiable intangible assets with respect to customer contracts, non-compete agreements and trademarks as summarized in the following table. UIP MAER (Dollars in millions) Amount Remaining Life Amount Remaining Life Customer contracts $ 58.2 15 years $ 18.7 15 years Non-compete agreements 1.2 5 years Trademarks 0.6 2 years Total $ 59.4 $ 19.3
Summary of Revenue and Income from Continuing Operations	The following unaudited pro forma information presents a summary of the Company’s revenues and income from continuing operations as if the UIP acquisition occurred on January 1, 2017 (the first day of the most recently completed fiscal year). The unaudited pro forma information is not necessarily indicative of operating results that would have been achieved had the acquisition been completed as of January 1, 2017 and is not intended to project the future financial results of the Company after the acquisition. The unaudited pro forma information is based on certain assumptions, which management believes are reasonable, and does not reflect the cost of any integration activities or the benefits from the acquisition and synergies that may be derived from any integration activities. Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) Pro forma revenue $ 442.7 $ 422.7 $ 1,335.5 $ 1,221.6 Pro forma income from continuing operations attributable to Koppers 7.9 19.9 34.5 42.6 Pro forma income per share - continuing operations: Basic - $ 0.38 $ 0.96 $ 1.64 $ 2.05 Diluted - $ 0.36 $ 0.91 $ 1.58 $ 1.94

Revenue Recognition (Tables)

Revenue Recognition (Tables)	9 Months Ended
	Sep. 30, 2018
Revenue From Contract With Customer [Abstract]
Schedule of Revenue Recognized Net Impact of Adopting Topic 606 Decrease in Sales and Cost of Sales	Revenue recognized during the nine months ended September 30, 2018 from the adoption of Topic 606 was $3.2 million and, as such, the net impact of adopting Topic 606 for the three and nine months ending September 30, 2018 was a decrease in sales and cost of sales as follows: Three Months Ended Nine Months Ended September 30, September 30, 2018 2018 (Dollars in millions) Sales $ (0.1 ) $ (2.1 ) Cost of sales (0.1 ) (2.0 )

Comprehensive (Loss) Income a_2

Comprehensive (Loss) Income and Equity (Tables)	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Schedule of Comprehensive (Loss) Income	Total comprehensive (loss) income for the three and nine months ended September 30, 2018 and 2017 is summarized in the table below: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) Net income $ 6.9 $ 19.9 $ 31.6 $ 44.3 Other comprehensive (loss) income: Change in currency translation adjustment (2.9 ) 6.3 (18.9 ) 17.2 Unrealized (loss) gain on cash flow hedges, net of tax (benefit) expense of $(3.7), $1.1, $(12.0) and $1.4 (6.3 ) 4.1 (20.3 ) 5.0 Change in accounting standard 0.0 0.0 0.3 0.0 Unrecognized pension net loss, net of tax expense of $0.1, $0.2, $0.3 and $0.4 0.3 6.5 0.8 7.2 Total comprehensive (loss) income (2.0 ) 36.8 (6.5 ) 73.7 Less: Comprehensive (loss) income attributable to noncontrolling interests (1.1 ) 0.2 4.7 0.6 Comprehensive (loss) income attributable to Koppers $ (0.9 ) $ 36.6 $ (11.2 ) $ 73.1
Schedule of Change in Equity	The following tables present the change in equity for the nine months ended September 30, 2018 and 2017, respectively: (Dollars in millions) Total Koppers Shareholders’ Equity Noncontrolling Interests Total Equity Balance at December 31, 2017 $ 99.9 $ 5.9 $ 105.8 Net income 26.0 5.6 31.6 Issuance of common stock 2.5 0.0 2.5 Employee stock plans 9.3 0.0 9.3 Other comprehensive loss (37.2 ) (0.9 ) (38.1 ) Repurchases of common stock (31.7 ) 0.0 (31.7 ) Balance at September 30, 2018 $ 68.8 $ 10.6 $ 79.4 (Dollars in millions) Total Koppers Shareholders’ Equity Noncontrolling Interests Total Equity Balance at December 31, 2016 $ 30.4 $ 4.2 $ 34.6 Net income 43.9 0.4 44.3 Issuance of common stock 1.9 0.0 1.9 Employee stock plans 8.0 0.0 8.0 Other comprehensive income 29.2 0.2 29.4 Repurchases of common stock (5.1 ) 0.0 (5.1 ) Balance at September 30, 2017 $ 108.3 $ 4.8 $ 113.1

Earnings per Common Share (Tabl

Earnings per Common Share (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Schedule of Computation of Basic and Diluted Earnings per Common Share	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, 2018 2017 2018 2017 (Dollars in millions, except share amounts, in thousands, and per share amounts) Net income attributable to Koppers $ 7.6 $ 19.8 $ 26.0 $ 43.9 Less: Income (loss) from discontinued operations 0.0 (0.1 ) 0.4 (1.3 ) Income from continuing operations attributable to Koppers $ 7.6 $ 19.9 $ 25.6 $ 45.2 Weighted average common shares outstanding: Basic 20,946 20,746 20,992 20,750 Effect of dilutive securities 754 1,165 900 1,177 Diluted 21,700 21,911 21,892 21,927 Income per common share – continuing operations: Basic income per common share $ 0.36 $ 0.96 $ 1.22 $ 2.17 Diluted income per common share 0.35 0.91 1.17 2.06 Other data: Antidilutive securities excluded from computation of diluted earnings per common share 571 543 214 482

Stock-based Compensation (Table

Stock-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Options And Performance Award Fair Value Assumptions	The Company calculated the fair value of stock options on the date of grant using the Black-Scholes-Merton model and the assumptions listed below: May March March March 2016 Grant March 2015 Grant Grant date price per share $ 39.10 $ 41.60 $ 44.10 $ 18.11 $ 17.57 Expected dividend yield per share 0.00 % 0.00 % 0.00 % 0.00 % 3.40 % Expected life in years 5.73 5.73 5.77 5.96 5.75 Expected volatility 37.05 % 37.05 % 39.70 % 40.86 % 42.27 % Risk-free interest rate 2.82 % 2.67 % 2.13 % 1.45 % 1.73 % Grant date fair value per share $ 15.48 $ 16.38 $ 17.90 $ 7.41 $ 5.20
Summary of Performance Stock Units	The following table shows a summary of the performance stock units as of September 30, 2018: Performance Period Minimum Shares Target Shares Maximum Shares 2016 – 2018 0 255,172 510,344 2017 – 2019 0 113,675 227,350 2018 – 2020 0 130,782 261,564
Summary of Status and Activity of Non-Vested Stock Units	The following table shows a summary of the status and activity of non-vested stock units for the nine months ended September 30, 2018: Restricted Stock Units Performance Stock Units Total Stock Units Weighted Grant Date Fair Value per Unit Non-vested at December 31, 2017 238,140 581,551 819,691 $ 29.14 Granted 118,297 134,351 252,648 $ 43.91 Performance share adjustment 0 204,866 204,866 $ 17.57 Vested (94,589 ) (409,732 ) (504,321 ) $ 19.41 Forfeited (7,094 ) (11,407 ) (18,501 ) $ 38.77 Non-vested at September 30, 2018 254,754 499,629 754,383 $ 37.20
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, 2018: Options Weighted Exercise Price per Option Weighted Average Remaining Contractual Term (in years) Aggregate Value (in millions) Outstanding at December 31, 2017 942,537 $ 27.59 Granted 144,825 $ 41.50 Exercised (70,307 ) $ 27.75 Forfeited (12,329 ) $ 30.49 Outstanding at September 30, 2018 1,004,726 $ 29.54 6.14 $ 6.6 Exercisable at September 30, 2018 617,361 $ 28.49 4.89 $ 4.2
Schedule of Stock-based Compensation Expense Recognized	Stock Compensation Expense Total stock-based compensation expense recognized under the Company’s LTIP and employee stock purchase plan for three and nine months ended September 30, 2018 and 2017 is as follows: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) Stock-based compensation expense recognized: Selling, general and administrative expenses $ 3.2 $ 2.7 $ 9.3 $ 7.8 Less related income tax benefit 1.2 0.6 3.5 1.7 Decrease in net income attributable to Koppers $ 2.0 $ 2.1 $ 5.8 $ 6.1 Intrinsic value of exercised stock options $ 0.1 $ 0.0 $ 1.1 $ 1.1 Cash received from the exercise of stock options $ 0.3 $ 0.0 $ 2.5 $ 1.9
Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Options And Performance Award Fair Value Assumptions	The Company calculated the fair value of the awards on the date of grant using the Monte Carlo valuation model and the assumptions listed below: May March March March 2016 Grant Grant date price per share $ 39.10 $ 41.60 $ 44.10 $ 18.11 Expected dividend yield per share 0.00 % 0.00 % 0.00 % 0.00 % Expected volatility 39.40 % 39.40 % 43.50 % 40.86 % Risk-free interest rate 2.35 % 2.35 % 1.54 % 0.96 % Look-back period in years 2.84 2.84 2.83 2.84 Grant date fair value per share $ 44.29 $ 47.12 $ 64.02 $ 23.70

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2018
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 the Company’s segments for the periods indicated: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) Revenues from external customers: Railroad and Utility Products and Services $ 185.0 $ 131.7 $ 470.6 $ 403.1 Performance Chemicals 108.2 109.7 320.7 318.2 Carbon Materials and Chemicals 149.5 143.4 493.5 388.1 Total $ 442.7 $ 384.8 $ 1,284.8 $ 1,109.4 Intersegment revenues: Performance Chemicals $ 3.8 $ 1.8 $ 8.0 $ 5.0 Carbon Materials and Chemicals 21.4 22.1 58.8 60.0 Total $ 25.2 $ 23.9 $ 66.8 $ 65.0 Depreciation and amortization expense: Railroad and Utility Products and Services $ 4.9 $ 2.9 $ 12.8 $ 8.8 Performance Chemicals 4.4 4.4 13.3 13.3 Carbon Materials and Chemicals 3.7 4.8 12.4 12.9 Total $ 13.0 $ 12.1 $ 38.5 $ 35.0 Operating profit (loss): Railroad and Utility Products and Services (a) $ 5.8 $ 9.5 $ 5.9 $ 30.1 Performance Chemicals 11.0 18.4 28.2 56.6 Carbon Materials and Chemicals 14.9 16.3 64.6 24.9 Corporate (b) (0.5 ) (9.2 ) (1.9 ) (10.4 ) Total $ 31.2 $ 35.0 $ 96.8 $ 101.2 (a) Operating profit for the three and nine months ended September 30, 2018 includes $0.5 million and $6.0 million of inventory fair value purchase price adjustment related to the (b) Operating loss for Corporate includes primarily general and administrative costs for Koppers Holdings Inc., the parent company of Koppers Inc.
Schedule of Segment Revenues for Significant Product Lines	The following table sets forth revenues for significant product lines, net of all intersegment transactions, for the Company’s segments for the periods indicated: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) Railroad and Utility Products and Services: Railroad treated products $ 96.7 $ 96.1 $ 259.8 $ 299.8 Utility poles 59.5 12.9 129.9 34.0 Rail joints 8.5 7.1 26.5 22.1 Railroad infrastructure services 10.0 9.0 26.2 27.0 Other products 10.3 6.6 28.2 20.2 185.0 131.7 470.6 403.1 Performance Chemicals: Wood preservative products 101.5 101.7 299.0 295.5 Other products 6.7 8.0 21.7 22.7 108.2 109.7 320.7 318.2 Carbon Materials and Chemicals: Pitch and related products 88.9 78.6 304.1 195.4 Creosote and distillates 19.2 21.6 61.3 61.0 Phthalic anhydride and other chemicals 23.3 24.5 64.0 69.3 Naphthalene 8.8 9.9 31.5 27.8 Other products 9.3 8.8 32.6 34.6 149.5 143.4 493.5 388.1 Total $ 442.7 $ 384.8 $ 1,284.8 $ 1,109.4
Summary of Tangible and Intangible Assets by Segments	The following table sets forth certain tangible and intangible assets allocated to each of the Company’s segments as of the dates indicated: September 30, 2018 December 31, 2017 (Dollars in millions) Segment assets: Railroad and Utility Products and Services (a) $ 556.3 $ 249.7 Performance Chemicals 472.0 494.0 Carbon Materials and Chemicals 469.3 414.2 All other 38.0 42.3 Total $ 1,535.6 $ 1,200.2 Goodwill: Railroad and Utility Products and Services (a) $ 120.8 $ 10.5 Performance Chemicals 175.7 177.7 Total $ 296.5 $ 188.2 (a) The increase in RUPS assets and goodwill as of September 30, 2018 includes $279.3 million and $110.9 million, respectively, attributable to the Company’s acquisitions of UIP and MAER during 2018.

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Summary of Income Taxes Reconciled with Federal Statutory Rate	The estimated annual effective income tax rate differs from the U.S. federal statutory tax rate due to: September 30, September 30, 2018 2017 Federal income tax rate 21.0 % 35.0 % State income taxes, net of federal tax benefit (0.8 ) 1.1 Foreign earnings taxed at different rates 1.8 (16.8 ) GILTI inclusion (net of foreign tax credits) 16.7 0.0 Valuation allowance adjustments (11.5 ) 0.0 Interest expense deduction limitation 7.7 0.0 Nondeductible expenses 2.6 1.6 Change in tax contingency reserves 0.2 0.2 Other (0.6 ) 0.6 Estimated annual effective income tax rate 37.1 % 21.7 %

Inventories (Tables)

Inventories (Tables)	9 Months Ended
	Sep. 30, 2018
Inventory Disclosure [Abstract]
Components of Inventories	Net inventories as of September 30, 2018 and December 31, 2017 are summarized in the table below: September 30, 2018 December 31, 2017 (Dollars in millions) Raw materials $ 203.4 $ 173.6 Work in process 14.8 11.2 Finished goods 123.9 98.4 $ 342.1 $ 283.2 Less revaluation to LIFO 52.8 46.3 Net $ 289.3 $ 236.9

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	9 Months Ended
	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Property, Plant and Equipment	Property, plant and equipment as of September 30, 2018 and December 31, 2017 are summarized in the table below: September 30, 2018 December 31, 2017 (Dollars in millions) Land $ 17.0 $ 17.6 Buildings 61.9 63.4 Machinery and equipment 835.2 756.6 $ 914.1 $ 837.6 Less accumulated depreciation 515.7 509.6 Net $ 398.4 $ 328.0

Pensions and Post-Retirement _2

Pensions and Post-Retirement Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 and 2017: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) Service cost $ 0.5 $ 0.5 $ 1.4 $ 1.5 Interest cost 1.8 2.1 5.6 7.0 Expected return on plan assets (2.1 ) (2.4 ) (6.4 ) (7.4 ) Amortization of net loss 0.4 0.5 1.1 1.5 Settlements 0.0 8.8 0.0 8.8 Net periodic benefit cost $ 0.6 $ 9.5 $ 1.7 $ 11.4 Defined contribution plan expense $ 1.3 $ 2.1 $ 5.5 $ 6.1

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Schedule of Long-Term Debt Instruments	Debt as of September 30, 2018 and December 31, 2017 was as follows: Weighted Average Interest Rate Maturity September 30, 2018 December 31, 2017 (Dollars in millions) Term Loan 5.16 % 2023 $ 95.0 $ 0.0 Revolving Credit Facility 5.16 % 2023 437.8 155.0 Construction and other loans 4.69 % 2020 24.9 33.7 Senior Notes due 2025 6.00 % 2025 500.0 500.0 Debt 1,057.7 688.7 Less short-term debt and current maturities of long-term debt 15.1 11.4 Less unamortized debt issuance costs 12.8 11.7 Long-term debt $ 1,029.8 $ 665.6

Asset Retirement Obligations (T

Asset Retirement Obligations (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 December 31, 2017 (Dollars in millions) Asset retirement obligation at beginning of year $ 37.1 $ 36.0 Accretion expense 1.3 2.4 Revision in estimated cash flows 1.4 9.4 Cash expenditures (10.4 ) (10.9 ) Currency translation 0.0 0.2 Balance at end of period $ 29.4 $ 37.1

Deferred Revenue (Tables)

Deferred Revenue (Tables)	9 Months Ended
	Sep. 30, 2018
Deferred Revenue Disclosure [Abstract]
Deferred Revenue from Extended Product Warranty Liabilities	The following table reflects changes in the carrying values of deferred revenue: September 30, 2018 December 31, 2017 (Dollars in millions) Balance at beginning of year $ 28.1 $ 27.2 Revenue earned (0.5 ) (0.7 ) Currency translation (1.6 ) 1.6 Balance at end of period $ 26.0 $ 28.1

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Schedule of Outstanding Copper Swap Contracts	As of September 30, 2018 and December 31, 2017, the Company had outstanding copper swap contracts of the following amounts: Units Outstanding (in Pounds) Net Fair Value - (Liability) Asset September 30, 2018 December 31, 2017 September 30, 2018 December 31, 2017 (Amounts in millions) Cash flow hedges 39.6 37.8 $ (0.2 ) $ 25.5 Not designated as hedges 16.9 11.3 (1.0 ) 4.5 Total 56.5 49.1 $ (1.2 ) $ 30.0
Schedule of Fair Value of Outstanding Copper Swap Contracts Recorded in Balance Sheet	As of September 30, 2018 and December 31, 2017, the fair value of the outstanding copper swap contracts is recorded in the balance sheet as follows: September 30, 2018 December 31, 2017 (Dollars in millions) Other current assets $ 1.2 $ 21.8 Other assets 0.5 8.2 Accrued liabilities (2.6 ) 0.0 Other long-term liabilities (0.3 ) 0.0 Net (liability) asset on balance sheet $ (1.2 ) $ 30.0 Accumulated other comprehensive gain (loss), net of tax $ (0.3 ) $ 15.8
Summary of Amount Recognized in Earnings Related to Copper Swap Contracts	See Note 6 – Comprehensive Income (Loss) and Equity, for amounts recorded in other comprehensive income and for amounts reclassified from accumulated other comprehensive loss to net income for the periods specified below. For the three and nine months ended September 30, 2018 and 2017, the following amounts were recognized in earnings related to copper swap contracts: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (Dollars in millions) (Loss) gain from ineffectiveness of cash flow hedges $ 0.0 $ (2.5 ) $ 0.0 $ 0.5 (Loss) gain from contracts not designated as hedges (1.0 ) 1.0 (5.5 ) 2.0 Net $ (1.0 ) $ (1.5 ) $ (5.5 ) $ 2.5
Schedule of Fair Value of Outstanding Foreign Currency Forward Contracts	As of September 30, 2018 and December 31, 2017, the fair value of outstanding foreign currency forward contracts is recorded in the balance sheet as follows: September 30, 2018 December 31, 2017 (Dollars in millions) Other current assets $ 0.7 $ 0.3 Accrued liabilities (1.1 ) (0.2 ) Net (liability) asset on balance sheet $ (0.4 ) $ 0.1
Summary of Net Currency Units Outstanding	As of September 30, 2018 and December 31, 2017, the net currency units outstanding for these contracts were: September 30, 2018 December 31, 2017 (In millions) British Pounds GBP 5.7 GBP 7.0 New Zealand Dollars NZD 16.0 NZD 15.5 United States Dollars USD 4.6 USD 12.5 Canadian Dollars CAD 2.5 CAD 2.5 Euro EUR 1.3 EUR 0.0

Commitments and Contingent Li_2

Commitments and Contingent Liabilities (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Changes in Accrued Liability for Environmental Matters	The following table reflects changes in the accrued liability for environmental matters, of which $3.7 million and $5.1 million are classified as current liabilities at September 30, 2018 and December 31, 2017, respectively: Period ended September 30, 2018 December 31, 2017 (Dollars in millions) Balance at beginning of year $ 13.9 $ 12.9 Expense 1.0 3.2 Reversal of reserves (0.1 ) (0.7 ) Cash expenditures (3.8 ) (1.8 ) Acquisition 1.9 0.0 Currency translation (0.2 ) 0.3 Balance at end of period $ 12.7 $ 13.9

Subsidiary Guarantor Informat_2

Subsidiary Guarantor Information for Koppers Inc. Shelf Registration (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Condensed Consolidating Statement of Operations	Condensed Consolidating Statement of Operations For the Three Months Ended September 30, 2018 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) Net sales $ 0.0 $ 158.3 $ 92.1 $ 216.9 $ (24.6 ) $ 442.7 Cost of sales including depreciation and amortization 0.0 145.3 71.5 181.1 (24.3 ) 373.6 Selling, general and administrative 0.5 12.0 9.9 15.5 0.0 37.9 Operating profit (loss) (0.5 ) 1.0 10.7 20.3 (0.3 ) 31.2 Other (loss) income 0.0 (0.4 ) 0.5 (0.7 ) 0.0 (0.6 ) Equity income (loss) of subsidiaries 8.0 15.6 14.8 0.1 (38.5 ) 0.0 Interest expense 0.0 14.4 0.1 0.6 0.0 15.1 Income taxes (0.1 ) (6.2 ) 10.6 4.3 0.0 8.6 Income from continuing operations 7.6 8.0 15.3 14.8 (38.8 ) 6.9 Noncontrolling interests 0.0 0.0 0.0 (0.7 ) 0.0 (0.7 ) Net income attributable to Koppers $ 7.6 $ 8.0 $ 15.3 $ 15.5 $ (38.8 ) $ 7.6 Comprehensive income attributable to Koppers $ (0.9 ) $ (0.3 ) $ 6.5 $ 13.0 $ (19.2 ) $ (0.9 ) Condensed Consolidating Statement of Operations For the Three Months Ended September 30, 2017 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) Net sales $ 0.0 $ 151.1 $ 92.0 $ 162.9 $ (21.2 ) $ 384.8 Cost of sales including depreciation and amortization 0.0 136.2 64.6 128.2 (21.2 ) 307.8 Loss on pension settlement 0.0 8.8 0.0 0.0 0.0 8.8 Selling, general and administrative 0.5 12.1 10.2 10.4 0.0 33.2 Operating profit (loss) (0.5 ) (6.0 ) 17.2 24.3 0.0 35.0 Other income (loss) 0.0 (0.1 ) 0.6 0.1 (0.3 ) 0.3 Equity income (loss) of subsidiaries 19.9 29.4 17.0 0.1 (66.4 ) 0.0 Interest expense (0.1 ) 9.8 0.1 1.1 (0.4 ) 10.5 Income taxes (0.3 ) (6.4 ) 6.5 5.0 0.0 4.8 Income (loss) from continuing operations 19.8 19.9 28.2 18.4 (66.3 ) 20.0 Discontinued operations 0.0 0.0 0.0 (0.1 ) 0.0 (0.1 ) Noncontrolling interests 0.0 0.0 0.0 0.1 0.0 0.1 Net income (loss) attributable to Koppers $ 19.8 $ 19.9 $ 28.2 $ 18.2 $ (66.3 ) $ 19.8 Comprehensive income (loss) attributable to Koppers $ 36.6 $ 36.8 $ 38.7 $ 24.4 $ (99.9 ) $ 36.6 Condensed Consolidating Statement of Operations For the Nine Months Ended September 30, 2018 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) Net sales $ 0.0 $ 429.6 $ 265.5 $ 651.4 $ (61.7 ) $ 1,284.8 Cost of sales including depreciation and amortization 0.0 397.5 205.9 525.4 (62.6 ) 1,066.2 Selling, general and administrative 1.9 39.3 33.8 46.8 0.0 121.8 Operating profit (loss) (1.9 ) (7.2 ) 25.8 79.2 0.9 96.8 Other (loss) income 0.0 (3.2 ) 1.3 0.8 0.0 (1.1 ) Equity income (loss) of subsidiaries 27.5 74.0 64.7 0.0 (166.2 ) 0.0 Interest expense 0.0 38.2 0.1 1.8 0.0 40.1 Income taxes (0.4 ) (2.1 ) 14.2 12.7 0.0 24.4 Income from continuing operations 26.0 27.5 77.5 65.5 (165.3 ) 31.2 Discontinued operations 0.0 0.0 0.0 0.4 0.0 0.4 Noncontrolling interests 0.0 0.0 0.0 5.6 0.0 5.6 Net income attributable to Koppers $ 26.0 $ 27.5 $ 77.5 $ 60.3 $ (165.3 ) $ 26.0 Comprehensive income attributable to Koppers $ (11.2 ) $ (9.7 ) $ 39.2 $ 41.7 $ (71.2 ) $ (11.2 ) Condensed Consolidating Statement of Operations For the Nine Months Ended September 30, 2017 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) Net sales $ 0.0 $ 455.2 $ 267.4 $ 451.1 $ (64.3 ) $ 1,109.4 Cost of sales including depreciation and amortization 0.0 425.7 183.7 357.0 (62.6 ) 903.8 Loss on pension settlement 0.0 8.8 0.0 0.0 0.0 8.8 Selling, general and administrative 1.5 33.8 30.5 29.8 0.0 95.6 Operating profit (loss) (1.5 ) (13.1 ) 53.2 64.3 (1.7 ) 101.2 Other income 0.0 (0.9 ) 1.8 1.9 (0.8 ) 2.0 Equity income of subsidiaries 44.8 81.2 47.3 0.0 (173.3 ) 0.0 Interest expense (0.1 ) 29.5 0.1 3.2 (0.8 ) 31.9 Loss on extinguishment of debt 0.0 13.3 0.0 0.0 0.0 13.3 Income taxes (0.5 ) (20.4 ) 20.2 13.1 0.0 12.4 Income (loss) from continuing operations 43.9 44.8 82.0 49.9 (175.0 ) 45.6 Discontinued operations 0.0 0.0 0.0 (1.3 ) 0.0 (1.3 ) Noncontrolling interests 0.0 0.0 0.0 0.4 0.0 0.4 Net income (loss) attributable to Koppers $ 43.9 $ 44.8 $ 82.0 $ 48.2 $ (175.0 ) $ 43.9 Comprehensive income (loss) attributable to Koppers $ 73.1 $ 74.2 $ 104.5 $ 66.4 $ (245.1 ) $ 73.1
Condensed Consolidating Balance Sheet	Condensed Consolidating Balance Sheet September 30, 2018 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) ASSETS Cash and cash equivalents $ 0.0 $ 0.0 $ 0.1 $ 62.4 $ 0.0 $ 62.5 Receivables, net 0.0 70.7 31.7 119.9 0.0 222.3 Affiliated receivables (22.7 ) 39.4 17.3 4.8 (38.8 ) 0.0 Inventories, net 0.0 98.3 39.0 153.0 (1.0 ) 289.3 Other current assets 0.0 7.0 5.3 18.9 0.3 31.5 Total current assets (22.7 ) 215.4 93.4 359.0 (39.5 ) 605.6 Equity investments 91.5 1,020.8 322.9 0.1 (1,435.3 ) 0.0 Property, plant and equipment, net 0.0 203.6 47.6 147.2 0.0 398.4 Goodwill 0.0 0.8 153.1 142.6 0.0 296.5 Intangible assets, net 0.0 6.7 89.1 97.7 0.0 193.5 Deferred tax assets 0.0 23.6 (7.1 ) 1.9 0.0 18.4 Affiliated loan receivables 0.0 45.5 159.1 9.5 (214.1 ) 0.0 Other assets 0.0 4.6 6.4 12.2 0.0 23.2 Total assets $ 68.8 $ 1,521.0 $ 864.5 $ 770.2 $ (1,688.9 ) $ 1,535.6 LIABILITIES AND EQUITY Accounts payable 0.0 75.8 35.2 55.2 0.0 166.2 Affiliated payables 0.0 23.7 3.4 11.8 (38.9 ) 0.0 Accrued liabilities 0.0 50.1 14.8 36.8 0.0 101.7 Current maturities of long-term debt 0.0 10.0 0.1 5.0 0.0 15.1 Total current liabilities 0.0 159.6 53.5 108.8 (38.9 ) 283.0 Long-term debt 0.0 1,010.0 0.1 19.7 0.0 1,029.8 Affiliated debt 0.0 168.6 37.7 7.8 (214.1 ) 0.0 Other long-term liabilities 0.0 89.8 14.1 39.5 0.0 143.4 Total liabilities 0.0 1,428.0 105.4 175.8 (253.0 ) 1,456.2 Koppers shareholders’ equity 68.8 93.0 759.1 583.8 (1,435.9 ) 68.8 Noncontrolling interests 0.0 0.0 0.0 10.6 0.0 10.6 Total liabilities and equity $ 68.8 $ 1,521.0 $ 864.5 $ 770.2 $ (1,688.9 ) $ 1,535.6 Condensed Consolidating Balance Sheet December 31, 2017 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) ASSETS Cash and cash equivalents $ 0.0 $ 0.7 $ 0.0 $ 59.6 $ 0.0 $ 60.3 Receivables, net 0.0 48.6 27.7 84.6 0.0 160.9 Affiliated receivables 0.6 19.4 (83.0 ) (12.1 ) 75.1 0.0 Inventories, net 0.0 80.4 40.5 117.9 (1.9 ) 236.9 Deferred tax assets 0.0 0.0 0.0 (0.1 ) 0.1 0.0 Other current assets 0.0 6.6 23.0 18.8 0.2 48.6 Total current assets 0.6 155.7 8.2 268.7 73.5 506.7 Equity investments 99.3 716.3 276.8 (0.1 ) (1,092.3 ) 0.0 Property, plant and equipment, net 0.0 155.2 47.3 125.5 0.0 328.0 Goodwill 0.0 0.8 153.1 34.3 0.0 188.2 Intangible assets, net 0.0 7.2 96.7 25.7 0.0 129.6 Deferred tax assets 0.0 29.4 (13.2 ) 2.2 0.0 18.4 Affiliated loan receivables 0.0 34.9 224.3 21.4 (280.6 ) 0.0 Other assets 0.0 4.4 15.3 9.6 0.0 29.3 Total assets $ 99.9 $ 1,103.9 $ 808.5 $ 487.3 $ (1,299.4 ) $ 1,200.2 LIABILITIES AND EQUITY Accounts payable $ 0.0 $ 65.1 $ 32.4 $ 44.4 $ 0.0 $ 141.9 Affiliated payables 0.0 (90.3 ) 13.8 2.2 74.3 0.0 Accrued liabilities 0.0 59.9 16.4 51.6 0.0 127.9 Current maturities of long-term debt 0.0 0.1 0.0 11.3 0.0 11.4 Total current liabilities 0.0 34.8 62.6 109.5 74.3 281.2 Long-term debt 0.0 643.3 0.0 22.3 0.0 665.6 Affiliated debt 0.0 233.7 19.3 27.6 (280.6 ) 0.0 Other long-term liabilities 0.0 92.0 14.4 41.2 0.0 147.6 Total liabilities 0.0 1,003.8 96.3 200.6 (206.3 ) 1,094.4 Koppers shareholders’ equity 99.9 100.1 712.2 280.8 (1,093.1 ) 99.9 Noncontrolling interests 0.0 0.0 0.0 5.9 0.0 5.9 Total liabilities and equity $ 99.9 $ 1,103.9 $ 808.5 $ 487.3 $ (1,299.4 ) $ 1,200.2
Condensed Consolidating Statement of Cash Flows	Condensed Consolidating Statement of Cash Flows For the Nine Months Ended September 30, 2018 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) Cash provided by (used in) operating activities $ 29.2 $ (68.3 ) $ 17.1 $ 38.2 $ (8.2 ) $ 8.0 Cash provided by (used in) investing activities: Capital expenditures and acquisitions 0.0 (326.8 ) (3.9 ) (14.7 ) 0.0 (345.4 ) Repayments (loans to) from affiliates 0.0 (4.5 ) (18.1 ) (5.5 ) 28.1 0.0 Insurance proceeds received 0.0 0.0 0.0 1.5 0.0 1.5 Net cash provided by divestitures and asset sales 0.0 (2.0 ) 0.0 4.3 0.0 2.3 Net cash (used in) provided by investing activities 0.0 (333.3 ) (22.0 ) (14.4 ) 28.1 (341.6 ) Cash provided by (used in) financing activities: Borrowings (repayments) of long-term debt 0.0 377.7 (0.1 ) (7.7 ) 0.0 369.9 Borrowings (repayments) of affiliated debt 0.0 34.3 5.1 (11.3 ) (28.1 ) 0.0 Debt issuance costs 0.0 (2.9 ) 0.0 0.0 0.0 (2.9 ) Dividends paid 0.0 (8.2 ) 0.0 0.0 8.2 0.0 Stock repurchased (29.2 ) 0.0 0.0 0.0 0.0 (29.2 ) Net cash provided by (used in) financing activities (29.2 ) 400.9 5.0 (19.0 ) (19.9 ) 337.8 Effect of exchange rates on cash 0.0 0.0 0.0 (2.0 ) 0.0 (2.0 ) Net increase in cash and cash equivalents 0.0 (0.7 ) 0.1 2.8 0.0 2.2 Cash and cash equivalents at beginning of year 0.0 0.7 0.0 59.6 0.0 60.3 Cash and cash equivalents at end of period $ 0.0 $ 0.0 $ 0.1 $ 62.4 $ 0.0 $ 62.5 Condensed Consolidating Statement of Cash Flows For the Nine Months Ended September 30, 2017 Parent Koppers Inc. Domestic Guarantor Subsidiaries Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated (Dollars in millions) Cash provided by (used in) operating activities $ 3.2 $ (15.9 ) $ 33.9 $ 31.0 $ (3.7 ) $ 48.5 Cash provided by (used in) investing activities: Capital expenditures and acquisitions 0.0 (32.0 ) (11.3 ) (5.3 ) 0.0 (48.6 ) Repayments (loans to) from affiliates 0.0 2.0 (19.1 ) 0.6 16.5 0.0 Repayment of loan 0.0 0.0 0.0 9.5 0.0 9.5 Net cash proceeds from divestitures and asset sales 0.0 0.0 0.7 0.4 0.0 1.1 Net cash (used in) provided by investing activities 0.0 (30.0 ) (29.7 ) 5.2 16.5 (38.0 ) Cash provided by (used in) financing activities: Repayments of long-term debt 0.0 36.7 0.0 (4.0 ) 0.0 32.7 Borrowings (repayments) of affiliated debt 0.0 23.9 (4.2 ) (3.2 ) (16.5 ) 0.0 Deferred financing cost 0.0 (11.0 ) 0.0 0.0 0.0 (11.0 ) Dividends paid 0.0 (3.7 ) 0.0 0.0 3.7 0.0 Stock repurchased (3.2 ) 0.0 0.0 0.0 0.0 (3.2 ) Net cash provided by (used in) financing activities (3.2 ) 45.9 (4.2 ) (7.2 ) (12.8 ) 18.5 Effect of exchange rates on cash 0.0 0.0 0.0 0.4 0.0 0.4 Net increase in cash and cash equivalents 0.0 0.0 0.0 29.4 0.0 29.4 Cash and cash equivalents at beginning of year 0.0 0.0 0.0 20.8 0.0 20.8 Cash and cash equivalents at end of period $ 0.0 $ 0.0 $ 0.0 $ 50.2 $ 0.0 $ 50.2

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Schedule of Company's Financial Instruments	Carrying amounts and the related estimated fair values of the Company’s financial instruments as of September 30, 2018 and December 31, 2017 are as follows: September 30, 2018 December 31, 2017 Fair Value Carrying Value Fair Value Carrying Value (Dollars in millions) Financial assets: Cash and cash equivalents, including restricted cash $ 62.5 $ 62.5 $ 60.3 $ 60.3 Investments and other assets (a) 1.1 1.1 1.1 1.1 Financial liabilities: Total debt (including current portion) $ 1,049.1 $ 1,057.7 $ 706.9 $ 688.7 (a) Excludes equity method investments.

New Accounting Pronouncements -

New Accounting Pronouncements - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Summary Of Significant Accounting Policies [Line Items]
Cost of goods sold	$ (359.7)	$ (293.5)	$ (1,023.9)	$ (863)
Selling, general and administrative expenses	$ (37.9)	(33.2)	(121.8)	(95.6)
ASU No. 2017-12 [Member]
Summary Of Significant Accounting Policies [Line Items]
Reclassification of unrealized gain from retained earnings to accumulated other comprehensive loss			$ 3.6
ASU No. 2017-07 [Member] | Scenario, Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Cost of goods sold		0.1		0.6
Selling, general and administrative expenses		$ 0.2		$ 0.7

Plant Closures and Divestitur_3

Plant Closures and Divestitures - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2016	Nov. 30, 2016	Mar. 31, 2016	Jul. 31, 2015
Restructuring Cost and Reserve [Line Items]
Loss on sale of facility		$ 2.2	$ (1.4)
Discontinued Operations Disposed Of By Means Other Than Sale [Member] | China [Member] | Carbon Materials and Chemicals [Member]
Restructuring Cost and Reserve [Line Items]
Percentage of operations in Tangshan, China owned by the company						60.00%
Scenario, Forecast [Member]
Restructuring Cost and Reserve [Line Items]
Loss on sale of facility	$ 6.1
Tangshan Koppers Kailuan Carbon Chemical Company Limited [Member]
Restructuring Cost and Reserve [Line Items]
Ownership percentage				30.00%	30.00%
KSA Limited Partnership [Member]
Restructuring Cost and Reserve [Line Items]
Ownership percentage							50.00%

Plant Closures and Divestitur_4

Plant Closures and Divestitures - Summary of Restructuring Activities and Related Reserves (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Restructuring Cost and Reserve [Line Items]
Reserve, Beginning Balance	$ 18.3	$ 16.1
Accrual	4.7	14.6
Cost charged against assets	(2.1)	(6.3)
Reversal of accrued charges		(2.1)
Cash paid	(9.8)	(4.8)
Currency translation	(0.2)	0.8
Reserve, Ending Balance	10.9	18.3
Environmental Remediation [Member]
Restructuring Cost and Reserve [Line Items]
Reserve, Beginning Balance	2.7	1.5
Accrual	0.9	2.1
Cost charged against assets	0	0
Reversal of accrued charges		0
Cash paid	(3.4)	(1.1)
Currency translation	(0.2)	0.2
Reserve, Ending Balance	0	2.7
Severance and Employee Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Reserve, Beginning Balance	1.7	1.4
Accrual	0	0.9
Cost charged against assets	0	0
Reversal of accrued charges		(0.3)
Cash paid	0	(0.3)
Currency translation	0	0
Reserve, Ending Balance	1.7	1.7
Site Demolition [Member]
Restructuring Cost and Reserve [Line Items]
Reserve, Beginning Balance	10.6	10
Accrual	1.6	4.7
Cost charged against assets	0	0
Reversal of accrued charges		(1.8)
Cash paid	(6.3)	(2.4)
Currency translation	0	0.1
Reserve, Ending Balance	5.9	10.6
Other [Member]
Restructuring Cost and Reserve [Line Items]
Reserve, Beginning Balance	3.3	3.2
Accrual	2.2	6.9
Cost charged against assets	(2.1)	(6.3)
Reversal of accrued charges		0
Cash paid	(0.1)	(1)
Currency translation	0	0.5
Reserve, Ending Balance	$ 3.3	$ 3.3

Acquisitions - Additional Infor

Acquisitions - Additional Information (Detail) $ in Millions	Apr. 10, 2018USD ($)FacilityYard	Feb. 28, 2018USD ($)Facility	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Business Acquisition [Line Items]
Combined costs related to acquisitions			$ 0.6		$ 6.2
Net sales			442.7	$ 384.8	1,284.8	$ 1,109.4
Income (loss) before taxes			(15.5)	$ (24.8)	(55.6)	$ (58)
Koppers Utility and Industrial Products Inc. [Member]
Business Acquisition [Line Items]
Cash consideration	$ 201.2
Number of manufacturing facilities | Facility	8
Number of distribution yards | Yard	19
Revenues							$ 147
Net sales			50.5		97.4
Income (loss) before taxes			0.5		(3.8)
Inventory fair value purchase price accounting adjustments			$ 0.5		$ 6
M.A. Energy Resources, LLC [Member]
Business Acquisition [Line Items]
Cash consideration		$ 62.8
Revenues							$ 30
Number of processing facilities | Facility		2

Acquisitions - Summary of Preli

Acquisitions - Summary of Preliminary Valuation of Identifiable Assets Acquired and Liabilities Assumed Upon Acquisition (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Apr. 10, 2018	Feb. 28, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 296.5			$ 188.2
Koppers Utility and Industrial Products Inc. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		$ 1.5
Accounts receivable		20.2
Inventory		43.1
Other current assets		1.2
Property, plant and equipment		19.8
Intangible assets, net		59.4
Goodwill		76.9
Other non-current assets		0.2
Total assets acquired		222.3
Accounts payable & accrued expenses		18.7
Other non-current liabilities		0.8
Net assets acquired		$ 202.8
M.A. Energy Resources, LLC [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			$ 3.2
Accounts receivable			4.9
Inventory			0
Other current assets			0.3
Property, plant and equipment			6.6
Intangible assets, net			19.3
Goodwill			34
Other non-current assets			0
Total assets acquired			68.3
Accounts payable & accrued expenses			2.3
Other non-current liabilities			0
Net assets acquired			$ 66

Acquisitions - Schedule of Iden

Acquisitions - Schedule of Identifiable Intangible Assets (Detail) - USD ($) $ in Millions	Apr. 10, 2018	Feb. 28, 2018
Koppers Utility and Industrial Products Inc. [Member]
Business Acquisition [Line Items]
Amount	$ 59.4
M.A. Energy Resources, LLC [Member]
Business Acquisition [Line Items]
Amount		$ 19.3
Customer Contracts [Member] | Koppers Utility and Industrial Products Inc. [Member]
Business Acquisition [Line Items]
Amount	$ 58.2
Remaining Life	15 years
Customer Contracts [Member] | M.A. Energy Resources, LLC [Member]
Business Acquisition [Line Items]
Amount		$ 18.7
Remaining Life		15 years
Non-Compete Agreements [Member] | Koppers Utility and Industrial Products Inc. [Member]
Business Acquisition [Line Items]
Amount	$ 1.2
Remaining Life	5 years
Trademarks [Member] | M.A. Energy Resources, LLC [Member]
Business Acquisition [Line Items]
Amount		$ 0.6
Remaining Life		2 years

Acquisitions - Summary of Reven

Acquisitions - Summary of Revenue and Income from Continuing Operations (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Business Combinations [Abstract]
Pro forma revenue	$ 442.7	$ 422.7	$ 1,335.5	$ 1,221.6
Pro forma income from continuing operations attributable to Koppers	$ 7.9	$ 19.9	$ 34.5	$ 42.6
Pro forma income per share - continuing operations, Basic	$ 0.38	$ 0.96	$ 1.64	$ 2.05
Pro forma income per share - continuing operations, Diluted	$ 0.36	$ 0.91	$ 1.58	$ 1.94

Revenue Recognition - Additiona

Revenue Recognition - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Disaggregation Of Revenue [Line Items]
Effect on revenue	$ 442.7	$ 384.8	$ 1,284.8	$ 1,109.4
Accounting Standards Update 2014-09 [Member]
Disaggregation Of Revenue [Line Items]
Cumulative increase to opening balance of retained earnings						$ 0.3
Impact of restatement on revenue not previously recognized						5.3
Impact of restatement on cost of goods sold not previously recognized						$ 5
Effect on revenue			3.2
Accounting Standards Update 2014-09 [Member] | Accounts Receivable, Net
Disaggregation Of Revenue [Line Items]
Contract assets	$ 11.4		$ 11.4		$ 5.3

Revenue Recognition - Schedule

Revenue Recognition - Schedule of Revenue Recognized Net Impact of Adopting Topic 606 Decrease in Sales and Cost of Sales (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation Of Revenue [Line Items]
Sales	$ 442.7	$ 384.8	$ 1,284.8	$ 1,109.4
Cost of sales (excluding items below)	359.7	293.5	1,023.9	863
RUPS [Member]
Disaggregation Of Revenue [Line Items]
Sales	185	$ 131.7	470.6	$ 403.1
Accounting Standards Update 2014-09 [Member]
Disaggregation Of Revenue [Line Items]
Sales			3.2
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 | RUPS [Member]
Disaggregation Of Revenue [Line Items]
Sales	(0.1)		(2.1)
Cost of sales (excluding items below)	$ (0.1)		$ (2)

Comprehensive (Loss) Income a_3

Comprehensive (Loss) Income and Equity - Schedule of Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
Net income	$ 6.9	$ 19.9	$ 31.6	$ 44.3
Other comprehensive (loss) income:
Change in currency translation adjustment	(2.9)	6.3	(18.9)	17.2
Unrealized (loss) gain on cash flow hedges, net of tax (benefit) expense of $(3.7), $1.1, $(12.0) and $1.4	(6.3)	4.1	(20.3)	5
Change in accounting standard	0	0	0.3	0
Unrecognized pension net loss, net of tax expense of $0.1, $0.2, $0.3 and $0.4	0.3	6.5	0.8	7.2
Total comprehensive (loss) income	(2)	36.8	(6.5)	73.7
Less: Comprehensive (loss) income attributable to noncontrolling interests	(1.1)	0.2	4.7	0.6
Comprehensive (loss) income attributable to Koppers	$ (0.9)	$ 36.6	$ (11.2)	$ 73.1

Comprehensive (Loss) Income a_4

Comprehensive (Loss) Income and Equity - Schedule of Comprehensive (Loss) Income (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
Tax (benefit) expense, unrealized gains on cash flow hedges	$ (3.7)	$ 1.1	$ (12)	$ 1.4
Tax expense, unrecognized pension net income	$ 0.1	$ 0.2	$ 0.3	$ 0.4

Comprehensive (Loss) Income a_5

Comprehensive (Loss) Income and Equity - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
Income related to derivative instrument	$ 1.1	$ 2.3	$ 6.5	$ 4.9

Comprehensive (Loss) Income a_6

Comprehensive (Loss) Income and Equity - Schedule of Change in Equity (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			$ 105.8	$ 34.6
Net income	$ 6.9	$ 19.9	31.6	44.3
Issuance of common stock			2.5	1.9
Employee stock plans			9.3	8
Other comprehensive (loss) income			(38.1)	29.4
Repurchases of common stock			(31.7)	(5.1)
Balance	79.4	113.1	79.4	113.1
Total Koppers Shareholders' Equity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			99.9	30.4
Net income			26	43.9
Issuance of common stock			2.5	1.9
Employee stock plans			9.3	8
Other comprehensive (loss) income			(37.2)	29.2
Repurchases of common stock			(31.7)	(5.1)
Balance	68.8	108.3	68.8	108.3
Noncontrolling Interests [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			5.9	4.2
Net income			5.6	0.4
Issuance of common stock			0	0
Employee stock plans			0	0
Other comprehensive (loss) income			(0.9)	0.2
Repurchases of common stock			0	0
Balance	$ 10.6	$ 4.8	$ 10.6	$ 4.8

Earnings per Common Share - Sch

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income attributable to Koppers	$ 7.6	$ 19.8	$ 26	$ 43.9
Less: Income (loss) from discontinued operations	0	(0.1)	0.4	(1.3)
Income from continuing operations attributable to Koppers	$ 7.6	$ 19.9	$ 25.6	$ 45.2
Weighted average common shares outstanding:
Basic	20,946	20,746	20,992	20,750
Effect of dilutive securities	754	1,165	900	1,177
Diluted	21,700	21,911	21,892	21,927
Income per common share – continuing operations:
Basic income per common share	$ 0.36	$ 0.96	$ 1.22	$ 2.17
Diluted income per common share	$ 0.35	$ 0.91	$ 1.17	$ 2.06
Other data:
Antidilutive securities excluded from computation of diluted earnings per common share	571	543	214	482

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Future gross compensation expense related to non-vested stock-based compensation arrangements, which are expected to vest		$ 18.7
Future compensation expense, weighted-average expected period of recognition in months		26 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] | March 2016 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based awards, vesting period			3 years
Vesting performance stock units if minimum performance criteria are not achieved			0
Stock options, term in years			3 years
Vesting Percentage	200.00%
Performance Stock Units [Member] | March 2016 and Thereafter Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based awards, vesting period		3 years
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] | March 2016 Grant [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] | Maximum [Member] | March 2016 and Thereafter Grant [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] | March 2016 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of target award earned by participants			0.00%
Performance Stock Units [Member] | Minimum [Member] | March 2016 and Thereafter Grant [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 | 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

Stock-based Compensation - Perf

Stock-based Compensation - Performance Award Fair Value Assumptions (Detail)	9 Months Ended
	Sep. 30, 2018$ / shares
May 2018 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 39.10
Expected dividend yield per share	0.00%
Expected volatility	37.05%
Risk-free interest rate	2.82%
Grant date fair value per share	$ 15.48
May 2018 Grant [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 39.10
Expected dividend yield per share	0.00%
Expected volatility	39.40%
Risk-free interest rate	2.35%
Look-back period in years	2 years 10 months 2 days
Grant date fair value per share	$ 44.29
March 2018 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 41.60
Expected dividend yield per share	0.00%
Expected volatility	37.05%
Risk-free interest rate	2.67%
Grant date fair value per share	$ 16.38
March 2018 Grant [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 41.60
Expected dividend yield per share	0.00%
Expected volatility	39.40%
Risk-free interest rate	2.35%
Look-back period in years	2 years 10 months 2 days
Grant date fair value per share	$ 47.12
March 2017 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 44.10
Expected dividend yield per share	0.00%
Expected volatility	39.70%
Risk-free interest rate	2.13%
Grant date fair value per share	$ 17.90
March 2017 Grant [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 44.10
Expected dividend yield per share	0.00%
Expected volatility	43.50%
Risk-free interest rate	1.54%
Look-back period in years	2 years 9 months 29 days
Grant date fair value per share	$ 64.02
March 2016 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 18.11
Expected dividend yield per share	0.00%
Expected volatility	40.86%
Risk-free interest rate	1.45%
Grant date fair value per share	$ 7.41
March 2016 Grant [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 18.11
Expected dividend yield per share	0.00%
Expected volatility	40.86%
Risk-free interest rate	0.96%
Look-back period in years	2 years 10 months 2 days
Grant date fair value per share	$ 23.70

Stock-based Compensation - Summ

Stock-based Compensation - Summary of Performance Stock Units (Detail) - shares	Sep. 30, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	754,383	819,691
Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	499,629	581,551
Minimum [Member] | 2016 - 2018 [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	0
Minimum [Member] | 2017 - 2019 [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	0
Minimum [Member] | 2018 - 2020 [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	0
Target Shares [Member] | 2016 - 2018 [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	255,172
Target Shares [Member] | 2017 - 2019 [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	113,675
Target Shares [Member] | 2018 - 2020 [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	130,782
Maximum [Member] | 2016 - 2018 [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	510,344
Maximum [Member] | 2017 - 2019 [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	227,350
Maximum [Member] | 2018 - 2020 [Member] | Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding	261,564

Stock-based Compensation - Su_2

Stock-based Compensation - Summary of Status and Activity of Non-Vested Stock Units (Detail)	9 Months Ended
	Sep. 30, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested, Beginning Balance	819,691
Granted	252,648
Performance share adjustment	204,866
Vested	(504,321)
Forfeited	(18,501)
Non-vested, Ending Balance	754,383
Beginning Balance, Non-vested, Weighted Average Grant Date Fair Value per Unit | $ / shares	$ 29.14
Granted, Weighted Average Grant Date Fair Value per Unit | $ / shares	43.91
Performance share adjustment | $ / shares	17.57
Vested, Weighted Average Grant Date Fair Value per Unit | $ / shares	19.41
Forfeited, Weighted Average Grant Date Fair Value per Unit | $ / shares	38.77
Ending Balance, Non-vested, Weighted Average Grant Date Fair Value per Unit | $ / shares	$ 37.20
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested, Beginning Balance	238,140
Granted	118,297
Performance share adjustment	0
Vested	(94,589)
Forfeited	(7,094)
Non-vested, Ending Balance	254,754
Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested, Beginning Balance	581,551
Granted	134,351
Performance share adjustment	204,866
Vested	(409,732)
Forfeited	(11,407)
Non-vested, Ending Balance	499,629

Stock-based Compensation - Stoc

Stock-based Compensation - Stock Options Fair Value Assumptions (Detail)	9 Months Ended
	Sep. 30, 2018$ / shares
May 2018 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 39.10
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	2.82%
Grant date fair value per share	$ 15.48
March 2018 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 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	$ 16.38
March 2017 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 44.10
Expected dividend yield per share	0.00%
Expected life in years	5 years 9 months 7 days
Expected volatility	39.70%
Risk-free interest rate	2.13%
Grant date fair value per share	$ 17.90
March 2016 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 18.11
Expected dividend yield per share	0.00%
Expected life in years	5 years 11 months 15 days
Expected volatility	40.86%
Risk-free interest rate	1.45%
Grant date fair value per share	$ 7.41
March 2015 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date price per share	$ 17.57
Expected dividend yield per share	3.40%
Expected life in years	5 years 9 months
Expected volatility	42.27%
Risk-free interest rate	1.73%
Grant date fair value per share	$ 5.20

Stock-based Compensation - Su_3

Stock-based Compensation - Summary of Status and Activity of Stock Options (Detail) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options, Outstanding at December 31, 2017 | shares	942,537
Options, Granted | shares	144,825
Options, Exercised | shares	(70,307)
Options, Forfeited | shares	(12,329)
Options, Outstanding at June 30, 2018 | shares	1,004,726
Options, Exercisable at June 30, 2018 | shares	617,361
Weighted Average Exercise Price per Option, Outstanding at December 31, 2017 | $ / shares	$ 27.59
Weighted Average Exercise Price per Option, Granted | $ / shares	41.50
Weighted Average Exercise Price per Option, Exercised | $ / shares	27.75
Weighted Average Exercise Price per Option, Forfeited | $ / shares	30.49
Weighted Average Exercise Price per Option, Outstanding at June 30, 2018 | $ / shares	29.54
Weighted Average Exercise Price per Option, Exercisable at June 30, 2018 | $ / shares	$ 28.49
Weighted Average Remaining Contractual Term, Outstanding at June 30, 2018	6 years 1 month 20 days
Weighted Average Remaining Contractual Term, Exercisable at June 30, 2018	4 years 10 months 20 days
Aggregate Intrinsic Value, Outstanding at June 30, 2018 | $	$ 6.6
Aggregate Intrinsic Value, Exercisable at June 30, 2018 | $	$ 4.2

Stock-based Compensation - Sche

Stock-based Compensation - Schedule of Stock-Based Compensation Expense Recognized (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Stock-based compensation expense recognized:
Less related income tax benefit	$ 1.2	$ 0.6	$ 3.5	$ 1.7
Decrease in net income attributable to Koppers	2	2.1	5.8	6.1
Intrinsic value of exercised stock options	0.1	0	1.1	1.1
Cash received from the exercise of stock options	0.3	0	2.5	1.9
Selling, General and Administrative Expenses [Member]
Stock-based compensation expense recognized:
Stock-based compensation expense	$ 3.2	$ 2.7	$ 9.3	$ 7.8

Segment Information - Additiona

Segment Information - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018Segment
Segment Reporting [Abstract]
Number of reportable segments	3

Segment Information - Summary o

Segment Information - Summary of Results of Segment Operations (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues from external customers:
Net sales		$ 442.7	$ 384.8	$ 1,284.8	$ 1,109.4
Depreciation and amortization expense		13	12.1	38.5	35
Operating profit (loss)		31.2	35	96.8	101.2
Railroad and Utility Products and Services [Member]
Revenues from external customers:
Net sales		185	131.7	470.6	403.1
Performance Chemicals [Member]
Revenues from external customers:
Net sales		108.2	109.7	320.7	318.2
Carbon Materials and Chemicals [Member]
Revenues from external customers:
Net sales		149.5	143.4	493.5	388.1
Operating Segments [Member] | Railroad and Utility Products and Services [Member]
Revenues from external customers:
Net sales		185	131.7	470.6	403.1
Depreciation and amortization expense		4.9	2.9	12.8	8.8
Operating profit (loss)	[1]	5.8	9.5	5.9	30.1
Operating Segments [Member] | Performance Chemicals [Member]
Revenues from external customers:
Net sales		108.2	109.7	320.7	318.2
Depreciation and amortization expense		4.4	4.4	13.3	13.3
Operating profit (loss)		11	18.4	28.2	56.6
Operating Segments [Member] | Carbon Materials and Chemicals [Member]
Revenues from external customers:
Net sales		149.5	143.4	493.5	388.1
Depreciation and amortization expense		3.7	4.8	12.4	12.9
Operating profit (loss)		14.9	16.3	64.6	24.9
Intersegment [Member]
Revenues from external customers:
Net sales		25.2	23.9	66.8	65
Intersegment [Member] | Performance Chemicals [Member]
Revenues from external customers:
Net sales		3.8	1.8	8	5
Intersegment [Member] | Carbon Materials and Chemicals [Member]
Revenues from external customers:
Net sales		21.4	22.1	58.8	60
Corporate, Non-Segment [Member]
Revenues from external customers:
Operating profit (loss)	[2]	$ (0.5)	$ (9.2)	$ (1.9)	$ (10.4)
[1]	Operating profit for the three and nine months ended September 30, 2018 includes $0.5 million and $6.0 million of inventory fair value purchase price adjustment related to the Company’s UIP acquisition, respectively.
[2]	Operating loss for Corporate includes primarily general and administrative costs for Koppers Holdings Inc., the parent company of Koppers Inc.

Segment Information - Summary_2

Segment Information - Summary of Results of Segment Operations (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues from external customers:
Operating profit (loss)	$ 31.2	$ 35	$ 96.8	$ 101.2
Koppers Utility and Industrial Products [Member] | Inventory Fair Value Purchase Price Adjustment [Member]
Revenues from external customers:
Operating profit (loss)	$ 0.5		$ 6

Segment Information - Schedule

Segment Information - Schedule of Segment Revenues for Significant Product Lines (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net sales	$ 442.7	$ 384.8	$ 1,284.8	$ 1,109.4
Railroad and Utility Products and Services [Member]
Segment Reporting Information [Line Items]
Net sales	185	131.7	470.6	403.1
Railroad and Utility Products and Services [Member] | Railroad Treated Products [Member]
Segment Reporting Information [Line Items]
Net sales	96.7	96.1	259.8	299.8
Railroad and Utility Products and Services [Member] | Utility Poles [Member]
Segment Reporting Information [Line Items]
Net sales	59.5	12.9	129.9	34
Railroad and Utility Products and Services [Member] | Rail Joints [Member]
Segment Reporting Information [Line Items]
Net sales	8.5	7.1	26.5	22.1
Railroad and Utility Products and Services [Member] | Railroad Infrastructure Services [Member]
Segment Reporting Information [Line Items]
Net sales	10	9	26.2	27
Railroad and Utility Products and Services [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Net sales	10.3	6.6	28.2	20.2
Performance Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales	108.2	109.7	320.7	318.2
Performance Chemicals [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Net sales	6.7	8	21.7	22.7
Performance Chemicals [Member] | Wood Preservative Products [Member]
Segment Reporting Information [Line Items]
Net sales	101.5	101.7	299	295.5
Carbon Materials and Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales	149.5	143.4	493.5	388.1
Carbon Materials and Chemicals [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Net sales	9.3	8.8	32.6	34.6
Carbon Materials and Chemicals [Member] | Pitch and Related Products [Member]
Segment Reporting Information [Line Items]
Net sales	88.9	78.6	304.1	195.4
Carbon Materials and Chemicals [Member] | Phthalic Anhydride and Other Chemicals [Member]
Segment Reporting Information [Line Items]
Net sales	23.3	24.5	64	69.3
Carbon Materials and Chemicals [Member] | Creosote and Distillates [Member]
Segment Reporting Information [Line Items]
Net sales	19.2	21.6	61.3	61
Carbon Materials and Chemicals [Member] | Naphthalene [Member]
Segment Reporting Information [Line Items]
Net sales	$ 8.8	$ 9.9	$ 31.5	$ 27.8

Segment Information - Summary_3

Segment Information - Summary of Tangible and Intangible Assets by Segments (Detail) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Asset Reconciling Item [Line Items]
Assets		$ 1,535.6	$ 1,200.2
Goodwill		296.5	188.2
Railroad and Utility Products and Services [Member] | Operating Segments [Member]
Segment Reporting Asset Reconciling Item [Line Items]
Assets	[1]	556.3	249.7
Goodwill	[1]	120.8	10.5
Performance Chemicals [Member] | Operating Segments [Member]
Segment Reporting Asset Reconciling Item [Line Items]
Assets		472	494
Goodwill		175.7	177.7
Carbon Materials and Chemicals [Member] | Operating Segments [Member]
Segment Reporting Asset Reconciling Item [Line Items]
Assets		469.3	414.2
All Other [Member] | Operating Segments [Member]
Segment Reporting Asset Reconciling Item [Line Items]
Assets		$ 38	$ 42.3
[1]	The increase in RUPS assets and goodwill as of September 30, 2018 includes $279.3 million and $110.9 million, respectively, attributable to the Company’s acquisitions of UIP and MAER during 2018.

Segment Information - Summary_4

Segment Information - Summary of Tangible and Intangible Assets by Segments ((Parenthetical)) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Asset Reconciling Item [Line Items]
Assets	$ 1,535.6	$ 1,200.2
Goodwill	296.5	$ 188.2
Koppers Utility and Industrial Products And M.A. Energy Resources, LLC [Member] | Railroad and Utility Products and Services [Member]
Segment Reporting Asset Reconciling Item [Line Items]
Assets	279.3
Goodwill	$ 110.9

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Corporate income tax rate			21.00%	35.00%	35.00%
Tax cuts and jobs act of 2017 one time transition provisional income tax expense			$ 21.2
Provisional charge recorded to income tax expense related to one-time transition tax on unrepatriated foreign earnings					$ 13.1
Tax cuts and jobs act of 2017 increase in transition tax partially offset by revisions to carrying value of deferred tax assets and liabilities			3.8
Tax cuts and jobs act of 2017 one time transition tax cash payments			$ 5
Estimated annual effective income tax rate			37.10%	21.70%
Income taxes as a percentage of pretax income	55.50%	19.40%	43.90%	21.40%
Net tax benefit included in income taxes, discrete items	$ 1		$ 3.8	$ (0.2)
Impact of tax act domestic and foreign tax returns, discrete items			6.1
Excess tax benefits from stock based compensation	2.3	$ 0.9	2.3	0.9
Reversal of uncertain tax positions			0.5	0.5
Additional accruals for uncertain tax positions		$ 0.8		0.8
Adjustment to deferred tax assets				$ 0.4
Unrecognized tax benefits with impact on the effective tax rate	4.2		4.2		4.4
Unrecognized tax benefits	8.5		8.5		8.7
Accrued interest expense and penalties	$ 4.3		4.3		$ 3.6
Decrease in unrecognized tax benefits			$ 2

Income Taxes - Summary of Incom

Income Taxes - Summary of Income Taxes Reconciled with Federal Statutory Rate (Detail)	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal income tax rate	21.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	(0.80%)	1.10%
Foreign earnings taxed at different rates	1.80%	(16.80%)
GILTI inclusion (net of foreign tax credits)	16.70%	0.00%
Valuation allowance adjustments	(11.50%)	0.00%
Interest expense deduction limitation	7.70%	0.00%
Nondeductible expenses	2.60%	1.60%
Change in tax contingency reserves	0.20%	0.20%
Other	(0.60%)	0.60%
Estimated annual effective income tax rate	37.10%	21.70%

Inventories - Components of Net

Inventories - Components of Net Inventories (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials	$ 203.4	$ 173.6
Work in process	14.8	11.2
Finished goods	123.9	98.4
Inventories, gross	342.1	283.2
Less revaluation to LIFO	52.8	46.3
Net	$ 289.3	$ 236.9

Property, Plant and Equipment -

Property, Plant and Equipment - Property, Plant and Equipment (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 914.1	$ 837.6
Less accumulated depreciation	515.7	509.6
Net	398.4	328
Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	17	17.6
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	61.9	63.4
Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 835.2	$ 756.6

Property, Plant and Equipment_2

Property, Plant and Equipment - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Property Plant And Equipment [Abstract]
Impairment charges	$ 0	$ 0

Pensions and Post-Retirement _3

Pensions and Post-Retirement Benefit Plans - Additional Information (Detail)	Sep. 30, 2018Plan
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 _4

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Postemployment Benefits [Abstract]
Service cost	$ 0.5	$ 0.5	$ 1.4	$ 1.5
Interest cost	1.8	2.1	5.6	7
Expected return on plan assets	(2.1)	(2.4)	(6.4)	(7.4)
Amortization of net loss	0.4	0.5	1.1	1.5
Settlements	0	8.8	0	8.8
Net periodic benefit cost	0.6	9.5	1.7	11.4
Defined contribution plan expense	$ 1.3	$ 2.1	$ 5.5	$ 6.1

Debt - Schedule of Long-Term De

Debt - Schedule of Long-Term Debt Instruments (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Debt	$ 1,057.7	$ 688.7
Less short-term debt and current maturities of long-term debt	15.1	11.4
Less unamortized debt issuance costs	12.8	11.7
Long-term debt	$ 1,029.8	665.6
Term Loan [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.16%
Debt, Maturity	2,023
Debt	$ 95	0
Senior Secured Credit Facilities [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.16%
Debt, Maturity	2,023
Debt	$ 437.8	155
Committed Loan Facility Agreements [Member] | Construction Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.69%
Debt, Maturity	2,020
Debt	$ 24.9	33.7
6.00 percent Senior Notes due 2025 [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	6.00%
Debt, Maturity	2,025
Debt	$ 500	$ 500

Debt - Additional Information (

Debt - Additional Information (Detail)	Feb. 15, 2022	Apr. 10, 2018USD ($)	Feb. 28, 2017USD ($)	Sep. 30, 2018USD ($)Installment	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)Installment	Sep. 30, 2017USD ($)
Debt Instrument [Line Items]
Loss on extinguishment of debt			$ 10,000,000	$ 0	$ 0	$ 0	$ 13,300,000
Bond premium and bond tender expenses			7,300,000
Write-off unamortized debt issuance cost			2,700,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Revolving credit facility		$ 600,000,000
Revolving credit facility, extended maturity period		1 year
Revolving credit facility, extended maturity date		2023-04
Revolving credit facility, variable interest rate basis		LIBOR
Initial average borrowing rate		5.00%
Write-off unamortized debt issuance cost			$ 3,300,000
Secured Term Loan Facility [Member]
Debt Instrument [Line Items]
Secured term loan		$ 100,000,000
Quarterly amortization		$ 2,500,000
Secured term loan maturity period		5 years
Initial average borrowing rate		5.00%
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 beginning on August 15, 2017 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 Credit Facilities [Member]
Debt Instrument [Line Items]
Letters of credit, amount outstanding				32,000,000		$ 32,000,000
Senior Secured Credit Facilities [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt instrument, unused borrowing capacity				$ 131,300,000		$ 131,300,000
Committed Loan Facility Agreements [Member] | Construction Loans [Member]
Debt Instrument [Line Items]
Debt maturity date						Dec. 21, 2020
Number of installments for repayment of loans | Installment				6		6
Installment frequency for repayment of loans						6 months
Commencement period of debt repayment						2018-06
Committed Loan Facility Agreements [Member] | Construction Loans [Member] | Koppers Carbon and Chemical Company Limited [Member]
Debt Instrument [Line Items]
Debt maturity date						Nov. 30, 2018
Percentage ownership of subsidiary						75.00%
Construction loan facility borrowing capacity				$ 22,200,000		$ 22,200,000
Percentage of non-controlling shareholders				25.00%		25.00%

Asset Retirement Obligations -

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, 2018	Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]
Asset retirement obligation at beginning of year	$ 37.1	$ 36
Accretion expense	1.3	2.4
Revision in estimated cash flows	1.4	9.4
Cash expenditures	(10.4)	(10.9)
Currency translation	0	0.2
Balance at end of period	$ 29.4	$ 37.1

Deferred Revenue - Additional I

Deferred Revenue - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2018	Dec. 31, 2017
Disaggregation Of Revenue [Line Items]
Deferred revenue		$ 25.2	$ 27.2
Koppers Carbon and Chemical Company Limited [Member]
Disaggregation Of Revenue [Line Items]
Deferred Revenue Advance Payment	$ 30

Deferred Revenue - Summary of C

Deferred Revenue - Summary of Changes in Carrying Values of Deferred Revenue (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Deferred Revenue Disclosure [Abstract]
Deferred revenue at beginning of year	$ 28.1	$ 27.2
Revenue earned	(0.5)	(0.7)
Currency translation	(1.6)	1.6
Deferred revenue at end of year	$ 26	$ 28.1

Derivative Financial Instrume_3

Derivative Financial Instruments - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Maximum period hedged in cash flow hedge	36 months
Unrealized losses, reclassification period	12 months
Unrealized losses, net of tax benefit, expected to be reclassified from other comprehensive loss into earnings	$ (0.1)

Derivative Financial Instrume_4

Derivative Financial Instruments - Schedule of Outstanding Copper Swap Contracts (Detail) - Copper Swap Contracts [Member] £ in Millions, $ in Millions	Sep. 30, 2018USD ($)	Sep. 30, 2018GBP (£)	Dec. 31, 2017USD ($)	Dec. 31, 2017GBP (£)
Derivatives Fair Value [Line Items]
Copper Swap Contracts Units Outstanding | £		£ 56.5		£ 49.1
Net Fair Value - (Liability) Asset | $	$ (1.2)		$ 30
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivatives Fair Value [Line Items]
Copper Swap Contracts Units Outstanding | £		39.6		37.8
Net Fair Value - (Liability) Asset | $	(0.2)		25.5
Not Designated as Hedging Instrument
Derivatives Fair Value [Line Items]
Copper Swap Contracts Units Outstanding | £		£ 16.9		£ 11.3
Net Fair Value - (Liability) Asset | $	$ (1)		$ 4.5

Derivative Financial Instrume_5

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, 2018	Dec. 31, 2017
Derivative [Line Items]
Net Fair Value - Asset (Liability)	$ (1.2)	$ 30
Accumulated other comprehensive gain (loss), net of tax	(0.3)	15.8
Other Current Assets [Member]
Derivative [Line Items]
Net Fair Value - Asset (Liability)	1.2	21.8
Other Assets [Member]
Derivative [Line Items]
Net Fair Value - Asset (Liability)	0.5	8.2
Accrued Liabilities [Member]
Derivative [Line Items]
Net Fair Value - Asset (Liability)	(2.6)	0
Other Long-term Liabilities [Member]
Derivative [Line Items]
Net Fair Value - Asset (Liability)	$ (0.3)	$ 0

Derivative Financial Instrume_6

Derivative Financial Instruments - Schedule of Amounts Recognized In Earnings Related To Copper Swap Contracts (Detail) - Copper Swap Contracts [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
(Loss) gain from contracts not designated as hedges	$ (1)	$ 1	$ (5.5)	$ 2
Net	(1)	(1.5)	(5.5)	2.5
Cash Flow Hedging [Member]
Derivative [Line Items]
(Loss) gain from ineffectiveness of cash flow hedges	$ 0	$ (2.5)	$ 0	$ 0.5

Derivative Financial Instrume_7

Derivative Financial Instruments - Schedule of Fair Value of Outstanding Foreign Currency Forward Contracts (Detail) - Forward Contracts [Member] - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Not designated net fair value - asset (liability)	$ (0.4)	$ 0.1
Other Current Assets [Member]
Derivative [Line Items]
Not designated gross derivative assets	0.7	0.3
Accrued Liabilities [Member]
Derivative [Line Items]
Not designated gross derivative liability	$ (1.1)	$ (0.2)

Derivative Financial Instrume_8

Derivative Financial Instruments - Summary of Net Currency Units Outstanding (Detail)	Sep. 30, 2018USD ($)	Sep. 30, 2018GBP (£)	Sep. 30, 2018NZD ($)	Sep. 30, 2018CAD ($)	Sep. 30, 2018EUR (€)	Dec. 31, 2017USD ($)	Dec. 31, 2017GBP (£)	Dec. 31, 2017NZD ($)	Dec. 31, 2017CAD ($)	Dec. 31, 2017EUR (€)
Offsetting [Abstract]
Net currency units outstanding	$ 4,600,000	£ 5,700,000	$ 16,000,000	$ 2,500,000	€ 1,300,000	$ 12,500,000	£ 7,000,000	$ 15,500,000	$ 2,500,000	€ 0

Commitments and Contingent Li_3

Commitments and Contingent Liabilities - Additional Information (Detail)	1 Months Ended	9 Months Ended
	Jul. 31, 2004USD ($)	Sep. 30, 2018USD ($)StatePlaintiffCasesitePartyPlant	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Loss Contingencies [Line Items]
Number of states with new claims filed | State		2
Number of plaintiffs | Plaintiff		87
Environmental remediation and regulation liability		$ 12,700,000	$ 13,900,000	$ 12,900,000
Accrued liability for environmental matters, current		3,700,000	$ 5,100,000
Portland Harbor and Newark Bay CERCLA sites [Member]
Loss Contingencies [Line Items]
Costs of participating in PRP group		$ 2,100,000
Beazer East [Member]
Loss Contingencies [Line Items]
Environmental legal indemnification expense	$ 7,000,000
Sites listed on National Priorities List | site		1
Compensatory Damages [Member]
Loss Contingencies [Line Items]
Number of cases | Case		48
Punitive Damages [Member]
Loss Contingencies [Line Items]
Number of cases | Case		43
Indemnification Agreement [Member] | Beazer East [Member]
Loss Contingencies [Line Items]
Environmental remediation costs paid by others, per year		$ 10,000,000
Coal Tar Pitch Cases [Member]
Loss Contingencies [Line Items]
Number of cases | Case		48
Reserve for legal proceedings		$ 0
Pennsylvania [Member]
Loss Contingencies [Line Items]
Number of cases | Case		47
Pennsylvania [Member] | Coal Tar Pitch Cases [Member]
Loss Contingencies [Line Items]
Number of cases | Case		47
Tennessee [Member] | Coal Tar Pitch Cases [Member]
Loss Contingencies [Line Items]
Number of cases | Case		1
Compensatory damages		$ 15,000,000
Oregon [Member] | Portland Harbor CERCLA site [Member]
Loss Contingencies [Line Items]
Number of potential responsible parties | Party		80
Net present value of selected remedy estimation		$ 1,100,000,000
Undiscounted cost of selected remedy estimation		1,700,000,000
United States [Member] | Performance Chemicals [Member]
Loss Contingencies [Line Items]
Environmental remediation and regulation liability		$ 4,900,000
Number of plant sites | Plant		2
United States [Member] | Utility and Industrial Products [Member]
Loss Contingencies [Line Items]
Environmental remediation and regulation liability		$ 1,900,000
Number of plant sites | Plant		1
United Kingdom And Australia [Member] | Performance Chemicals [Member]
Loss Contingencies [Line Items]
Environmental remediation and regulation liability		$ 2,500,000
Number of plant sites | Plant		2

Commitments and Contingent Li_4

Commitments and Contingent Liabilities - Changes in Accrued Liability for Environmental Matters (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Balance at beginning of year	$ 13.9	$ 12.9
Expense	1	3.2
Reversal of reserves	(0.1)	(0.7)
Cash expenditures	(3.8)	(1.8)
Acquisition	1.9	0
Currency translation	(0.2)	0.3
Balance at end of period	$ 12.7	$ 13.9

Subsidiary Guarantor Informat_3

Subsidiary Guarantor Information for Koppers Inc. Shelf Registration - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Koppers Inc. [Member]
Debt Instrument [Line Items]
Amount of restricted net assets unavailable for distribution	$ 38.5
Domestic Guarantor Subsidiaries [Member]
Debt Instrument [Line Items]
Cash dividends paid	8.2	$ 3.7
Senior Notes [Member] | Koppers Inc. [Member]
Debt Instrument [Line Items]
Amount of assets exceeded liabilities	$ 93

Subsidiary Guarantor Informat_4

Subsidiary Guarantor Information for Koppers Inc. Shelf Registration - Condensed Consolidating Statement of Operations (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 28, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Condensed Statement Of Income Captions [Line Items]
Net sales		$ 442.7	$ 384.8	$ 1,284.8	$ 1,109.4
Cost of sales including depreciation and amortization		373.6	307.8	1,066.2	903.8
Loss on pension settlement		0	8.8	0	8.8
Selling, general and administrative		37.9	33.2	121.8	95.6
Operating profit		31.2	35	96.8	101.2
Other (loss) income		(0.6)	0.3	(1.1)	2
Equity income (loss) of subsidiaries		0	0	0	0
Interest expense		15.1	10.5	40.1	31.9
Loss on extinguishment of debt	$ 10	0	0	0	13.3
Income taxes		8.6	4.8	24.4	12.4
Income from continuing operations		6.9	20	31.2	45.6
Discontinued operations		0	(0.1)	0.4	(1.3)
Noncontrolling interests		(0.7)	0.1	5.6	0.4
Net income attributable to Koppers		7.6	19.8	26	43.9
Comprehensive income (loss) attributable to Koppers		(0.9)	36.6	(11.2)	73.1
Consolidation Eliminations [Member]
Condensed Statement Of Income Captions [Line Items]
Net sales		(24.6)	(21.2)	(61.7)	(64.3)
Cost of sales including depreciation and amortization		(24.3)	(21.2)	(62.6)	(62.6)
Loss on pension settlement			0		0
Selling, general and administrative		0	0	0	0
Operating profit		(0.3)	0	0.9	(1.7)
Other (loss) income		0	(0.3)	0	(0.8)
Equity income (loss) of subsidiaries		(38.5)	(66.4)	(166.2)	(173.3)
Interest expense		0	(0.4)	0	(0.8)
Loss on extinguishment of debt					0
Income taxes		0	0	0	0
Income from continuing operations		(38.8)	(66.3)	(165.3)	(175)
Discontinued operations			0	0	0
Noncontrolling interests		0	0	0	0
Net income attributable to Koppers		(38.8)	(66.3)	(165.3)	(175)
Comprehensive income (loss) attributable to Koppers		(19.2)	(99.9)	(71.2)	(245.1)
Parent [Member] | Reportable Legal Entities [Member]
Condensed Statement Of Income Captions [Line Items]
Net sales		0	0	0	0
Cost of sales including depreciation and amortization		0	0	0	0
Loss on pension settlement			0		0
Selling, general and administrative		0.5	0.5	1.9	1.5
Operating profit		(0.5)	(0.5)	(1.9)	(1.5)
Other (loss) income		0	0	0	0
Equity income (loss) of subsidiaries		8	19.9	27.5	44.8
Interest expense		0	(0.1)	0	(0.1)
Loss on extinguishment of debt					0
Income taxes		(0.1)	(0.3)	(0.4)	(0.5)
Income from continuing operations		7.6	19.8	26	43.9
Discontinued operations			0	0	0
Noncontrolling interests		0	0	0	0
Net income attributable to Koppers		7.6	19.8	26	43.9
Comprehensive income (loss) attributable to Koppers		(0.9)	36.6	(11.2)	73.1
Koppers Inc. [Member] | Reportable Legal Entities [Member]
Condensed Statement Of Income Captions [Line Items]
Net sales		158.3	151.1	429.6	455.2
Cost of sales including depreciation and amortization		145.3	136.2	397.5	425.7
Loss on pension settlement			8.8		8.8
Selling, general and administrative		12	12.1	39.3	33.8
Operating profit		1	(6)	(7.2)	(13.1)
Other (loss) income		(0.4)	(0.1)	(3.2)	(0.9)
Equity income (loss) of subsidiaries		15.6	29.4	74	81.2
Interest expense		14.4	9.8	38.2	29.5
Loss on extinguishment of debt					13.3
Income taxes		(6.2)	(6.4)	(2.1)	(20.4)
Income from continuing operations		8	19.9	27.5	44.8
Discontinued operations			0	0	0
Noncontrolling interests		0	0	0	0
Net income attributable to Koppers		8	19.9	27.5	44.8
Comprehensive income (loss) attributable to Koppers		(0.3)	36.8	(9.7)	74.2
Domestic Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Statement Of Income Captions [Line Items]
Net sales		92.1	92	265.5	267.4
Cost of sales including depreciation and amortization		71.5	64.6	205.9	183.7
Loss on pension settlement			0		0
Selling, general and administrative		9.9	10.2	33.8	30.5
Operating profit		10.7	17.2	25.8	53.2
Other (loss) income		0.5	0.6	1.3	1.8
Equity income (loss) of subsidiaries		14.8	17	64.7	47.3
Interest expense		0.1	0.1	0.1	0.1
Loss on extinguishment of debt					0
Income taxes		10.6	6.5	14.2	20.2
Income from continuing operations		15.3	28.2	77.5	82
Discontinued operations			0	0	0
Noncontrolling interests		0	0	0	0
Net income attributable to Koppers		15.3	28.2	77.5	82
Comprehensive income (loss) attributable to Koppers		6.5	38.7	39.2	104.5
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Statement Of Income Captions [Line Items]
Net sales		216.9	162.9	651.4	451.1
Cost of sales including depreciation and amortization		181.1	128.2	525.4	357
Loss on pension settlement			0		0
Selling, general and administrative		15.5	10.4	46.8	29.8
Operating profit		20.3	24.3	79.2	64.3
Other (loss) income		(0.7)	0.1	0.8	1.9
Equity income (loss) of subsidiaries		0.1	0.1	0	0
Interest expense		0.6	1.1	1.8	3.2
Loss on extinguishment of debt					0
Income taxes		4.3	5	12.7	13.1
Income from continuing operations		14.8	18.4	65.5	49.9
Discontinued operations			(0.1)	0.4	(1.3)
Noncontrolling interests		(0.7)	0.1	5.6	0.4
Net income attributable to Koppers		15.5	18.2	60.3	48.2
Comprehensive income (loss) attributable to Koppers		$ 13	$ 24.4	$ 41.7	$ 66.4

Subsidiary Guarantor Informat_5

Subsidiary Guarantor Information for Koppers Inc. Shelf Registration - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Assets
Cash and cash equivalents	$ 62.5	$ 60.3	$ 50.2	$ 20.8
Receivables, net	222.3	160.9
Affiliated receivables	0	0
Inventories, net	289.3	236.9
Deferred tax assets		0
Other current assets	31.5	48.6
Total current assets	605.6	506.7
Equity investments	0	0
Property, plant and equipment, net	398.4	328
Goodwill	296.5	188.2
Intangible assets, net	193.5	129.6
Deferred tax assets	18.4	18.4
Affiliated loan receivables	0	0
Other assets	23.2	29.3
Total assets	1,535.6	1,200.2
LIABILITIES AND EQUITY
Accounts payable	166.2	141.9
Affiliated payables	0	0
Accrued liabilities	101.7	127.9
Current maturities of long-term debt	15.1	11.4
Total current liabilities	283	281.2
Long-term debt	1,029.8	665.6
Affiliated debt	0	0
Other long-term liabilities	143.4	147.6
Total liabilities	1,456.2	1,094.4
Koppers shareholders’ equity	68.8	99.9
Noncontrolling interests	10.6	5.9
Total liabilities and equity	1,535.6	1,200.2
Consolidation Eliminations [Member]
Assets
Cash and cash equivalents	0	0	0	0
Receivables, net	0	0
Affiliated receivables	(38.8)	75.1
Inventories, net	(1)	(1.9)
Deferred tax assets		0.1
Other current assets	0.3	0.2
Total current assets	(39.5)	73.5
Equity investments	(1,435.3)	(1,092.3)
Property, plant and equipment, net	0	0
Goodwill	0	0
Intangible assets, net	0	0
Deferred tax assets	0	0
Affiliated loan receivables	(214.1)	(280.6)
Other assets	0	0
Total assets	(1,688.9)	(1,299.4)
LIABILITIES AND EQUITY
Accounts payable	0	0
Affiliated payables	(38.9)	74.3
Accrued liabilities	0	0
Current maturities of long-term debt	0	0
Total current liabilities	(38.9)	74.3
Long-term debt	0	0
Affiliated debt	(214.1)	(280.6)
Other long-term liabilities	0	0
Total liabilities	(253)	(206.3)
Koppers shareholders’ equity	(1,435.9)	(1,093.1)
Noncontrolling interests	0	0
Total liabilities and equity	(1,688.9)	(1,299.4)
Parent [Member] | Reportable Legal Entities [Member]
Assets
Cash and cash equivalents	0	0	0	0
Receivables, net	0	0
Affiliated receivables	(22.7)	0.6
Inventories, net	0	0
Deferred tax assets		0
Other current assets	0	0
Total current assets	(22.7)	0.6
Equity investments	91.5	99.3
Property, plant and equipment, net	0	0
Goodwill	0	0
Intangible assets, net	0	0
Deferred tax assets	0	0
Affiliated loan receivables	0	0
Other assets	0	0
Total assets	68.8	99.9
LIABILITIES AND EQUITY
Accounts payable	0	0
Affiliated payables	0	0
Accrued liabilities	0	0
Current maturities of long-term debt	0	0
Total current liabilities	0	0
Long-term debt	0	0
Affiliated debt	0	0
Other long-term liabilities	0	0
Total liabilities	0	0
Koppers shareholders’ equity	68.8	99.9
Noncontrolling interests	0	0
Total liabilities and equity	68.8	99.9
Koppers Inc. [Member] | Reportable Legal Entities [Member]
Assets
Cash and cash equivalents	0	0.7	0	0
Receivables, net	70.7	48.6
Affiliated receivables	39.4	19.4
Inventories, net	98.3	80.4
Deferred tax assets		0
Other current assets	7	6.6
Total current assets	215.4	155.7
Equity investments	1,020.8	716.3
Property, plant and equipment, net	203.6	155.2
Goodwill	0.8	0.8
Intangible assets, net	6.7	7.2
Deferred tax assets	23.6	29.4
Affiliated loan receivables	45.5	34.9
Other assets	4.6	4.4
Total assets	1,521	1,103.9
LIABILITIES AND EQUITY
Accounts payable	75.8	65.1
Affiliated payables	23.7	(90.3)
Accrued liabilities	50.1	59.9
Current maturities of long-term debt	10	0.1
Total current liabilities	159.6	34.8
Long-term debt	1,010	643.3
Affiliated debt	168.6	233.7
Other long-term liabilities	89.8	92
Total liabilities	1,428	1,003.8
Koppers shareholders’ equity	93	100.1
Noncontrolling interests	0	0
Total liabilities and equity	1,521	1,103.9
Domestic Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Assets
Cash and cash equivalents	0.1	0	0	0
Receivables, net	31.7	27.7
Affiliated receivables	17.3	(83)
Inventories, net	39	40.5
Deferred tax assets		0
Other current assets	5.3	23
Total current assets	93.4	8.2
Equity investments	322.9	276.8
Property, plant and equipment, net	47.6	47.3
Goodwill	153.1	153.1
Intangible assets, net	89.1	96.7
Deferred tax assets	(7.1)	(13.2)
Affiliated loan receivables	159.1	224.3
Other assets	6.4	15.3
Total assets	864.5	808.5
LIABILITIES AND EQUITY
Accounts payable	35.2	32.4
Affiliated payables	3.4	13.8
Accrued liabilities	14.8	16.4
Current maturities of long-term debt	0.1	0
Total current liabilities	53.5	62.6
Long-term debt	0.1	0
Affiliated debt	37.7	19.3
Other long-term liabilities	14.1	14.4
Total liabilities	105.4	96.3
Koppers shareholders’ equity	759.1	712.2
Noncontrolling interests	0	0
Total liabilities and equity	864.5	808.5
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Assets
Cash and cash equivalents	62.4	59.6	$ 50.2	$ 20.8
Receivables, net	119.9	84.6
Affiliated receivables	4.8	(12.1)
Inventories, net	153	117.9
Deferred tax assets		(0.1)
Other current assets	18.9	18.8
Total current assets	359	268.7
Equity investments	0.1	(0.1)
Property, plant and equipment, net	147.2	125.5
Goodwill	142.6	34.3
Intangible assets, net	97.7	25.7
Deferred tax assets	1.9	2.2
Affiliated loan receivables	9.5	21.4
Other assets	12.2	9.6
Total assets	770.2	487.3
LIABILITIES AND EQUITY
Accounts payable	55.2	44.4
Affiliated payables	11.8	2.2
Accrued liabilities	36.8	51.6
Current maturities of long-term debt	5	11.3
Total current liabilities	108.8	109.5
Long-term debt	19.7	22.3
Affiliated debt	7.8	27.6
Other long-term liabilities	39.5	41.2
Total liabilities	175.8	200.6
Koppers shareholders’ equity	583.8	280.8
Noncontrolling interests	10.6	5.9
Total liabilities and equity	$ 770.2	$ 487.3

Subsidiary Guarantor Informat_6

Subsidiary Guarantor Information for Koppers Inc. Shelf Registration - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	$ 8	$ 48.5
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(345.4)	(48.6)
Repayments (loans to) from affiliates	0	0
Repayment of loan	0	9.5
Insurance proceeds received	1.5	0
Net cash proceeds from divestitures and asset sales	2.3	1.1
Net cash used in investing activities	(341.6)	(38)
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt	369.9	32.7
Borrowings (repayments) of affiliated debt	0	0
Deferred financing cost		(11)
Payment of debt issuance costs	(2.9)	(11)
Dividends paid	0	0
Stock repurchased	(29.2)	(3.2)
Net cash (used in) provided by financing activities	337.8	18.5
Effect of exchange rates on cash	(2)	0.4
Net increase in cash and cash equivalents	2.2	29.4
Cash and cash equivalents at beginning of period	60.3	20.8
Cash and cash equivalents at end of period	62.5	50.2
Consolidation Eliminations [Member]
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	(8.2)	(3.7)
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	0	0
Repayments (loans to) from affiliates	28.1	16.5
Repayment of loan		0
Insurance proceeds received	0
Net cash proceeds from divestitures and asset sales	0	0
Net cash used in investing activities	28.1	16.5
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt	0	0
Borrowings (repayments) of affiliated debt	(28.1)	(16.5)
Deferred financing cost		0
Payment of debt issuance costs	0
Dividends paid	8.2	3.7
Stock repurchased	0	0
Net cash (used in) provided by financing activities	(19.9)	(12.8)
Effect of exchange rates on cash	0	0
Net increase in cash and cash equivalents	0	0
Cash and cash equivalents at beginning of period	0	0
Cash and cash equivalents at end of period	0	0
Parent [Member] | Reportable Legal Entities [Member]
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	29.2	3.2
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	0	0
Repayments (loans to) from affiliates	0	0
Repayment of loan		0
Insurance proceeds received	0
Net cash proceeds from divestitures and asset sales	0	0
Net cash used in investing activities	0	0
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt	0	0
Borrowings (repayments) of affiliated debt	0	0
Deferred financing cost		0
Payment of debt issuance costs	0
Dividends paid	0	0
Stock repurchased	(29.2)	(3.2)
Net cash (used in) provided by financing activities	(29.2)	(3.2)
Effect of exchange rates on cash	0	0
Net increase in cash and cash equivalents	0	0
Cash and cash equivalents at beginning of period	0	0
Cash and cash equivalents at end of period	0	0
Koppers Inc. [Member] | Reportable Legal Entities [Member]
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	(68.3)	(15.9)
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(326.8)	(32)
Repayments (loans to) from affiliates	(4.5)	2
Repayment of loan		0
Insurance proceeds received	0
Net cash proceeds from divestitures and asset sales	(2)	0
Net cash used in investing activities	(333.3)	(30)
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt	377.7	36.7
Borrowings (repayments) of affiliated debt	34.3	23.9
Deferred financing cost		(11)
Payment of debt issuance costs	(2.9)
Dividends paid	(8.2)	(3.7)
Stock repurchased	0	0
Net cash (used in) provided by financing activities	400.9	45.9
Effect of exchange rates on cash	0	0
Net increase in cash and cash equivalents	(0.7)	0
Cash and cash equivalents at beginning of period	0.7	0
Cash and cash equivalents at end of period	0	0
Domestic Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	17.1	33.9
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(3.9)	(11.3)
Repayments (loans to) from affiliates	(18.1)	(19.1)
Repayment of loan		0
Insurance proceeds received	0
Net cash proceeds from divestitures and asset sales	0	0.7
Net cash used in investing activities	(22)	(29.7)
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt	(0.1)	0
Borrowings (repayments) of affiliated debt	5.1	(4.2)
Deferred financing cost		0
Payment of debt issuance costs	0
Dividends paid	0	0
Stock repurchased	0	0
Net cash (used in) provided by financing activities	5	(4.2)
Effect of exchange rates on cash	0	0
Net increase in cash and cash equivalents	0.1	0
Cash and cash equivalents at beginning of period	0	0
Cash and cash equivalents at end of period	0.1	0
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	38.2	31
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(14.7)	(5.3)
Repayments (loans to) from affiliates	(5.5)	0.6
Repayment of loan		9.5
Insurance proceeds received	1.5
Net cash proceeds from divestitures and asset sales	4.3	0.4
Net cash used in investing activities	(14.4)	5.2
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt	(7.7)	(4)
Borrowings (repayments) of affiliated debt	(11.3)	(3.2)
Deferred financing cost		0
Payment of debt issuance costs	0
Dividends paid	0	0
Stock repurchased	0	0
Net cash (used in) provided by financing activities	(19)	(7.2)
Effect of exchange rates on cash	(2)	0.4
Net increase in cash and cash equivalents	2.8	29.4
Cash and cash equivalents at beginning of period	59.6	20.8
Cash and cash equivalents at end of period	$ 62.4	$ 50.2

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Nov. 30, 2016
Committed Loan Facility Agreements [Member] | Construction Loans [Member]
Related Party Transaction [Line Items]
Debt maturity date	Dec. 21, 2020
Committed Loan Facility Agreements [Member] | Construction Loans [Member] | Koppers Carbon and Chemical Company Limited [Member]
Related Party Transaction [Line Items]
Percentage of non-controlling shareholders	25.00%
Loan outstanding from 25-percent non-controlling share holder	$ 2.5
Debt maturity date	Nov. 30, 2018
TKK [Member]
Related Party Transaction [Line Items]
Ownership percentage			30.00%	30.00%
Loan to related party			$ 10
Gain (loss) on sale of equity method investment			$ (1.1)
Gain on repayment of loan by related party		$ 1.3

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Company's Financial Instruments (Detail) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Fair Value [Member]
Financial assets:
Cash and cash equivalents, including restricted cash		$ 62.5	$ 60.3
Investments and other assets	[1]	1.1	1.1
Financial liabilities:
Total debt (including current portion)		1,049.1	706.9
Carrying Value [Member]
Financial assets:
Cash and cash equivalents, including restricted cash		62.5	60.3
Investments and other assets	[1]	1.1	1.1
Financial liabilities:
Total debt (including current portion)		$ 1,057.7	$ 688.7
[1]	Excludes equity method investments.