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
Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net (loss) income attributable to Koppers	$ (1.4)	$ 11.6
(Loss) income from discontinued operations	(4.4)	2.7
Non-controlling loss (income) from discontinued operations	1.1	(0.9)
Income from continuing operations attributable to Koppers	$ 1.9	$ 9.8
Weighted average common shares outstanding:
Basic	20,852	20,575
Effect of dilutive securities	463	306
Diluted	21,315	20,881
Earnings per common share – continuing operations:
Basic earnings per common share	$ 0.09	$ 0.47
Diluted earnings per common share	$ 0.09	$ 0.47
Other data:
Antidilutive securities excluded from computation of diluted earnings per common share	682	534

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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