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		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net income attributable to Koppers	$ 26.9	$ 29.2	$ 52.8	$ 27.8
Less: Income (loss) from discontinued operations, net of tax	0.5	0.0	0.3	(4.4)
Gain on sale of discontinued operations	0.5	0.0	0.3	0.0
Noncontrolling interest related to discontinued operations	0.0	(0.3)	0.0	0.8
Income from continuing operations attributable to Koppers	$ 25.9	$ 29.5	$ 52.2	$ 31.4
Weighted average common shares outstanding:
Basic	21,289	21,001	21,217	20,927
Effect of dilutive securities	678	67	732	157
Diluted	21,967	21,068	21,949	21,084
Earnings per common share – continuing operations:
Basic earnings per common share	$ 1.22	$ 1.40	$ 2.46	$ 1.50
Diluted earnings per common share	$ 1.18	$ 1.40	$ 2.38	$ 1.49
Other data:
Antidilutive securities excluded from computation of diluted earnings per common share	434	1,773	439	1,022