Earnings and Dividends per Common Share - Schedule of Computation of Basic and Diluted Earnings per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Earnings Per Share [Abstract]
Net income attributable to Koppers	$ 89.2	$ 63.4	$ 85.2
Loss on sale of discontinued operations	0.0	(0.6)	(0.2)
Income from continuing operations attributable to Koppers	$ 89.2	$ 64.0	$ 85.4
Weighted average common shares outstanding:
Basic	20,835	20,977	21,238
Effect of dilutive securities	704	336	687
Diluted	21,539	21,313	21,925
Earnings per common share – continuing operations:
Basic earnings per common share	$ 4.28	$ 3.05	$ 4.02
Diluted earnings per common share	$ 4.14	$ 3.00	$ 3.90
Other data:
Antidilutive securities excluded from computation of diluted earnings per common share	315	952	436