Stock-based Compensation - Schedule of Stock-Based Compensation Expense Recognized (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Stock-based compensation expense recognized:
Selling, general and administrative expenses	$ 1.3	$ 2.0	$ 2.0	$ 3.2
Less related income tax benefit	0.5	0.8	0.8	1.3
Decrease in net income attributable to Koppers	$ 0.8	$ 1.2	$ 1.2	$ 1.9