Condensed Consolidated Statement of Operations and Comprehensive Income (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Net sales	$ 401.9	$ 376.9
Cost of sales	340.3	301.8
Depreciation and amortization	13.5	13.6
Impairment and restructuring charges	(0.2)	0.3
Selling, general and administrative expenses	34.7	36.9
Operating profit	13.6	24.3
Other income, net	0.5	0.5
Interest expense	14.0	16.2
Income from continuing operations before income taxes	0.1	8.6
Income tax provision	(1.8)	(1.2)
Income from continuing operations	1.9	9.8
(Loss) income from discontinued operations, net of tax benefit (expense) of $0.8 and $(1.1)	(4.4)	2.7
Net (loss) income	(2.5)	12.5
Net (loss) income attributable to noncontrolling interests	(1.1)	0.9
Net (loss) income attributable to Koppers	$ (1.4)	$ 11.6
Basic -
Continuing operations	$ 0.09	$ 0.47
Discontinued operations	(0.16)	0.09
(Loss) earnings per basic common share	(0.07)	0.56
Diluted -
Continuing operations	0.09	0.47
Discontinued operations	(0.16)	0.09
(Loss) earnings per diluted common share	$ (0.07)	$ 0.56
Comprehensive (loss) income	$ (51.0)	$ 20.7
Comprehensive (loss) income attributable to noncontrolling interests	(1.3)	1.1
Comprehensive (loss) income attributable to Koppers	$ (49.7)	$ 19.6
Weighted average shares outstanding (in thousands):
Basic	20,852	20,575
Diluted	21,315	20,881