Condensed Consolidated Statement of Operations - USD ($) shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Net sales	$ 431.6	$ 356.8	$ 829.4	$ 688.2
Cost of sales (excluding items below)	356.8	311.9	700.3	597.0
Depreciation and amortization	17.7	9.7	32.7	18.6
Gain on sale of business	0.0	0.0	(3.2)	0.0
Impairment and restructuring charges	0.0	0.0	2.7	15.5
Selling, general and administrative expenses	31.1	21.9	62.9	43.3
Operating (loss) profit	26.0	13.3	34.0	13.8
Other income (loss)	0.2	(0.3)	0.4	(0.1)
Interest expense	12.9	6.6	25.9	13.4
Income before income taxes	13.3	6.4	8.5	0.3
Income tax provision (benefit)	5.4	5.6	4.8	(0.4)
Income (loss) from continuing operations	7.9	0.8	3.7	0.7
Loss from discontinued operations, net of tax benefit of $0.0, $0.1, $0.0 and $0.1	0.0	(0.1)	0.0	(0.1)
Net (loss) income	7.9	0.7	3.7	0.6
Net loss attributable to noncontrolling interests	(1.1)	(0.9)	(1.9)	(3.2)
Net income (loss) attributable to Koppers	$ 9.0	$ 1.6	$ 5.6	$ 3.8
Earnings per common share attributable to Koppers common shareholders:
Basic	$ 0.44	$ 0.08	$ 0.28	$ 0.19
Diluted	$ 0.44	$ 0.08	$ 0.27	$ 0.19
Comprehensive income (loss)	$ 11.1	$ 3.2	$ (3.1)	$ 5.8
Comprehensive loss attributable to noncontrolling interests	(1.1)	(0.9)	(1.9)	(3.6)
Comprehensive income (loss) attributable to Koppers	$ 12.2	$ 4.1	$ (1.2)	$ 9.4
Weighted average shares outstanding (in thousands):
Basic	20,545	20,475	20,529	20,430
Diluted	20,640	20,582	20,603	20,584
Dividends declared per common share	$ 0.00	$ 0.25	$ 0.00	$ 0.50