Selected Quarterly Financial Data - Schedule of Selected Quarterly Financial Data (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of operations data:
Net sales	$ 426.7	$ 440.1	$ 356.8	$ 331.4	$ 341.8	$ 395.2	$ 370.9	$ 370.4	$ 1,555.0	$ 1,478.3	$ 1,555.0
Operating profit	2.1	17.3	13.3	0.5	7.8	39.1	28.5	24.9	33.2	100.3	126.6
Income (loss) from continuing operations	(36.4)	(4.3)	0.8	(0.1)	(5.6)	19.6	14.8	11.4	(40.0)	40.2	67.3
Net income	(35.8)	(4.2)	0.7	(0.1)	(5.6)	19.5	14.7	11.5	(39.4)	40.1	67.2
Net income (loss) attributable to Koppers	$ (33.5)	$ (2.7)	$ 1.6	$ 2.2	$ (4.1)	$ 19.1	$ 14.4	$ 11.0	$ (32.4)	$ 40.4	$ 65.6
Basic -
Continuing operations	$ (1.66)	$ (0.14)	$ 0.08	$ 0.11	$ (0.20)	$ 0.93	$ 0.70	$ 0.53	$ (1.61)	$ 1.96	$ 3.18
Discontinued operations	$ 0.03	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ 0.00	$ 0.03	$ 0.00	$ (0.01)
Earnings (loss) per basic common share	$ (1.63)	$ (0.14)	$ 0.08	$ 0.11	$ (0.20)	$ 0.93	$ 0.69	$ 0.53	$ (1.58)	$ 1.96	$ 3.17
Diluted -
Continuing operations	$ (1.66)	$ (0.14)	$ 0.08	$ 0.11	$ (0.20)	$ 0.92	$ 0.70	$ 0.53	$ (1.61)	$ 1.94	$ 3.14
Discontinued operations	$ 0.03	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ 0.00	$ 0.03	$ 0.00	$ (0.01)
Earnings (loss) per diluted common share	$ (1.63)	$ (0.14)	$ 0.08	$ 0.11	$ (0.20)	$ 0.92	$ 0.69	$ 0.53	$ (1.58)	$ 1.94	$ 3.13
Dividends declared per common share	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 1.00	$ 1.00	$ 0.96
Maximum [Member]
Price range of common stock:
Price range of common stock	$ 39.88	$ 39.45	$ 45.51	$ 45.92	$ 49.99	$ 42.43	$ 46.48	$ 45.72	$ 45.92	$ 49.99
Minimum [Member]
Price range of common stock:
Price range of common stock	$ 22.52	$ 33.02	$ 35.53	$ 35.51	$ 41.61	$ 34.69	$ 37.45	$ 38.70	$ 22.52	$ 34.69