Consolidated Statement of Shareholders' Equity - USD ($) $ in Millions	Total	Senior Convertible Preferred Stock [Member]	Common Stock [Member]	Noncontrolling Interests [Member]	Retained Earnings (Accumulated Deficit) [Member]	Currency Translation Adjustment [Member]	Unrecognized Gains (Losses) on Cash Flow Hedges [Member]	Unrecognized Pension Prior Service Cost [Member]	Unrecognized Pension Net Loss [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Koppers Shareholder's Equity (Deficit) [Member]
Balance at Dec. 31, 2016		$ 0.0	$ 0.2	$ 4.2	$ (24.7)	$ (30.6)	$ 6.9	$ 0.0	$ (44.9)	$ 176.5	$ (53.0)
Net income attributable to noncontrolling interests	$ 1.4			1.4
Net income (loss) attributable to Koppers	29.1				29.1
Tax reform rate change reclassification					3.2		2.8		(6.0)
Change in accounting standard, net of tax expense of $0.0, $1.3 and $0.0					(0.2)		0.0
Purchases											(5.2)
Change in currency translation adjustment	17.0			0.3		16.8
Reclassification of unrealized losses (gains) on cash flow hedges to expense, net of tax (expense) benefit of $(1.5), $2.5 and $5.0							(7.1)
Change in cash flow hedges, net of tax (expense) benefit of $(2.6), $7.5 and $(8.5)	6.1						13.2
Revaluation of unrecognized prior service benefit,net of tax benefit of $0.0, $0.1 and $0.0								0.0
Reclassification of unrecognized pension net loss to expense, net of tax benefit of $0.3, $0.4 and $4.4									7.3
Revaluation of unrecognized pension net loss, net of tax benefit of $0.4, $0.5 and $0.2									1.5
Employee stock plans										11.4
Issuance of common stock										2.7
Balance at Dec. 31, 2017	105.8	0.0	0.2	5.9	7.4	(13.8)	15.8	0.0	(42.1)	190.6	(58.2)	$ (40.1)	$ 99.9
Net income attributable to noncontrolling interests	5.8			5.8
Net income (loss) attributable to Koppers	23.4				23.4
Tax reform rate change reclassification					0.0		0.0		0.0
Change in accounting standard, net of tax expense of $0.0, $1.3 and $0.0					(3.6)		3.9
Purchases											(31.8)
Change in currency translation adjustment	(25.6)			(0.9)		(24.7)
Reclassification of unrealized losses (gains) on cash flow hedges to expense, net of tax (expense) benefit of $(1.5), $2.5 and $5.0							(7.4)
Change in cash flow hedges, net of tax (expense) benefit of $(2.6), $7.5 and $(8.5)	(25.2)						(17.8)
Revaluation of unrecognized prior service benefit,net of tax benefit of $0.0, $0.1 and $0.0								(0.6)
Reclassification of unrecognized pension net loss to expense, net of tax benefit of $0.3, $0.4 and $4.4									1.1
Revaluation of unrecognized pension net loss, net of tax benefit of $0.4, $0.5 and $0.2									(1.6)
Employee stock plans										12.5
Issuance of common stock										2.9
Balance at Dec. 31, 2018	67.0	$ 0.0	$ 0.2	10.8	27.2	(38.5)	(5.5)	(0.6)	(42.6)	206.0	(90.0)	(87.2)	56.2
Net income attributable to noncontrolling interests	0.8			0.8
Net income (loss) attributable to Koppers	66.6				66.6
Tax reform rate change reclassification					0.0		0.0		0.0
Change in accounting standard, net of tax expense of $0.0, $1.3 and $0.0					0.0		0.0
Purchases											(0.9)
Change in currency translation adjustment	(1.3)			(0.2)		(1.3)
Reclassification of unrealized losses (gains) on cash flow hedges to expense, net of tax (expense) benefit of $(1.5), $2.5 and $5.0							4.6
Change in cash flow hedges, net of tax (expense) benefit of $(2.6), $7.5 and $(8.5)	8.7						4.1
Revaluation of unrecognized prior service benefit,net of tax benefit of $0.0, $0.1 and $0.0								0.0
Reclassification of unrecognized pension net loss to expense, net of tax benefit of $0.3, $0.4 and $4.4									1.1
Revaluation of unrecognized pension net loss, net of tax benefit of $0.4, $0.5 and $0.2									1.0
Employee stock plans										12.1
Issuance of common stock										3.8
Balance at Dec. 31, 2019	$ 158.7			$ 11.4	$ 93.8	$ (39.8)	$ 3.2	$ (0.6)	$ (40.5)	$ 221.9	$ (90.9)	$ (77.7)	$ 147.3