Subsidiary Guarantor Information for Koppers Inc. Senior Notes - Condensed Consolidating Statement of Comprehensive Income (Detail) (USD $) In Millions, 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
Subsidiary Guarantor Information For Subsidiary Senior Notes [Line Items]
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
Selling, general and administrative									116.2	71.7	75.6
Operating profit (loss)	2.1	17.3	13.3	0.5	7.8	39.1	28.5	24.9	33.2	100.3	126.6
Other income (expense)									0	3.5	1.9
Equity income of subsidiaries									(1.6)	0.8	0.8
Interest expense									39.1	26.8	27.9
Income taxes									34.1	36.8	33.3
(Loss) income 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
Discontinued operations									0.6	(0.1)	(0.1)
Noncontrolling interests									(7.0)	(0.3)	1.6
Net (loss) income 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
Comprehensive (loss) income attributable to Koppers									(82.5)	52.3	73.7
Intersegment [Member]
Subsidiary Guarantor Information For Subsidiary Senior Notes [Line Items]
Net sales									87.8	94.2	106.5
Senior Notes [Member]
Subsidiary Guarantor Information For Subsidiary Senior Notes [Line Items]
Net sales									1,555.0	1,478.3	1,555.0
Cost of sales including depreciation and amortization									1,405.6	1,306.3	1,352.8
Selling, general and administrative									116.2	71.7	75.6
Operating profit (loss)									33.2	100.3	126.6
Other income (expense)									0	3.5	1.9
Equity income of subsidiaries									0	0	0
Interest expense									39.1	26.8	27.9
Income taxes									34.1	36.8	33.3
(Loss) income from continuing operations									(40.0)	40.2	67.3
Discontinued operations									0.6	(0.1)	(0.1)
Noncontrolling interests									(7.0)	(0.3)	1.6
Net (loss) income attributable to Koppers									(32.4)	40.4	65.6
Comprehensive (loss) income attributable to Koppers									(82.5)	52.3	73.7
Senior Notes [Member] | Intersegment [Member]
Subsidiary Guarantor Information For Subsidiary Senior Notes [Line Items]
Net sales									(67.4)	(49.4)	(74.1)
Cost of sales including depreciation and amortization									(66.7)	(49.5)	(73.8)
Selling, general and administrative									0	0	0
Operating profit (loss)									(0.7)	0.1	(0.3)
Other income (expense)									(4.3)	(4.1)	(5.2)
Equity income of subsidiaries									53.3	(70.6)	(136.3)
Interest expense									(4.3)	(4.1)	(5.3)
Income taxes									0	0	0
(Loss) income from continuing operations									52.6	(70.5)	(136.5)
Discontinued operations									0	0	0
Noncontrolling interests									0	0	0
Net (loss) income attributable to Koppers									52.6	(70.5)	(136.5)
Comprehensive (loss) income attributable to Koppers									170.6	(75.7)	(151.2)
Senior Notes [Member] | Parent [Member]
Subsidiary Guarantor Information For Subsidiary Senior Notes [Line Items]
Net sales									0	0	0
Cost of sales including depreciation and amortization									0	0	0
Selling, general and administrative									2.2	1.9	1.6
Operating profit (loss)									(2.2)	(1.9)	(1.6)
Other income (expense)									0	0	0
Equity income of subsidiaries									(31.0)	41.7	66.6
Interest expense									0	0	0
Income taxes									(0.8)	(0.6)	(0.6)
(Loss) income from continuing operations									(32.4)	40.4	65.6
Discontinued operations									0	0	0
Noncontrolling interests									0	0	0
Net (loss) income attributable to Koppers									(32.4)	40.4	65.6
Comprehensive (loss) income attributable to Koppers									(82.5)	52.3	73.7
Senior Notes [Member] | Koppers Inc. [Member]
Subsidiary Guarantor Information For Subsidiary Senior Notes [Line Items]
Net sales									795.9	843.7	893.2
Cost of sales including depreciation and amortization									724.5	745.4	786.6
Selling, general and administrative									59.2	41.5	45.1
Operating profit (loss)									12.2	56.8	61.5
Other income (expense)									0.2	1.3	0.4
Equity income of subsidiaries									(15.9)	17.5	40.8
Interest expense									36.5	26.7	27.4
Income taxes									(9.0)	7.3	8.7
(Loss) income from continuing operations									(31.0)	41.6	66.6
Discontinued operations									0	0.1	0
Noncontrolling interests									0	0	0
Net (loss) income attributable to Koppers									(31.0)	41.7	66.6
Comprehensive (loss) income attributable to Koppers									(81.1)	53.5	74.8
Senior Notes [Member] | Domestic Guarantor Subsidiaries [Member]
Subsidiary Guarantor Information For Subsidiary Senior Notes [Line Items]
Net sales									143.0	61.8	97.5
Cost of sales including depreciation and amortization									108.8	43.5	78.0
Selling, general and administrative									17.4	0.7	2.1
Operating profit (loss)									16.8	17.6	17.4
Other income (expense)									4.7	4.2	6.1
Equity income of subsidiaries									(6.4)	11.4	28.8
Interest expense									0	0	0
Income taxes									30.5	16.4	12.5
(Loss) income from continuing operations									(15.4)	16.8	39.8
Discontinued operations									(0.6)	0	0
Noncontrolling interests									0	0	0
Net (loss) income attributable to Koppers									(16.0)	16.8	39.8
Comprehensive (loss) income attributable to Koppers									(54.4)	8.4	44.0
Senior Notes [Member] | Non-Guarantor Subsidiaries [Member]
Subsidiary Guarantor Information For Subsidiary Senior Notes [Line Items]
Net sales									683.5	622.2	638.4
Cost of sales including depreciation and amortization									639.0	566.9	562.0
Selling, general and administrative									37.4	27.6	26.8
Operating profit (loss)									7.1	27.7	49.6
Other income (expense)									(0.6)	2.1	0.6
Equity income of subsidiaries									0	0	0.1
Interest expense									6.9	4.2	5.8
Income taxes									13.4	13.7	12.7
(Loss) income from continuing operations									(13.8)	11.9	31.8
Discontinued operations									1.2	(0.2)	(0.1)
Noncontrolling interests									(7.0)	(0.3)	1.6
Net (loss) income attributable to Koppers									(5.6)	12.0	30.1
Comprehensive (loss) income attributable to Koppers									$ (35.1)	$ 13.8	$ 32.4