Subsidiary Guarantor Information for Shelf Registration - Condensed Consolidating Statement of Cash Flows (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Shelf Registration [Member]	Jun. 30, 2012 Shelf Registration [Member]	Jun. 30, 2013 Parent [Member] Shelf Registration [Member]	Jun. 30, 2012 Parent [Member] Shelf Registration [Member]	Jun. 30, 2013 Koppers Inc. [Member] Shelf Registration [Member]	Jun. 30, 2012 Koppers Inc. [Member] Shelf Registration [Member]	Jun. 30, 2013 Domestic Guarantor Subsidiaries [Member] Shelf Registration [Member]	Jun. 30, 2012 Domestic Guarantor Subsidiaries [Member] Shelf Registration [Member]	Jun. 30, 2013 Foreign Guarantor Subsidiaries [Member] Shelf Registration [Member]	Jun. 30, 2012 Foreign Guarantor Subsidiaries [Member] Shelf Registration [Member]	Jun. 30, 2013 Non-Guarantor Subsidiaries [Member] Shelf Registration [Member]	Jun. 30, 2012 Non-Guarantor Subsidiaries [Member] Shelf Registration [Member]	Jun. 30, 2013 Consolidating Adjustments [Member] Shelf Registration [Member]	Jun. 30, 2012 Consolidating Adjustments [Member] Shelf Registration [Member]
Subsidiary Guarantor Information for Shelf Registration [Line Items]
Cash provided by (used in) operating activities	$ 20.0	$ 3.4	$ 20.0	$ 3.4	$ 11.5	$ 10.6	$ 9.1	$ 0.7	$ 0.1	$ 0	$ 3.0	$ 3.7	$ 6.9	$ (6.6)	$ (10.6)	$ (5.0)
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(15.7)	(9.3)	(15.7)	(9.3)	0	0	(8.1)	(7.7)	0	0	(3.0)	(1.6)	(4.6)	0	0	0
Net cash proceeds (payments) from divestitures and asset sales	0.9	0.4	0.9	0.4	0	0	0.8	0.1	0	0	0.1	0.4	0	(0.1)	0	0
Net cash provided by (used in) investing activities	(14.8)	(8.9)	(14.8)	(8.9)	0	0	(7.3)	(7.6)	0	0	(2.9)	(1.2)	(4.6)	(0.1)	0	0
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt			5.1	12.0	0	0	5.1	12.0	0	0	0	0	0	0	0	0
Stock issued (repurchased)			(1.4)	(1.1)	(1.4)	(1.1)	0	0	0	0	0	0	0	0	0	0
Deferred financing costs	(1.2)	0	(1.2)		0		(1.2)		0		0		0		0
Other financing activities			2.3		0		0		0		0		2.3		0
Dividends paid	(10.2)	(9.5)	(10.2)	(9.5)	(10.1)	(9.5)	(10.6)	(5.0)	0	0	0	0	(0.1)	0	10.6	5.0
Net cash provided by (used in) financing activities	(5.4)	1.4	(5.4)	1.4	(11.5)	(10.6)	(6.7)	7.0	0	0	0	0	2.2	0	10.6	5.0
Effect of exchange rates on cash	(3.3)	0.3	(3.3)	0.3	0	0	0.1	(0.1)	0	0	(3.4)	0.5	0	(0.1)	0	0
Net increase (decrease) in cash and cash equivalents	(3.5)	(3.8)	(3.5)	(3.8)	0	0	(4.8)	0	0.1	0	(3.3)	3.0	4.5	(6.8)	0	0
Cash and cash equivalents at beginning of year	66.7	54.1	66.7	54.1	0	0	4.8	0	0	0	40.7	45.1	21.2	9.0	0	0
Cash and cash equivalents at end of period	$ 63.2	$ 50.3	$ 63.2	$ 50.3	$ 0	$ 0	$ 0	$ 0	$ 0.1	$ 0	$ 37.4	$ 48.1	$ 25.7	$ 2.2	$ 0	$ 0