Subsidiary Guarantor Information for Shelf Registration - Condensed Consolidating Statement of Cash Flows (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Subsidiary Guarantor Information for Shelf Registration [Line Items]
Cash provided by (used in) operating activities	$ 65.6	$ 54.5
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(29.4)	(15.8)
Net cash proceeds (payments) from divestitures and asset sales	2.4	0.7
Net cash provided by (used in) investing activities	(27.0)	(15.1)
Cash provided by (used in) financing activities:
Deferred financing costs	(1.2)	(0.1)
Dividends paid	(15.3)	(14.5)
Net cash provided by (used in) financing activities	(31.5)	(27.0)
Effect of exchange rates on cash	(1.4)	2.4
Net increase (decrease) in cash and cash equivalents	5.7	14.8
Cash and cash equivalents at beginning of year	66.7	54.1
Cash and cash equivalents at end of period	72.4	68.9
Shelf Registration [Member]
Subsidiary Guarantor Information for Shelf Registration [Line Items]
Cash provided by (used in) operating activities	65.6	54.5
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(29.4)	(15.8)
Net cash proceeds (payments) from divestitures and asset sales	2.4	0.7
Net cash provided by (used in) investing activities	(27.0)	(15.1)
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt		(5.0)
Deferred financing costs	(1.2)	(0.1)
Dividends paid	(15.3)	(14.5)
Other financing activities	2.3
Stock issued (repurchased)	(17.3)	(7.4)
Net cash provided by (used in) financing activities	(31.5)	(27.0)
Effect of exchange rates on cash	(1.4)	2.4
Net increase (decrease) in cash and cash equivalents	5.7	14.8
Cash and cash equivalents at beginning of year	66.7	54.1
Cash and cash equivalents at end of period	72.4	68.9
Parent [Member] | Shelf Registration [Member]
Subsidiary Guarantor Information for Shelf Registration [Line Items]
Cash provided by (used in) operating activities	32.6	21.8
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	0	0
Net cash proceeds (payments) from divestitures and asset sales	0	0
Net cash provided by (used in) investing activities	0	0
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt		0.1
Deferred financing costs	0	0
Dividends paid	(15.3)	(14.5)
Other financing activities	0
Stock issued (repurchased)	(17.3)	(7.4)
Net cash provided by (used in) financing activities	(32.6)	(21.8)
Effect of exchange rates on cash	0	0
Net increase (decrease) in cash and cash equivalents	0	0
Cash and cash equivalents at beginning of year	0	0
Cash and cash equivalents at end of period	0	0
Koppers Inc. [Member] | Shelf Registration [Member]
Subsidiary Guarantor Information for Shelf Registration [Line Items]
Cash provided by (used in) operating activities	39.7	32.0
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(13.3)	(12.2)
Net cash proceeds (payments) from divestitures and asset sales	1.4	0.3
Net cash provided by (used in) investing activities	(11.9)	(11.9)
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt		(5.0)
Deferred financing costs	(1.2)	(0.1)
Dividends paid	(23.5)	(14.8)
Other financing activities	0
Stock issued (repurchased)	0	0
Net cash provided by (used in) financing activities	(24.7)	(19.9)
Effect of exchange rates on cash	0.1	(0.1)
Net increase (decrease) in cash and cash equivalents	3.2	0.1
Cash and cash equivalents at beginning of year	4.8	0
Cash and cash equivalents at end of period	8.0	0.1
Domestic Guarantor Subsidiaries [Member] | Shelf Registration [Member]
Subsidiary Guarantor Information for Shelf Registration [Line Items]
Cash provided by (used in) operating activities	0	0
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	0	0
Net cash proceeds (payments) from divestitures and asset sales	0	0
Net cash provided by (used in) investing activities	0	0
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt		0
Deferred financing costs	0	0
Dividends paid	0	0
Other financing activities	0
Stock issued (repurchased)	0	0
Net cash provided by (used in) financing activities	0	0
Effect of exchange rates on cash	0	0
Net increase (decrease) in cash and cash equivalents	0	0
Cash and cash equivalents at beginning of year	0	0
Cash and cash equivalents at end of period	0	0
Foreign Guarantor Subsidiaries [Member] | Shelf Registration [Member]
Subsidiary Guarantor Information for Shelf Registration [Line Items]
Cash provided by (used in) operating activities	14.9	21.5
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(6.6)	(3.2)
Net cash proceeds (payments) from divestitures and asset sales	1.1	0.3
Net cash provided by (used in) investing activities	(5.5)	(2.9)
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt		0
Deferred financing costs	0	0
Dividends paid	0	0
Other financing activities	0
Stock issued (repurchased)	0	0
Net cash provided by (used in) financing activities	0	0
Effect of exchange rates on cash	(1.8)	2.5
Net increase (decrease) in cash and cash equivalents	7.6	21.1
Cash and cash equivalents at beginning of year	40.7	45.1
Cash and cash equivalents at end of period	48.3	66.2
Non-Guarantor Subsidiaries [Member] | Shelf Registration [Member]
Subsidiary Guarantor Information for Shelf Registration [Line Items]
Cash provided by (used in) operating activities	1.9	(6.0)
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(9.5)	(0.4)
Net cash proceeds (payments) from divestitures and asset sales	(0.1)	0.1
Net cash provided by (used in) investing activities	(9.6)	(0.3)
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt		(0.1)
Deferred financing costs	0	0
Dividends paid	0	0
Other financing activities	2.3
Stock issued (repurchased)	0	0
Net cash provided by (used in) financing activities	2.3	(0.1)
Effect of exchange rates on cash	0.3	0
Net increase (decrease) in cash and cash equivalents	(5.1)	(6.4)
Cash and cash equivalents at beginning of year	21.2	9.0
Cash and cash equivalents at end of period	16.1	2.6
Consolidating Adjustments [Member] | Shelf Registration [Member]
Subsidiary Guarantor Information for Shelf Registration [Line Items]
Cash provided by (used in) operating activities	(23.5)	(14.8)
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	0	0
Net cash proceeds (payments) from divestitures and asset sales	0	0
Net cash provided by (used in) investing activities	0	0
Cash provided by (used in) financing activities:
Borrowings (repayments) of long-term debt		0
Deferred financing costs	0	0
Dividends paid	23.5	14.8
Other financing activities	0
Stock issued (repurchased)	0	0
Net cash provided by (used in) financing activities	23.5	14.8
Effect of exchange rates on cash	0	0
Net increase (decrease) in cash and cash equivalents	0	0
Cash and cash equivalents at beginning of year	0	0
Cash and cash equivalents at end of period	$ 0	$ 0