Subsidiary Guarantor Information for Koppers Inc. Senior Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	$ 127.7	$ 35.5	$ 117.6
Cash provided by (used in) investing activities:
Net cash proceeds (payments) from divestitures and asset sales	14.9	0.3	2.9
Net cash used in investing activities	(41.1)	(580.0)	(72.3)
Cash provided by (used in) financing activities:
Deferred financing costs	(1.0)	(11.1)	(1.3)
Dividends paid	(8.7)	(20.4)	(21.1)
Net cash (used in) provided by financing activities	(123.4)	516.1	(30.3)
Effect of exchange rates on cash	7.5	(2.7)	0.5
Net (decrease) increase in cash and cash equivalents	(29.3)	(31.1)	15.5
Cash and cash equivalents at beginning of period	51.1	82.2	66.7
Cash and cash equivalents at end of period	21.8	51.1	82.2
Senior Notes [Member]
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	127.7	35.5	117.6
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(56.0)	(580.3)	(75.2)
Repayments (loans to) from affiliates	0.0	0.0	0.0
Net cash proceeds (payments) from divestitures and asset sales	14.9	0.3	2.9
Net cash used in investing activities	(41.1)	(580.0)	(72.3)
Cash provided by (used in) financing activities:
(Repayments) borrowings of long-term debt	(113.4)	547.5	6.6
Borrowings (repayments) of affiliated debt	0.0	0.0	0.0
Deferred financing costs	(1.0)	(11.1)	(1.3)
Other financing activities	0.0	1.4	2.8
Dividends paid	(8.7)	(20.4)	(21.1)
Stock repurchased	(0.3)	(1.3)	(17.3)
Net cash (used in) provided by financing activities	(123.4)	516.1	(30.3)
Effect of exchange rates on cash	7.5	(2.7)	0.5
Net (decrease) increase in cash and cash equivalents	(29.3)	(31.1)	15.5
Cash and cash equivalents at beginning of period	51.1	82.2	82.2
Cash and cash equivalents at end of period	21.8	51.1	82.2
Senior Notes [Member] | Intersegment [Member]
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	(56.9)	(54.3)	(376.6)
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	0.0	14.8	0.0
Repayments (loans to) from affiliates	(10.4)	71.1	(190.7)
Net cash proceeds (payments) from divestitures and asset sales	0.0	0.0	0.0
Net cash used in investing activities	(10.4)	85.9	(190.7)
Cash provided by (used in) financing activities:
(Repayments) borrowings of long-term debt	0.0	0.0	0.0
Borrowings (repayments) of affiliated debt	10.4	(71.1)	190.7
Deferred financing costs	0.0	0.0	0.0
Other financing activities	0.0	0.0	0.0
Dividends paid	56.9	54.3	376.6
Stock repurchased	0.0	(14.8)	0.0
Net cash (used in) provided by financing activities	67.3	(31.6)	567.3
Effect of exchange rates on cash	0.0	0.0	0.0
Net (decrease) increase in cash and cash equivalents	0.0	0.0	0.0
Cash and cash equivalents at beginning of period	0.0	0.0	0.0
Cash and cash equivalents at end of period	0.0	0.0	0.0
Senior Notes [Member] | Parent [Member]
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	5.4	21.8	37.8
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	0.0	0.0	0.0
Repayments (loans to) from affiliates	0.0	0.0	0.0
Net cash proceeds (payments) from divestitures and asset sales	0.0	0.0	0.0
Net cash used in investing activities	0.0	0.0	0.0
Cash provided by (used in) financing activities:
(Repayments) borrowings of long-term debt	0.0	0.0	0.0
Borrowings (repayments) of affiliated debt	0.0	0.0	0.0
Deferred financing costs	0.0	0.0	0.0
Other financing activities	0.0	0.0	0.0
Dividends paid	(5.1)	(20.5)	(20.5)
Stock repurchased	(0.3)	(1.3)	(17.3)
Net cash (used in) provided by financing activities	(5.4)	(21.8)	(37.8)
Effect of exchange rates on cash	0.0	0.0	0.0
Net (decrease) increase in cash and cash equivalents	0.0	0.0	0.0
Cash and cash equivalents at beginning of period	0.0	0.0	0.0
Cash and cash equivalents at end of period	0.0	0.0	0.0
Senior Notes [Member] | Koppers Inc. [Member]
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	63.0	23.8	278.7
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(41.2)	(518.5)	(23.3)
Repayments (loans to) from affiliates	6.3	(32.2)	(0.8)
Net cash proceeds (payments) from divestitures and asset sales	12.3	0.1	1.3
Net cash used in investing activities	(22.6)	(550.6)	(22.8)
Cash provided by (used in) financing activities:
(Repayments) borrowings of long-term debt	(104.2)	497.0	0.0
Borrowings (repayments) of affiliated debt	71.0	35.8	(191.5)
Deferred financing costs	(1.0)	(11.1)	(1.3)
Other financing activities	0.0	0.0	0.5
Dividends paid	(6.2)	(23.6)	(38.5)
Stock repurchased	0.0	0.0	0.0
Net cash (used in) provided by financing activities	(40.4)	498.1	(230.8)
Effect of exchange rates on cash	0.1	(1.2)	0.0
Net (decrease) increase in cash and cash equivalents	0.1	(29.9)	25.1
Cash and cash equivalents at beginning of period	0.0	29.9	29.9
Cash and cash equivalents at end of period	0.1	0.0	29.9
Senior Notes [Member] | Domestic Guarantor Subsidiaries [Member]
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	55.9	33.8	130.1
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(5.3)	(16.7)	0.0
Repayments (loans to) from affiliates	(5.1)	(38.9)	193.2
Net cash proceeds (payments) from divestitures and asset sales	2.1	0.1	0.0
Net cash used in investing activities	(8.3)	(55.5)	193.2
Cash provided by (used in) financing activities:
(Repayments) borrowings of long-term debt	0.1	0.0	0.0
Borrowings (repayments) of affiliated debt	(6.4)	27.6	0.8
Deferred financing costs	0.0	0.0	0.0
Other financing activities	0.0	0.0	0.0
Dividends paid	(40.8)	(6.1)	(324.0)
Stock repurchased	0.0	0.0	0.0
Net cash (used in) provided by financing activities	(47.1)	21.5	(323.2)
Effect of exchange rates on cash	(0.7)	1.0	0.0
Net (decrease) increase in cash and cash equivalents	(0.2)	0.8	0.1
Cash and cash equivalents at beginning of period	0.9	0.1	0.1
Cash and cash equivalents at end of period	0.7	0.9	0.1
Senior Notes [Member] | Non-Guarantor Subsidiaries [Member]
Subsidiary, Sale of Stock [Line Items]
Cash provided by (used in) operating activities	60.3	10.4	47.6
Cash provided by (used in) investing activities:
Capital expenditures and acquisitions	(9.5)	(59.9)	(51.9)
Repayments (loans to) from affiliates	9.2	0.0	(1.7)
Net cash proceeds (payments) from divestitures and asset sales	0.5	0.1	1.6
Net cash used in investing activities	0.2	(59.8)	(52.0)
Cash provided by (used in) financing activities:
(Repayments) borrowings of long-term debt	(9.3)	50.5	6.6
Borrowings (repayments) of affiliated debt	(75.0)	7.7	0.0
Deferred financing costs	0.0	0.0	0.0
Other financing activities	0.0	1.4	2.3
Dividends paid	(13.5)	(24.5)	(14.7)
Stock repurchased	0.0	14.8	0.0
Net cash (used in) provided by financing activities	(97.8)	49.9	(5.8)
Effect of exchange rates on cash	8.1	(2.5)	0.5
Net (decrease) increase in cash and cash equivalents	(29.2)	(2.0)	(9.7)
Cash and cash equivalents at beginning of period	50.2	52.2	52.2
Cash and cash equivalents at end of period	$ 21.0	$ 50.2	$ 52.2