Consolidated Statement of Operations - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Income Statement [Abstract]
Net sales	$ 1,980.5	$ 1,678.6	$ 1,669.1
Cost of sales	1,635.9	1,344.5	1,308.7
Depreciation and amortization	56.1	57.7	54.1
Selling, general and administrative expenses	153.3	148.9	143.1
Impairment and restructuring charges	0.0	2.2	6.5
(Gain) on sale of assets	(2.5)	(31.2)	0.0
Operating profit	137.7	156.5	156.7
Other income, net	2.5	3.6	2.3
Interest expense	44.8	40.5	48.9
Income from continuing operations before income taxes	95.4	119.6	110.1
Income tax provision	31.6	34.5	21.0
Income from continuing operations	63.8	85.1	89.1
(Loss) gain on sale of discontinued operations, net of tax benefit (expense) of $0.0, $0.1 and $(7.4)	(0.6)	(0.2)	31.9
Net income	63.2	84.9	121.0
Net (loss) attributable to noncontrolling interests	(0.2)	(0.3)	(1.0)
Net income attributable to Koppers	$ 63.4	$ 85.2	$ 122.0
Basic -
Continuing operations	$ 3.05	$ 4.02	$ 4.25
Discontinued operations	(0.03)	(0.02)	1.56
Earnings per basic common share	3.02	4.00	5.81
Diluted -
Continuing operations	3.00	3.90	4.17
Discontinued operations	(0.02)	(0.02)	1.54
Earnings per diluted common share	$ 2.98	$ 3.88	$ 5.71
Weighted average shares outstanding (in thousands):
Basic	20,977	21,238	20,992
Diluted	21,313	21,925	21,374