Adoption of Revenue Recognition Guidance - Effect of Revenue Recognition on Condensed Consolidated Statement of Operations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ 556.5	$ 527.7	$ 512.4	$ 476.1	$ 523.0	$ 481.8	$ 485.0	$ 491.7	$ 2,072.7	$ 1,981.5	$ 1,895.9
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)	449.6	441.6	415.2	394.9	443.5	392.9	379.6	397.6	1,701.3	1,613.6	1,549.4
Selling, general and administrative expense									56.1	53.6	43.8
Depreciation and amortization									65.3	74.5	72.3
Other income, net											(40.2)
Total operating costs and expenses									1,832.1	1,743.3	1,625.3
Operating income									240.6	238.2	270.6
Total non-operating expense, net									60.4	46.3	82.0
Income before income taxes	43.3	40.7	53.5	42.7	37.2	43.5	59.4	51.8	180.2	191.9	188.6
Income tax provision									42.5	50.9	51.3
Net income	$ 35.4	$ 31.7	$ 39.2	$ 31.4	$ 25.3	$ 32.9	$ 44.1	$ 38.7	$ 137.7	$ 141.0	$ 137.3
Earnings per share of common stock
Basic earnings per share (in USD per share)	$ 0.46	$ 0.40	$ 0.50	$ 0.40	$ 0.32	$ 0.42	$ 0.56	$ 0.49	$ 1.76	$ 1.79	$ 1.80
Diluted earnings per share (in USD per share)	$ 0.43	$ 0.38	$ 0.47	$ 0.37	$ 0.30	$ 0.39	$ 0.54	$ 0.47	$ 1.66	$ 1.69	$ 1.75
Weighted average shares of common stock outstanding, basic (in shares)									77.4	77.2	74.8
Weighted average shares of common stock outstanding, diluted (in shares)									81.9	81.7	76.8
Effect of Adoption | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales											$ 14.1
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)											27.2
Selling, general and administrative expense											0.0
Depreciation and amortization											0.0
Other income, net											0.0
Total operating costs and expenses											27.2
Operating income											(13.1)
Income before income taxes											(13.1)
Income tax provision											(3.5)
Net income											$ (9.6)
Earnings per share of common stock
Basic earnings per share (in USD per share)											$ (0.13)
Diluted earnings per share (in USD per share)											$ (0.12)
Weighted average shares of common stock outstanding, basic (in shares)											0.0
Weighted average shares of common stock outstanding, diluted (in shares)											0.0
Amounts Excluding Effect of Adoption
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales											$ 1,910.0
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)											1,576.6
Selling, general and administrative expense											43.8
Depreciation and amortization											72.3
Other income, net											(40.2)
Total operating costs and expenses											1,652.5
Operating income											257.5
Total non-operating expense, net											82.0
Income before income taxes											175.5
Income tax provision											47.8
Net income											$ 127.7
Earnings per share of common stock
Basic earnings per share (in USD per share)											$ 1.67
Diluted earnings per share (in USD per share)											$ 1.63
Weighted average shares of common stock outstanding, basic (in shares)											74.8
Weighted average shares of common stock outstanding, diluted (in shares)											76.8