Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Net sales	$ 2,072.7	$ 1,981.5	$ 1,895.9
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)	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 expense (income), net:
Environmental remediation provision adjustments	4.3	2.1	(36.9)
Other	5.1	(0.5)	(3.3)
Total operating costs and expenses	1,832.1	1,743.3	1,625.3
Operating income	240.6	238.2	270.6
Non-operating:
Retirement benefits expense	36.6	26.1	57.6
Interest income	(6.3)	(15.5)	(10.0)
Interest expense	30.1	35.7	34.4
Total non-operating expense, net	60.4	46.3	82.0
Income before income taxes	180.2	191.9	188.6
Income tax provision	42.5	50.9	51.3
Net income	$ 137.7	$ 141.0	$ 137.3
Earnings per share of common stock
Basic earnings per share (in USD per share)	$ 1.76	$ 1.79	$ 1.80
Diluted earnings per share (in USD per share)	$ 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
Cash dividends declared per share (in USD per share)	$ 5.00	$ 0	$ 0