Condensed Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Net sales	$ 527.7	$ 481.8	$ 1,516.2	$ 1,458.5
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)	441.6	392.9	1,251.7	1,170.1
Selling, general and administrative expense	11.2	15.1	31.0	44.2
Depreciation and amortization	15.1	18.4	49.7	53.4
Other expense, net	3.3	0.3	1.2	1.3
Total operating costs and expenses	471.2	426.7	1,333.6	1,269.0
Operating income	56.5	55.1	182.6	189.5
Non-operating:
Retirement benefits expense	9.1	6.5	27.4	19.5
Interest income	(0.5)	(3.9)	(4.7)	(11.9)
Interest expense	7.2	9.0	23.0	27.2
Total non-operating expense, net	15.8	11.6	45.7	34.8
Income before income taxes	40.7	43.5	136.9	154.7
Income tax provision	9.0	10.6	34.6	39.0
Net income	$ 31.7	$ 32.9	$ 102.3	$ 115.7
Earnings per share of common stock
Basic earnings per share (in dollars per share)	$ 0.40	$ 0.42	$ 1.30	$ 1.47
Diluted earnings per share (in dollars per share)	$ 0.38	$ 0.39	$ 1.23	$ 1.39
Weighted average shares of common stock outstanding, basic (in shares)	77.6	77.3	77.6	77.1
Weighted average shares of common stock outstanding, diluted (in shares)	81.9	82.8	82.4	81.5