Condensed Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Net sales	$ 545.3	$ 527.7	$ 1,598.3	$ 1,516.2
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)	446.5	441.6	1,310.6	1,251.7
Selling, general and administrative expense	5.2	11.2	27.8	31.0
Depreciation and amortization	13.5	15.1	44.4	49.7
Other expense, net	8.7	3.3	24.0	1.2
Total operating costs and expenses	473.9	471.2	1,406.8	1,333.6
Operating income	71.4	56.5	191.5	182.6
Non-operating:
Retirement benefits expense	8.5	9.1	25.4	27.4
Loss on debt	0.9	0.0	10.5	0.0
Interest income	(0.2)	(0.5)	(1.5)	(4.7)
Interest expense	4.9	7.2	15.1	23.0
Total non-operating expense, net	14.1	15.8	49.5	45.7
Income before income taxes	57.3	40.7	142.0	136.9
Income tax provision	14.7	9.0	36.3	34.6
Net income	$ 42.6	$ 31.7	$ 105.7	$ 102.3
Earnings per share of common stock
Basic earnings per share (in dollars per share)	$ 0.53	$ 0.40	$ 1.33	$ 1.30
Diluted earnings per share (in dollars per share)	$ 0.52	$ 0.38	$ 1.29	$ 1.23
Weighted average shares of common stock outstanding, basic (in shares)	79.9	77.6	79.0	77.6
Weighted average shares of common stock outstanding, diluted (in shares)	82.3	81.9	81.5	82.4
Cash dividends paid per share (in USD per share)	$ 0	$ 0	$ 5.00	$ 0