Condensed Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Net sales	$ 512.4	$ 485.0	$ 988.5	$ 976.7
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)	415.2	379.6	810.1	777.2
Selling, general and administrative expense	10.7	16.9	19.8	29.1
Depreciation and amortization	17.4	17.5	34.6	35.0
Other expense (income), net	0.1	(0.1)	(2.1)	1.0
Total operating costs and expenses	443.4	413.9	862.4	842.3
Operating income	69.0	71.1	126.1	134.4
Non-operating:
Retirement benefits expense	9.1	6.5	18.3	13.0
Interest income	(1.0)	(4.0)	(4.2)	(8.0)
Interest expense	7.4	9.2	15.8	18.2
Total non-operating expense, net	15.5	11.7	29.9	23.2
Income before income taxes	53.5	59.4	96.2	111.2
Income tax provision	14.3	15.3	25.6	28.4
Net income	$ 39.2	$ 44.1	$ 70.6	$ 82.8
Earnings per share of common stock
Basic earnings per share (in dollars per share)	$ 0.50	$ 0.56	$ 0.90	$ 1.05
Diluted earnings per share (in dollars per share)	$ 0.47	$ 0.54	$ 0.84	$ 1.01
Weighted average shares of common stock outstanding, basic (in shares)	77.6	77.1	77.5	77.1
Weighted average shares of common stock outstanding, diluted (in shares)	82.1	80.7	82.6	80.6