Adoption of Revenue Recognition - Effect of Revenue Recognition on Condensed Consolidated Statement of Operations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ 492.0	$ 405.3
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)	426.8	339.4
Selling, general and administrative expense	6.7	13.5
Depreciation and amortization	17.7	16.3
Other expense, net	1.2	1.7
Total operating costs and expenses	452.4	370.9
Operating income	39.6	34.4
Non-operating (income) expense:
Retirement benefits expense	14.4	18.3
Interest income	(1.6)	(0.5)
Interest expense	8.1	7.4
Total non-operating expense, net	20.9	25.2
Income before income taxes	18.7	9.2
Income tax provision	4.7	3.3
Net income	$ 14.0	$ 5.9
Basic
Basic net income per share (in USD per share)	$ 0.19	$ 0.08
Diluted
Diluted net income per share (in USD per share)	$ 0.18	$ 0.08
Weighted average shares of common stock outstanding, basic (in shares)	73.7	72.3
Weighted average shares of common stock outstanding, diluted (in shares)	74.7	72.3
Effect of Adoption | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ (22.6)
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)	(14.7)
Selling, general and administrative expense	0.0
Depreciation and amortization	0.0
Other expense, net	0.0
Total operating costs and expenses	(14.7)
Operating income	(7.9)
Non-operating (income) expense:
Retirement benefits expense	0.0
Interest income	0.0
Interest expense	0.0
Total non-operating expense, net	0.0
Income before income taxes	(7.9)
Income tax provision	(2.0)
Net income	$ (5.9)
Basic
Basic net income per share (in USD per share)	$ (0.08)
Diluted
Diluted net income per share (in USD per share)	$ (0.07)
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	$ 469.4
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)	412.1
Selling, general and administrative expense	6.7
Depreciation and amortization	17.7
Other expense, net	1.2
Total operating costs and expenses	437.7
Operating income	31.7
Non-operating (income) expense:
Retirement benefits expense	14.4
Interest income	(1.6)
Interest expense	8.1
Total non-operating expense, net	20.9
Income before income taxes	10.8
Income tax provision	2.7
Net income	$ 8.1
Basic
Basic net income per share (in USD per share)	$ 0.11
Diluted
Diluted net income per share (in USD per share)	$ 0.11
Weighted average shares of common stock outstanding, basic (in shares)	73.7
Weighted average shares of common stock outstanding, diluted (in shares)	74.7