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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ 467.2	$ 459.6	$ 959.2	$ 864.9
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)	369.5	370.6	796.3	710.0
Selling, general and administrative expense	11.1	9.7	17.8	23.2
Depreciation and amortization	17.7	19.1	35.4	35.4
Other expense (income), net	0.9	(1.3)	2.1	0.4
Total operating costs and expenses	399.2	398.1	851.6	769.0
Operating income	68.0	61.5	107.6	95.9
Non-operating (income) expense:
Retirement benefits expense	14.4	18.3	28.8	36.6
Interest income	(2.0)	(0.8)	(3.6)	(1.3)
Interest expense	8.3	7.8	16.4	15.2
Total non-operating expense, net	20.7	25.3	41.6	50.5
Income before income taxes	47.3	36.2	66.0	45.4
Income tax provision	12.5	11.9	17.2	15.2
Net income	$ 34.8	$ 24.3	$ 48.8	$ 30.2
Basic
Net favorable effect of the changes in contract estimates on basic net income per share	$ 0.46	$ 0.32	$ 0.65	$ 0.41
Diluted
Net favorable effect of the changes in contract estimates on diluted net income per share	$ 0.45	$ 0.32	$ 0.64	$ 0.41
Weighted average shares of common stock outstanding, basic (in shares)	73.9	73.2	73.8	72.6
Weighted average shares of common stock outstanding, diluted (in shares)	75.1	73.2	75.0	72.7
Effect of Adoption | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ 9.4		$ (13.2)
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)	12.9		(1.8)
Selling, general and administrative expense	0.0		0.0
Depreciation and amortization	0.0		0.0
Other expense (income), net	0.0		0.0
Total operating costs and expenses	12.9		(1.8)
Operating income	(3.5)		(11.4)
Non-operating (income) expense:
Retirement benefits expense	0.0		0.0
Interest income	0.0		0.0
Interest expense	0.0		0.0
Total non-operating expense, net	0.0		0.0
Income before income taxes	(3.5)		(11.4)
Income tax provision	(1.1)		(3.1)
Net income	$ (2.4)		$ (8.3)
Basic
Net favorable effect of the changes in contract estimates on basic net income per share	$ (0.03)		$ (0.11)
Diluted
Net favorable effect of the changes in contract estimates on diluted net income per share	$ (0.03)		$ (0.11)
Weighted average shares of common stock outstanding, basic (in shares)	0.0		0.0
Weighted average shares of common stock outstanding, diluted (in shares)	0.0		0.0
Amounts Excluding Effect of Adoption
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ 476.6		$ 946.0
Operating costs and expenses:
Cost of sales (exclusive of items shown separately below)	382.4		794.5
Selling, general and administrative expense	11.1		17.8
Depreciation and amortization	17.7		35.4
Other expense (income), net	0.9		2.1
Total operating costs and expenses	412.1		849.8
Operating income	64.5		96.2
Non-operating (income) expense:
Retirement benefits expense	14.4		28.8
Interest income	(2.0)		(3.6)
Interest expense	8.3		16.4
Total non-operating expense, net	20.7		41.6
Income before income taxes	43.8		54.6
Income tax provision	11.4		14.1
Net income	$ 32.4		$ 40.5
Basic
Net favorable effect of the changes in contract estimates on basic net income per share	$ 0.43		$ 0.54
Diluted
Net favorable effect of the changes in contract estimates on diluted net income per share	$ 0.42		$ 0.53
Weighted average shares of common stock outstanding, basic (in shares)	73.9		73.8
Weighted average shares of common stock outstanding, diluted (in shares)	75.1		75.0