Income Per Share of Common Stock - Reconciliation of Numerator and Denominator (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
Numerator:
Net income	$ 34.8	$ 24.3	$ 48.8	$ 30.2
Income allocated to participating securities, basic	(0.8)	(0.5)	(1.0)	(0.7)
Net income for basic earnings per share	34.0	23.8	47.8	29.5
Interest on 4 1/16% Convertible Subordinated Debentures (“4 1/16% Debentures”)	0.0	0.0	0.0	0.1
Net income for diluted earnings per share	$ 34.0	$ 23.8	$ 47.8	$ 29.6
Denominator:
Basic weighted average shares (in shares)	73.9	73.2	73.8	72.6
Effect of:
Employee stock options and stock purchase plan (in shares)	0.1	0.0	0.1	0.0
Diluted weighted average shares (in shares)	75.1	73.2	75.0	72.7
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
4.0625% Debentures
Effect of:
Convertible subordinated notes	0.0	0.0	0.0	0.1
2 1/4% Notes
Effect of:
Convertible subordinated notes	1.1	0.0	1.1	0.0
Diluted
Debt instrument interest rate stated percentage	2.25%		2.25%
Convertible Debt Securities | 4.0625% Debentures
Diluted
Debt instrument interest rate stated percentage	4.0625%		4.0625%