Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income	$ 249.8	$ 237.2	$ 209.4
Basic net income per common share:
Weighted average common shares outstanding — Basic (in shares)	92.8	95.8	96.8
Basic net income per common share:	$ 2.69	$ 2.48	$ 2.16
Diluted net income per common share:
Weighted average common shares outstanding — Basic (in shares)	92.8	95.8	96.8
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.4	0.5	0.7
Stock options (in shares)	1.0	0.9	1.2
Weighted average common shares outstanding — Diluted (in shares)	94.2	97.2	98.7
Diluted net income per common share:	$ 2.65	$ 2.44	$ 2.12
Anti-dilutive shares outstanding, excluded from computation (in shares)	0.4		0.2