Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic net income per common share:
Net income	$ 284.0	$ 249.8	$ 237.2
Weighted average common shares outstanding — Basic (in shares)	90.6	92.8	95.8
Basic net income per common share	$ 3.13	$ 2.69	$ 2.48
Diluted net income per common share:
Weighted average common shares outstanding — Basic (in shares)	90.6	92.8	95.8
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.4	0.4	0.5
Stock options (in shares)	0.9	1.0	0.9
Weighted average common shares outstanding — Dilutive (in shares)	91.9	94.2	97.2
Dilutive net income per common share	$ 3.09	$ 2.65	$ 2.44
Anti-dilutive shares outstanding, excluded from computation (in shares)	0.2	0.4