Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Basic net income per common share:
Net income	$ 64.6	$ 56.0
Weighted average common shares outstanding — Basic (in shares)	91.4	93.4
Basic net income per common share	$ 0.71	$ 0.60
Diluted net income per common share:
Net income	$ 64.6	$ 56.0
Weighted average common shares outstanding — Basic (in shares)	91.4	93.4
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.5	0.4
Stock options (in shares)	1.0	1.0
Weighted average common shares outstanding — Dilutive (in shares)	92.9	94.8
Dilutive net income per common share	$ 0.70	$ 0.59