Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Basic net income per common share:
Net Income	$ 68.2	$ 53.5	$ 190.3	$ 183.3
Weighted average common shares outstanding — Basic (in shares)	92.7	95.8	93.1	96.2
Basic net income per common share (in dollars per share)	$ 0.74	$ 0.56	$ 2.04	$ 1.91
Diluted net income per common share:
Net income	$ 68.2	$ 53.5	$ 190.3	$ 183.3
Weighted average common shares outstanding — Basic (in shares)	92.7	95.8	93.1	96.2
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.4	0.5	0.5	0.5
Stock options (in shares)	1.0	1.0	1.0	1.0
Weighted average common shares outstanding — Dilutive (in shares)	94.1	97.3	94.6	97.7
Dilutive net income per common share (in dollars per share)	$ 0.72	$ 0.55	$ 2.01	$ 1.88