Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Basic net income per common share:
Net Income	$ 53.5	$ 55.8	$ 183.3	$ 156.5
Weighted average common shares outstanding (in shares)	95.8	95.8	96.2	97.2
Basic net income per common share ( in dollars per share)	$ 0.56	$ 0.58	$ 1.91	$ 1.61
Diluted net income per common share:
Net income	$ 53.5	$ 55.8	$ 183.3	$ 156.5
Weighted average common shares outstanding (in shares)	95.8	95.8	96.2	97.2
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.5	0.7	0.5	0.8
Stock options (in shares)	1.0	1.2	1.0	1.2
Weighted average common shares outstanding - Dilutive (in shares)	97.3	97.7	97.7	99.2
Dilutive net income per common share (in dollars per share)	$ 0.55	$ 0.57	$ 1.88	$ 1.58