Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Basic net income per common share:
Net Income	$ 61.7	$ 50.6	$ 129.8	$ 100.7
Weighted average common shares outstanding (in shares)	96.6	97.3	96.4	97.9
Basic net income per common share ( in dollars per share)	$ 0.64	$ 0.52	$ 1.35	$ 1.03
Diluted net income per common share:
Net income	$ 61.7	$ 50.6	$ 129.8	$ 100.7
Weighted average common shares outstanding (in shares)	96.6	97.3	96.4	97.9
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.6	0.7	0.6	0.7
Stock options (in shares)	1.0	1.2	1.0	1.3
Weighted average common shares outstanding - Dilutive (in shares)	98.2	99.2	98.0	99.9
Diluted net income per common share (in dollars per share)	$ 0.63	$ 0.51	$ 1.32	$ 1.01