Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Basic net income per common share:
Net Income	$ 66.1	$ 61.7	$ 122.1	$ 129.8
Weighted average common shares outstanding — Basic (in shares)	93.1	96.6	93.3	96.4
Basic net income per common share ( in dollars per share)	$ 0.71	$ 0.64	$ 1.31	$ 1.35
Diluted net income per common share:
Net income	$ 66.1	$ 61.7	$ 122.1	$ 129.8
Weighted average common shares outstanding — Basic (in shares)	93.1	96.6	93.3	96.4
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.5	0.6	0.4	0.6
Stock options (in shares)	1.0	1.0	1.0	1.0
Weighted average common shares outstanding — Dilutive (in shares)	94.6	98.2	94.7	98.0
Dilutive net income per common share (in dollars per share)	$ 0.70	$ 0.63	$ 1.29	$ 1.32