Earnings Per Share - Computation of Basic and Diluted 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
Numerator:
Net income	$ 115.9	$ 114.0	$ 219.6	$ 204.8
Less: distributed and undistributed earnings allocated to participating securities	(1.2)	(1.4)	(2.3)	(2.6)
Net income attributable to common shareholders	$ 114.7	$ 112.6	$ 217.3	$ 202.2
Denominator:
Weighted average basic common shares outstanding	93.2	96.8	93.6	97.0
Effect of dilutive securities	0.1	0.1	0.1	0.1
Weighted average diluted common shares outstanding	93.3	96.9	93.7	97.1
Basic income per common share (in dollars per share)	$ 1.23	$ 1.16	$ 2.32	$ 2.09
Diluted income per common share (in dollars per share)	$ 1.23	$ 1.16	$ 2.32	$ 2.08