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
	Mar. 31, 2016	Mar. 31, 2015
Numerator:
Net income	$ 103.7	$ 90.8
Less: distributed and undistributed earnings allocated to participating securities	(1.1)	(1.2)
Net income attributable to common shareholders	$ 102.6	$ 89.6
Denominator:
Weighted average basic common shares outstanding	94.1	97.1
Effect of dilutive securities	0.1	0.1
Diluted common shares outstanding	94.2	97.2
Basic income per common share (in dollars per share)	$ 1.09	$ 0.92
Diluted income per common share (in dollars per share)	$ 1.09	$ 0.92