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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income	$ 179.8	$ 206.7	$ 560.2	$ 533.4
Less: Distributed and undistributed earnings allocated to participating securities	(1.3)	(1.6)	(4.2)	(4.1)
Net income attributable to common shareholders	$ 178.5	$ 205.1	$ 556.0	$ 529.3
Denominator:
Weighted average basic common shares outstanding (in shares)	93.9	93.7	93.8	93.7
Effect of dilutive securities (in shares)	0.4	0.3	0.3	0.2
Weighted average diluted common shares outstanding (in shares)	94.3	94.0	94.1	93.9
Basic income per common share (in dollars per share)	$ 1.90	$ 2.19	$ 5.93	$ 5.65
Diluted income per common share (in dollars per share)	$ 1.89	$ 2.18	$ 5.91	$ 5.64