Earnings Per 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	$ 71.0	$ 58.6	$ 132.1	$ 111.8
Less: Net income attributable to participating securities	(1.7)	(1.4)	(3.0)	(2.6)
Net income attributable to common shareholders - Basic	69.3	57.2	129.1	109.2
Undistributed earnings reallocated to common shareholders under two step dilutive method	0.0	0.0	0.0	0.0
Net income attributable to common shareholders - Dilutive	$ 69.3	$ 57.2	$ 129.1	$ 109.2
Denominators
Basic - Weighted-average shares outstanding	78.3	77.8	78.1	77.7
Basic - Less: Participating securities	(1.8)	(1.9)	(1.8)	(1.9)
Basic - Weighted-average common shares outstanding	76.5	75.9	76.3	75.8
Effect of dilutive securities - dilutive stock-based compensation	0.4	0.4	0.4	0.4
Diluted - Adjusted weighted-average shares after assumed conversions	76.9	76.3	76.7	76.2
Basic EPS	$ 0.91	$ 0.75	$ 1.69	$ 1.44
Diluted EPS	$ 0.90	$ 0.75	$ 1.68	$ 1.43