Earnings Per Share Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator [Abstract]
Net income									$ 235.6	$ 143.0	$ 94.5
Less: Net income attributable to participating securities									(5.3)	(2.9)	(1.9)
Net income attributable to common shareholders - Basic									230.3	140.1	92.6
Undistributed earnings reallocated to common shareholders under two step dilutive method									0.0	0.0	0.0
Net income attributable to common shareholders - Dilutive									$ 230.3	$ 140.1	$ 92.6
Denominator [Abstract]
Weighted-average shares outstanding									77.7	77.3	76.7
Less: Participating securities									(1.8)	(1.6)	(1.5)
Weighted-average common shares outstanding									75.9	75.7	75.2
Effect of dilutive securities - dilutive stock options									0.4	0.2	0.1
Diluted shares outstanding - adjusted weighted-average shares after assumed conversions									76.3	75.9	75.3
Basic earnings per share	$ 0.81	$ 0.78	$ 0.75	$ 0.69	$ 0.17	$ 0.57	$ 0.67	$ 0.44	$ 3.03	$ 1.85	$ 1.23
Diluted earnings per share	$ 0.81	$ 0.78	$ 0.75	$ 0.68	$ 0.17	$ 0.57	$ 0.67	$ 0.44	$ 3.02	$ 1.85	$ 1.23