Earnings Per Share Computations (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
Net income	$ 56.8	$ 62.5	$ 65.6	$ 54.4	$ 59.1	$ 17.0	$ 65.8	$ 49.2	$ 239.3	$ 191.1	$ 212.6
Undistributed Earnings (Loss) Allocated to Participating Securities, Diluted									0.0	0.0	0.2
Net income available to common shareholders (in dollars)									$ 239.3	$ 191.1	$ 212.4
Denominator
Basic weighted-average common shares outstanding									96.7	100.2	102.9
Dilutive shares									1.2	1.0	1.0
Weighted-average common and common equivalent shares outstanding - diluted									97.9	101.2	103.9
Per common share income
Net income, per basic share	$ 0.59	$ 0.65	$ 0.68	$ 0.55	$ 0.60	$ 0.17	$ 0.66	$ 0.48	$ 2.47	$ 1.91	$ 2.06
Net income, per diluted share	$ 0.58	$ 0.64	$ 0.67	$ 0.55	$ 0.59	$ 0.17	$ 0.65	$ 0.48	$ 2.44	$ 1.89	$ 2.04
Basic weighted-average common shares outstanding									96.7	100.2	102.9
Basic weighted-average common shares, outstanding and participating securities									96.7	100.2	103.0
Percentage allocated to common shareholders									100.00%	100.00%	99.90%
Antidilutive stock options and stock awards										0.3	0.3