Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator
Net income	$ 60.5	$ 68.6	$ 50.9	$ 56.2	$ 56.8	$ 62.5	$ 65.6	$ 54.4	$ 236.2	$ 239.3	$ 191.1
Denominator
Basic weighted-average common shares outstanding									94.3	96.7	100.2
Dilutive shares									0.8	1.2	1.0
Weighted-average common and common equivalent shares outstanding - diluted									95.1	97.9	101.2
Per common share income
Net income, per basic share	$ 0.65	$ 0.73	$ 0.54	$ 0.59	$ 0.59	$ 0.65	$ 0.68	$ 0.55	$ 2.51	$ 2.47	$ 1.91
Net income, per diluted share	$ 0.64	$ 0.72	$ 0.53	$ 0.59	$ 0.58	$ 0.64	$ 0.67	$ 0.55	$ 2.48	$ 2.44	$ 1.89
Basic weighted-average common shares outstanding									94.3	96.7	100.2
Antidilutive stock options and stock awards										0.0	0.3