Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator
Net income	$ 65.6	$ 65.8	$ 120.0	$ 115.0
Denominator
Basic weighted-average common shares outstanding	96.9	100.4	97.3	101.0
Dilutive shares	1.2	0.9	1.1	0.9
Weighted-average common and common equivalent shares outstanding - diluted	98.1	101.3	98.4	101.9
Per common share income
Net income, per basic share	$ 0.68	$ 0.66	$ 1.23	$ 1.14
Net income, per diluted share	$ 0.67	$ 0.65	$ 1.22	$ 1.13
Antidilutive stock options and stock awards	0.0	0.4	0.0	0.4