Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator
Net income	$ 68.6	$ 62.5	$ 175.7	$ 182.5
Denominator
Basic weighted-average common shares outstanding	94.3	96.4	94.6	97.0
Dilutive shares	0.7	1.3	0.9	1.2
Weighted-average common and common equivalent shares outstanding - diluted	95.0	97.7	95.5	98.2
Per common share income
Net income, per basic share	$ 0.73	$ 0.65	$ 1.86	$ 1.88
Net income, per diluted share	$ 0.72	$ 0.64	$ 1.84	$ 1.86
Antidilutive stock options and stock awards	0.0	0.0	0.0	0.0