Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Numerator
Net income	$ 56.2	$ 54.4
Denominator
Basic weighted-average common shares outstanding	94.9	97.7
Dilutive shares	1.0	1.1
Weighted-average common and common equivalent shares outstanding - diluted	95.9	98.8
Per common share income
Net income, per basic share	$ 0.59	$ 0.55
Net income, per diluted share	$ 0.59	$ 0.55
Antidilutive stock options and stock awards	0.0	0.0