Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator
Net income	$ 47.6	$ 51.1
Denominator
Basic weighted-average common shares outstanding	91.0	92.4
Dilutive shares	0.2	0.4
Weighted-average common and common equivalent shares outstanding - diluted	91.2	92.8
Per common share income
Net income, per basic share	$ 0.52	$ 0.55
Net income, per diluted share	$ 0.52	$ 0.55
Antidilutive stock options and stock awards	0.8	0.5