Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator
Net income	$ 46.7	$ 28.0	$ 94.3	$ 79.1
Denominator
Basic weighted-average common shares outstanding	91.0	92.0	91.0	92.2
Dilutive shares	0.2	0.3	0.2	0.3
Weighted-average common and common equivalent shares outstanding - diluted	91.2	92.3	91.2	92.5
Per common share income
Net income, per basic share	$ 0.51	$ 0.31	$ 1.03	$ 0.86
Net income, per diluted share	$ 0.51	$ 0.30	$ 1.03	$ 0.85
Antidilutive stock options and stock awards	0.7	0.6	0.7	0.6