Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator
Net income	$ 55.6	$ 68.6	$ 134.7	$ 175.7
Denominator
Basic weighted-average common shares outstanding	90.9	94.3	91.8	94.6
Dilutive shares	0.3	0.7	0.3	0.9
Weighted-average common and common equivalent shares outstanding - diluted	91.2	95.0	92.1	95.5
Per common share income
Net income, per basic share	$ 0.61	$ 0.73	$ 1.47	$ 1.86
Net income, per diluted share	$ 0.61	$ 0.72	$ 1.46	$ 1.84
Antidilutive stock options and stock awards	0.6	0.0	0.6	0.0