Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator
Net income	$ 28.0	$ 50.9	$ 79.1	$ 107.1
Denominator
Basic weighted-average common shares outstanding	92.0	94.7	92.2	94.8
Dilutive shares	0.3	1.0	0.3	1.0
Weighted-average common and common equivalent shares outstanding - diluted	92.3	95.7	92.5	95.8
Per common share income
Net income, per basic share	$ 0.31	$ 0.54	$ 0.86	$ 1.13
Net income, per diluted share	$ 0.30	$ 0.53	$ 0.85	$ 1.12
Antidilutive stock options and stock awards	0.6	0.0	0.6	0.0