Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator
Net income	$ (40.7)	$ 55.6	$ 28.0	$ 51.1	$ 60.5	$ 68.6	$ 50.9	$ 56.2	$ 94.0	$ 236.2	$ 239.3
Denominator
Basic weighted-average common shares outstanding									91.5	94.3	96.7
Dilutive shares									0.4	0.8	1.2
Weighted-average common and common equivalent shares outstanding - diluted									91.9	95.1	97.9
Per common share income
Net income, per basic share	$ (0.44)	$ 0.61	$ 0.31	$ 0.55	$ 0.65	$ 0.73	$ 0.54	$ 0.59	$ 1.03	$ 2.51	$ 2.47
Net income, per diluted share	$ (0.44)	$ 0.61	$ 0.30	$ 0.55	$ 0.64	$ 0.72	$ 0.53	$ 0.59	$ 1.02	$ 2.48	$ 2.44
Basic weighted-average common shares outstanding									91.5	94.3	96.7
Antidilutive stock options and stock awards									0.4	0.0	0.0