Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator
Net income	$ 62.5	$ 17.0	$ 182.5	$ 132.0
Denominator
Basic weighted-average common shares outstanding	96.4	99.9	97.0	100.6
Dilutive shares	1.3	1.0	1.2	0.9
Weighted-average common and common equivalent shares outstanding - diluted	97.7	100.9	98.2	101.5
Per common share income
Net income, per basic share	$ 0.65	$ 0.17	$ 1.88	$ 1.31
Net income, per diluted share	$ 0.64	$ 0.17	$ 1.86	$ 1.30
Antidilutive stock options and stock awards	0.0	0.4	0.0	0.4