Earnings Per Share Computations (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Numerator
Net income	$ 17.0	$ 54.0	$ 132.0	$ 156.4
Income allocated to participating securities	0	(0.1)	0	(0.2)
Net income available to common shareholders (in dollars)	$ 17.0	$ 53.9	$ 132.0	$ 156.2
Denominator
Basic weighted-average common shares outstanding	99.9	102.9	100.6	103.0
Dilutive shares	1.0	0.9	0.9	1.0
Weighted-average common and common equivalent shares outstanding - diluted	100.9	103.8	101.5	104.0
Per common share income
Net income, per basic share	$ 0.17	$ 0.52	$ 1.31	$ 1.52
Net income, per diluted share	$ 0.17	$ 0.52	$ 1.30	$ 1.50
Basic weighted-average common shares outstanding	99.9	102.9	100.6	103.0
Basic weighted-average common shares, outstanding and participating securities	99.9	103.0	100.6	103.1
Percentage allocated to common shareholders	100.00%	99.90%	100.00%	99.90%
Antidilutive stock options and stock awards	0.4	0.1	0.4	0.1