Earnings Per Share Computations (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Numerator
Net income	$ 49.2	$ 49.3
Income allocated to participating securities (in dollars)	0	0
Net income available to common shareholders (in dollars)	$ 49.2	$ 49.3
Denominator
Basic weighted-average common shares outstanding	101.5	103.2
Dilutive shares	0.9	1.1
Weighted-average common and common equivalent shares outstanding - diluted	102.4	104.3
Per common share income
Basic (in dollars per share)	$ 0.48	$ 0.48
Diluted (in dollars per share)	$ 0.48	$ 0.47
Basic weighted-average common shares outstanding	101.5	103.2
Basic weighted-average common shares, outstanding and participating securities	101.5	103.3
Percentage allocated to common shareholders	100.00%	99.90%
Antidilutive stock options and stock awards	0.4