Earnings Per Share Computations (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Numerator
Net income	$ 49.3	$ 44.0
Income allocated to participating securities (in dollars)	0	(0.5)
Net income available to common shareholders (in dollars)	$ 49.3	$ 43.5
Denominator
Basic weighted-average common shares outstanding	103.2	103.1
Dilutive shares	1.1	0.6
Weighted-average common and common equivalent shares outstanding - diluted	104.3	103.7
Per common share income
Basic (in dollars per share)	$ 0.48	$ 0.42
Diluted (in dollars per share)	$ 0.47	$ 0.42
Basic weighted-average common shares outstanding	103.2	103.1
Basic weighted-average common shares, outstanding and participating securities	103.3	104.4
Percentage allocated to common shareholders	99.90%	98.80%
Antidilutive stock options and stock awards		0.1