Earnings Per Share Computations (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Numerator
Net income	$ 54.0	$ 47.4	$ 156.4	$ 133.7
Income allocated to participating securities (in dollars)	(0.1)	(0.4)	(0.2)	(1.5)
Net income available to common shareholders (in dollars)	$ 53.9	$ 47.0	$ 156.2	$ 132.2
Denominator
Basic weighted-average common shares outstanding	102.9	103.3	103.0	103.2
Dilutive shares	0.9	0.6	1.0	0.7
Weighted-average common and common equivalent shares outstanding - diluted	103.8	103.9	104.0	103.9
Per common share income
Basic (in dollars per share)	$ 0.52	$ 0.46	$ 1.52	$ 1.28
Diluted (in dollars per share)	$ 0.52	$ 0.45	$ 1.50	$ 1.27
Basic weighted-average common shares outstanding	102.9	103.3	103.0	103.2
Basic weighted-average common shares, outstanding and participating securities	103.0	104.2	103.1	104.3
Percentage allocated to common shareholders	99.90%	99.10%	99.90%	98.90%
Antidilutive stock options and stock awards	0.1