Earnings Per Share Computations (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Numerator
Net income	$ 53.1	$ 42.3	$ 102.4	$ 86.3
Income allocated to participating securities (in dollars)	(0.1)	(0.5)	(0.2)	(1.0)
Net income available to common shareholders (in dollars)	$ 53.0	$ 41.8	$ 102.2	$ 85.3
Denominator
Basic weighted-average common shares outstanding	102.9	103.1	103.0	103.1
Dilutive shares	1.0	0.7	1.1	0.7
Weighted-average common and common equivalent shares outstanding - diluted	103.9	103.8	104.1	103.8
Per common share income
Basic (in dollars per share)	$ 0.51	$ 0.41	$ 0.99	$ 0.83
Diluted (in dollars per share)	$ 0.51	$ 0.40	$ 0.98	$ 0.82
Basic weighted-average common shares outstanding	102.9	103.1	103.0	103.1
Basic weighted-average common shares, outstanding and participating securities	103.0	104.3	103.2	104.3
Percentage allocated to common shareholders	99.90%	98.80%	99.80%	98.80%
Antidilutive stock options and stock awards				0.1