Earnings Per Share Computations (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Numerator
Net income	$ 65.8	$ 53.1	$ 115.0	$ 102.4
Income allocated to participating securities (in dollars)	0	(0.1)	0	(0.2)
Net income available to common shareholders (in dollars)	$ 65.8	$ 53.0	$ 115.0	$ 102.2
Denominator
Basic weighted-average common shares outstanding	100.4	102.9	101.0	103.0
Dilutive shares	0.9	1.0	0.9	1.1
Weighted-average common and common equivalent shares outstanding - diluted	101.3	103.9	101.9	104.1
Per common share income
Basic (in dollars per share)	$ 0.66	$ 0.51	$ 1.14	$ 0.99
Diluted (in dollars per share)	$ 0.65	$ 0.51	$ 1.13	$ 0.98
Basic weighted-average common shares outstanding	100.4	102.9	101.0	103.0
Basic weighted-average common shares, outstanding and participating securities	100.4	103.0	101.0	103.2
Percentage allocated to common shareholders	100.00%	99.90%	100.00%	99.80%
Antidilutive stock options and stock awards	0.4		0.4