Earnings Per Share Computation (Detail) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 129,300,000	$ 130,100,000	$ 401,900,000	$ 214,200,000
Less: Income allocated to participating securities	2,400,000	2,300,000	7,700,000	4,000,000
Net income available to common shareholders	$ 126,900,000	$ 127,800,000	$ 394,200,000	$ 210,200,000
Weighted average common shares outstanding	83.4	86.6	83.8	86.0
Dilutive common share equivalents from share-based compensation plans	1.2	1.2	1.1	1.7
Weighted average common and common equivalent shares outstanding, assuming dilution	84.6	87.8	84.9	87.7