Earnings Per Share Computation (Detail) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 90,700,000	$ 138,700,000
Less: Income allocated to participating securities	1,700,000	2,700,000
Net income available to common shareholders	$ 89,000,000	$ 136,000,000
Weighted average common shares outstanding	81.3	84.1
Dilutive common share equivalents from share-based compensation plans	1.2	1.0
Weighted average common and common equivalent shares outstanding, assuming dilution	82.5	85.1