Earnings Per Share Computation (Detail) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 148,400,000	$ 90,700,000
Less: Income allocated to participating securities	2,500,000	1,700,000
Net income available to common shareholders	$ 145,900,000	$ 89,000,000
Weighted average common shares outstanding	77.0	81.3
Dilutive common share equivalents from share-based compensation plans	1.5	1.2
Weighted average common and common equivalent shares outstanding, assuming dilution	78.5	82.5