Earnings Per Share Computation (Detail) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 93,200,000	$ 129,300,000	$ 22,300,000	$ 401,900,000
Less: Income allocated to participating securities	1,600,000	2,400,000	300,000	7,700,000
Net income available to common shareholders	$ 91,600,000	$ 126,900,000	$ 22,000,000	$ 394,200,000
Weighted average common shares outstanding	78.5	83.4	79.9	83.8
Dilutive common share equivalents from share-based compensation plans	1.5	1.2	1.3	1.1
Weighted average common and common equivalent shares outstanding, assuming dilution	80.0	84.6	81.2	84.9