Earnings Per Share Computation (Detail) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net (loss) income	$ (161,600,000)	$ 133,900,000	$ (70,900,000)	$ 272,600,000
Less: Income allocated to participating securities	0	2,600,000	0	5,300,000
Net (loss) income available to common shareholders	$ (161,600,000)	$ 131,300,000	$ (70,900,000)	$ 267,300,000
Weighted average common shares outstanding	79.8	83.9	80.6	84.0
Dilutive common share equivalents from share-based compensation plans	0	1.2	0	1.1
Weighted average common and common equivalent shares outstanding, assuming dilution	79.8	85.1	80.6	85.1