Earnings Per Common Share (Earnings Per Share Computation) (Detail) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income (loss)	$ 133,900,000	$ (47,800,000)	$ 272,600,000	$ 84,100,000
Less: Income allocated to participating securities	2,600,000		5,300,000	1,700,000
Net income (loss) available to common shareholders	$ 131,300,000	$ (47,800,000)	$ 267,300,000	$ 82,400,000
Weighted average common shares outstanding	83.9	86.2	84.0	85.8
Dilutive common share equivalents from share-based compensation plans	1.2		1.1	2.0
Weighted average common and common equivalent shares outstanding, assuming dilution	85.1	86.2	85.1	87.8