Net (Loss) Earnings per Common Share - Calculation of Basic and Diluted Net (Loss) Earnings Per Common Share (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share Basic And Diluted By Common Class Including Two Class Method [Line Items]
Net (loss) earnings available to common stockholders	$ (1,410.3)	$ 149.1	$ 255.9
Distributed and allocated undistributed net (loss) earnings to non-vested restricted stockholders	(0.5)	(0.9)	(1.6)
Distributed and allocated undistributed net (loss) earnings to common stockholders	(1,410.8)	148.2	254.3
Distributed net (loss) earnings - dividends paid to common stockholders	(100.4)	(86.8)	(79.2)
Allocation of undistributed net (loss) earnings to common stockholders	(1,511.2)	61.4	175.1
Weighted average number of common shares outstanding - basic	192.8	167.0	158.3
Basic net (loss) earnings per common share:
Distributed net (loss) earnings to common stockholders	$ 0.52	$ 0.52	$ 0.50
Allocated undistributed net (loss) earnings to common stockholders	$ (7.83)	$ 0.37	$ 1.11
Basic net (loss) earnings per common share:	$ (7.31)	$ 0.89	$ 1.61
Distributed and allocated undistributed net (loss) earnings to common stockholders	(1,410.8)	148.2	254.3
Add: Allocated undistributed net (loss) earnings to non-vested restricted stockholders		0.4	1.1
Less: Undistributed net (loss) earnings reallocated to non-vested restricted stockholders		(0.3)	(1.0)
Net (loss) earnings available to common stockholders - diluted	$ (1,410.8)	$ 148.3	$ 254.4
Weighted average number of common shares outstanding - basic	192.8	167.0	158.3
Effect of assumed issuance of Settlement agreement shares		18.0	18.0
Effect of non-vested restricted stock units		0.4	0.4
Weighted average number of common shares outstanding - diluted	192.8	185.4	176.7
Diluted net (loss) earnings per common share	$ (7.31)	$ 0.80	$ 1.44