Net Earnings (Loss) Per Common Share - Calculation of Basic and Diluted Net Earnings (Loss) Per Common Share (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Numerator
Net earnings (loss) available to common stockholders	$ 37.7	$ (1,232.4)	$ 96.7	$ (1,252.0)
Distributed and allocated undistributed net earnings (loss) to non-vested restricted stockholders	(0.2)	(0.1)	(0.5)	(0.4)
Distributed and allocated undistributed net earnings (loss) to common stockholders	37.5	(1,232.5)	96.2	(1,252.4)
Distributed net earnings - dividends paid to common stockholders	(25.3)	(25.1)	(75.9)	(75.3)
Allocation of undistributed net earnings (loss) to common stockholders	12.2	(1,257.6)	20.3	(1,327.7)
Denominator
Weighted average number of common shares outstanding - basic	194.9	193.2	194.5	192.7
Basic net earnings (loss) per common share:
Distributed net earnings (loss) to common stockholders	$ 0.13	$ 0.13	$ 0.39	$ 0.39
Allocated undistributed net earnings (loss) to common stockholders	$ 0.06	$ (6.51)	$ 0.11	$ (6.89)
Net earnings (loss) per common share - basic	$ 0.19	$ (6.38)	$ 0.50	$ (6.50)
Numerator
Distributed and allocated undistributed net earnings (loss) to common stockholders	37.5	(1,232.5)	96.2	(1,252.4)
Add: Allocated undistributed net earnings (loss) to non-vested restricted stockholders	0.1		0.1
Less: Undistributed net earnings (loss) reallocated to non-vested restricted stockholders	(0.1)		(0.1)
Net earnings (loss) available to common stockholders - diluted	$ 37.5	$ (1,232.5)	$ 96.2	$ (1,252.4)
Weighted average number of common shares outstanding:
Weighted average number of common shares outstanding - basic	194.9	193.2	194.5	192.7
Effect of assumed issuance of Settlement agreement shares	18.0		18.0
Effect of non-vested restricted stock units	0.8		0.9
Weighted average number of common shares outstanding - diluted	213.7	193.2	213.4	192.7
Diluted net earnings (loss) per common share	$ 0.18	$ (6.38)	$ 0.45	$ (6.50)