Net Earnings (Loss) Per Common Share - Calculation of Basic and Diluted Net Earnings Per Common Share (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Numerator
Net earnings available to common stockholders	$ 60.3	$ 56.3	$ 132.1	$ 59.0
Distributed and allocated undistributed net earnings to non-vested restricted stockholders	(0.3)	(0.2)	(0.7)	(0.3)
Distributed and allocated undistributed net earnings to common stockholders	60.0	56.1	131.4	58.7
Distributed net earnings - dividends paid to common stockholders	(27.9)	(25.3)	(55.6)	(50.6)
Allocation of undistributed net earnings to common stockholders	32.1	30.8	75.8	8.1
Denominator
Weighted average number of common shares outstanding - basic	213.5	194.8	210.1	194.3
Basic net earnings per common share:
Distributed net earnings to common stockholders	$ 0.13	$ 0.13	$ 0.26	$ 0.26
Allocated undistributed net earnings (loss) to common stockholders	$ 0.15	$ 0.16	$ 0.37	$ 0.04
Net earnings per common share-basic	$ 0.28	$ 0.29	$ 0.63	$ 0.30
Numerator
Distributed and allocated undistributed net earnings to common stockholders	60.0	56.1	131.4	58.7
Add: Allocated undistributed net earnings to non-vested restricted stockholders	0.2	0.1	0.5	0.1
Less: Undistributed net earnings reallocated to non-vested restricted stockholders	(0.2)	(0.1)	(0.5)	(0.1)
Net earnings available to common stockholders - diluted	$ 60.0	$ 56.1	$ 131.4	$ 58.7
Denominator
Weighted average number of common shares outstanding - basic	213.5	194.8	210.1	194.3
Effect of assumed issuance of Settlement agreement shares		18.0	3.3	18.0
Effect of non-vested restricted stock units	1.2	0.8	1.2	0.9
Weighted average number of common shares outstanding - diluted	214.7	213.6	214.6	213.2
Diluted net earnings per common share	$ 0.28	$ 0.26	$ 0.61	$ 0.28