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		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Numerator
Net earnings available to common stockholders	$ 59.3	$ 37.6	$ 191.3	$ 96.6
Distributed and allocated undistributed net earnings to non-vested restricted stockholders	(0.4)	(0.2)	(1.1)	(0.5)
Distributed and allocated undistributed net earnings to common stockholders	58.9	37.4	190.2	96.1
Distributed net earnings - dividends paid to common stockholders	(27.4)	(25.3)	(82.9)	(75.9)
Allocation of undistributed net earnings to common stockholders	31.5	12.1	107.3	20.2
Denominator
Weighted average number of common shares outstanding - basic	210.4	194.9	210.2	194.5
Basic net earnings per common share:
Distributed net earnings to common stockholders	$ 0.13	$ 0.13	$ 0.39	$ 0.39
Allocated undistributed net earnings to common stockholders	$ 0.15	$ 0.06	$ 0.52	$ 0.10
Net earnings per common share - basic	$ 0.28	$ 0.19	$ 0.91	$ 0.49
Numerator
Distributed and allocated undistributed net earnings to common stockholders	58.9	37.4	190.2	96.1
Add: Allocated undistributed net earnings to non-vested restricted stockholders	0.2	0.1	0.6	0.1
Less: Undistributed net earnings reallocated to non-vested restricted stockholders	(0.2)	(0.1)	(0.6)	(0.1)
Net earnings available to common stockholders - diluted	$ 58.9	$ 37.4	$ 190.2	$ 96.1
Denominator
Weighted average number of common shares outstanding - basic	210.4	194.9	210.2	194.5
Effect of assumed issuance of Settlement agreement shares		18.0	2.2	18.0
Effect of contingently issuable shares	1.9	1.6	1.8	1.5
Effect of non-vested restricted stock units	1.2	0.8	1.1	0.9
Weighted average number of common shares outstanding - diluted	213.5	215.3	215.3	214.9
Diluted net earnings per common share	$ 0.28	$ 0.18	$ 0.88	$ 0.45