Net Earnings 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
	Mar. 31, 2015	Mar. 31, 2014
Numerator
Net earnings available to common stockholders	$ 97.2	$ 70.9
Distributed and allocated undistributed net loss to non-vested restricted stockholders	(0.6)	(0.4)
Distributed and allocated undistributed net earnings to common Stockholders	96.6	70.5
Distributed net earnings - dividends paid to common stockholders	(27.3)	(27.7)
Allocation of undistributed net earnings to common Stockholders	69.3	42.8
Denominator
Weighted average number of common shares outstanding - basic	208.9	206.7
Basic net earning per common share:
Distributed net earnings to common stockholders	$ 0.13	$ 0.13
Allocated undistributed net earnings to common stockholders	$ 0.33	$ 0.21
Net earnings (loss) per common share - basic	$ 0.46	$ 0.34
Numerator
Distributed and allocated undistributed net earnings to common Stockholders	96.6	70.5
Add: Allocated undistributed net earnings to non-vested restricted Stockholders	0.5	0.3
Less: Undistributed net earnings reallocated to non-vested restricted Stockholders	(0.5)	(0.3)
Net earnings available to common stockholders - diluted	$ 96.6	$ 70.5
Denominator
Weighted average number of common shares outstanding - basic	208.9	206.7
Effect of assumed issuance of Settlement agreement shares		6.6
Effect of contingently issuable shares	1.2	0.6
Effect of non-vested restricted stock units	1.6	1.2
Weighted average number of common shares outstanding - diluted	211.7	215.1
Diluted net earnings per common share	$ 0.46	$ 0.33