Net Earnings Per Common Share - Calculation of Basic and Diluted Net Earnings (Loss) Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator
Net earnings available to common stockholders	$ 28.1	$ 60.1	$ 125.3	$ 131.0
Distributed and allocated undistributed net loss to non-vested restricted stockholders	(0.3)	(0.3)	(0.9)	(0.7)
Distributed and allocated undistributed net earnings to common stockholders	27.8	59.8	124.4	130.3
Distributed net earnings - dividends paid to common stockholders	(27.2)	(27.9)	(54.5)	(55.6)
Allocation of undistributed net earnings to common stockholders	$ 0.6	$ 31.9	$ 69.9	$ 74.7
Denominator
Weighted average number of common shares outstanding - basic	208.5	213.5	208.7	210.1
Basic net earnings per common share:
Distributed net earnings to common stockholders	$ 0.13	$ 0.13	$ 0.26	$ 0.26
Allocated undistributed net earnings to common stockholders		0.15	0.34	0.36
Basic net earnings per common share:	$ 0.13	$ 0.28	$ 0.60	$ 0.62
Numerator
Distributed and allocated undistributed net earnings to common stockholders	$ 27.8	$ 59.8	$ 124.4	$ 130.3
Add: Allocated undistributed net earnings to non-vested restricted stockholders		0.2	0.5	0.5
Less: Undistributed net earnings (loss) reallocated to non-vested restricted stockholders		(0.2)	(0.5)	(0.5)
Net earnings available to common stockholders - diluted	$ 27.8	$ 59.8	$ 124.4	$ 130.3
Denominator
Weighted average number of common shares outstanding - basic	208.5	213.5	208.7	210.1
Effect of assumed issuance of Settlement agreement shares				3.3
Effect of contingently issuable shares	1.3	0.8	1.3	0.8
Effect of non-vested restricted stock units	0.7	0.5	0.7	0.5
Weighted average number of common shares outstanding - diluted under two-class	210.5	214.8	210.7	214.7
Effect of nonvested restricted stock - participating security	0.8	0.7	0.8	0.7
Weighted average number of common shares outstanding - diluted under treasury stock	211.3	215.5	211.5	215.4
Diluted net earnings per common share	$ 0.13	$ 0.28	$ 0.59	$ 0.61