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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator
Net earnings available to common stockholders	$ 86.6	$ 60.8	$ 211.9	$ 191.8
Distributed and allocated undistributed net loss to non-vested restricted stockholders	(0.6)	(0.4)	(1.5)	(1.1)
Distributed and allocated undistributed net earnings to common stockholders	86.0	60.4	210.4	190.7
Distributed net earnings - dividends paid to common stockholders	(26.1)	(27.3)	(80.6)	(82.9)
Allocation of undistributed net earnings to common stockholders	$ 59.9	$ 33.1	$ 129.8	$ 107.8
Denominator
Weighted average number of common shares outstanding - basic	202.9	210.4	206.7	210.2
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	0.30	0.16	0.63	0.52
Basic net earnings per common share:	$ 0.43	$ 0.29	$ 1.02	$ 0.91
Numerator
Distributed and allocated undistributed net earnings to common stockholders	$ 86.0	$ 60.4	$ 210.4	$ 190.7
Add: Allocated undistributed net earnings to unvested restricted stockholders	0.5	0.1	1.0	0.6
Less: Undistributed net earnings (loss) reallocated to non-vested restricted stockholders	(0.5)	(0.1)	(1.0)	(0.6)
Net earnings available to common stockholders - diluted	$ 86.0	$ 60.4	$ 210.4	$ 190.7
Denominator
Weighted average number of common shares outstanding - basic	202.9	210.4	206.7	210.2
Effect of assumed issuance of Settlement agreement shares				2.2
Effect of contingently issuable shares	1.4	1.9	1.3	1.8
Effect of unvested restricted stock units	0.7	0.5	0.7	0.4
Weighted average number of common shares outstanding - diluted under two-class	205.0	212.8	208.7	214.6
Effect of unvested restricted stock - participating security	0.8	0.7	0.8	0.7
Weighted average number of common shares outstanding - diluted under treasury stock	205.8	213.5	209.5	215.3
Diluted net earnings per common share	$ 0.42	$ 0.28	$ 1.01	$ 0.89