Net Earnings per Common Share - Calculation of Basic and Diluted Net Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator
Net earnings	$ 200.3	$ 79.0	$ 114.4	$ (200.6)	$ (33.5)	$ 787.5	$ 104.1	$ (43.2)	$ 193.1	$ 814.9	$ 486.4
Distributed and allocated undistributed net earnings to unvested restricted stockholders									(0.9)	(4.9)	(3.4)
Distributed and allocated undistributed net earnings									192.2	810.0	483.0
Distributed net earnings - dividends paid to common stockholders									(101.7)	(118.7)	(118.7)
Allocation of undistributed net earnings to common stockholders									$ 90.5	$ 691.3	$ 364.3
Denominator
Weighted average number of common shares outstanding - basic									159.4	186.9	194.3
Basic net earnings per common share
Distributed net earnings (in dollars per share)									$ 0.64	$ 0.64	$ 0.61
Allocated undistributed net earnings to common stockholders (in dollars per share)									0.57	3.69	1.88
Net earnings per common share - basic (in dollars per share)	$ 1.29	$ 0.50	$ 0.71	$ (1.21)	$ (0.19)	$ 4.19	$ 0.53	$ (0.22)	$ 1.21	$ 4.33	$ 2.49
Numerator
Distributed and allocated undistributed net earnings									$ 192.2	$ 810.0	$ 483.0
Add: Allocated undistributed net earnings to unvested restricted stockholders									0.5	4.3	2.6
Less: Undistributed net earnings reallocated to unvested restricted stockholders									(0.5)	(4.3)	(2.6)
Net earnings available to common stockholders - diluted									$ 192.2	$ 810.0	$ 483.0
Denominator
Weighted average number of common shares outstanding - basic									159.4	186.9	194.3
Effect of contingently issuable shares									0.1	0.7	1.2
Effect of unvested restricted stock units (in shares)									0.3	0.7	0.7
Weighted average number of common shares outstanding - diluted under two-class									159.8	188.3	196.2
Effect of unvested restricted stock - participating security (in shares)									0.4	0.6	1.0
Weighted average number of common shares outstanding - diluted under treasury stock									160.2	188.9	197.2
Net earnings per common share - diluted (in dollars per share)	$ 1.28	$ 0.50	$ 0.71	$ (1.21)	$ (0.19)	$ 4.15	$ 0.52	$ (0.22)	$ 1.20	$ 4.29	$ 2.46