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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator
Net earnings available to common stockholders	$ (33.5)	$ 787.5	$ 104.1	$ (43.2)	$ 171.1	$ 163.2	$ 49.6	$ 102.5	$ 814.9	$ 486.4	$ 335.4	[1]
Distributed and allocated undistributed net earnings to non-vested restricted stockholders									(4.9)	(3.4)	(2.2)
Distributed and allocated undistributed net earnings to common stockholders									810.0	483.0	333.2
Distributed net earnings - dividends paid to common stockholders									(118.7)	(118.7)	(106.2)
Allocation of undistributed net earnings to common stockholders									$ 691.3	$ 364.3	$ 227.0
Denominator
Weighted average number of common shares outstanding - basic									186.9	194.3	203.9
Basic net earnings per common share
Distributed net earnings to common stockholders (in dollars per share)									$ 0.64	$ 0.61	$ 0.52
Allocated undistributed net earnings to common stockholders (in dollars per share)									3.69	1.88	1.11
Net earnings per common share - basic (in dollars per share)	$ (0.19)	$ 4.19	$ 0.53	$ (0.22)	$ 0.88	$ 0.84	$ 0.25	$ 0.52	$ 4.33	$ 2.49	$ 1.63
Numerator
Distributed and allocated undistributed net earnings to common stockholders									$ 810.0	$ 483.0	$ 333.2
Add: Allocated undistributed net earnings to unvested restricted stockholders									4.3	2.6	1.6
Less: Undistributed net earnings reallocated to non-vested restricted stockholders									(4.3)	(2.6)	(1.6)
Net earnings available to common stockholders - diluted									$ 810.0	$ 483.0	$ 333.2
Denominator
Weighted average number of common shares outstanding - basic									186.9	194.3	203.9
Effect of contingently issuable shares									0.7	1.2	1.3
Effect of unvested restricted stock units (in shares)									0.7	0.7	0.7
Weighted average number of common shares outstanding - diluted under two-class									188.3	196.2	205.9
Effect of unvested restricted stock - participating security (in shares)									0.6	1.0	0.8
Weighted average number of common shares outstanding - diluted under treasury stock									188.9	197.2	206.7
Net earnings per common share - diluted (in dollars per share)	$ (0.19)	$ 4.15	$ 0.52	$ (0.22)	$ 0.87	$ 0.83	$ 0.25	$ 0.52	$ 4.29	$ 2.46	$ 1.62

[1] During the first quarter of 2015, the Company received the tax refund of $235.2 million related to the Settlement agreement payment. See Note 16 “Income Taxes” of the Notes to Consolidated Financial Statements for further discussion of the out-of-period adjustment.