Supplemental Information about Changes to Accumulated Other Comprehensive Income - Schedule of Change in Balance by Component of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands			12 Months Ended
		Dec. 22, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in AOCI, Net of Tax
Beginning balance			$ 4,566,861	$ 4,055,552	$ 4,697,466
Reclassifications, net of tax			(14,668)	10,574	11,517
Reclassification, Tax Reform		$ 252,000
Ending balance			6,231,421	4,566,861	4,055,552
Total [Member]
Increase (Decrease) in AOCI, Net of Tax
Beginning balance			577,574	231,947	997,452
Other comprehensive income (loss) before reclassifications, net of tax			608,968	335,053	(777,022)
Reclassifications, net of tax			(14,668)	10,574	11,517
Other comprehensive income (loss)			594,300	345,627	(765,505)
Reclassification, Tax Reform	[1]		252,400
Ending balance			1,424,274	577,574	231,947
Available for Sale Assets
Increase (Decrease) in AOCI, Net of Tax
Beginning balance			692,314	332,333	1,090,273
Other comprehensive income (loss) before reclassifications, net of tax			621,619	356,016	(759,976)
Reclassifications, net of tax			(22,751)	3,965	2,036
Other comprehensive income (loss)			598,868	359,981	(757,940)
Reclassification, Tax Reform			278,107
Ending balance			1,569,289	692,314	332,333
Deferred Acquisition Costs
Increase (Decrease) in AOCI, Net of Tax
Beginning balance			(6,682)	(5,115)	(10,758)
Other comprehensive income (loss) before reclassifications, net of tax			(350)	(1,567)	5,643
Reclassifications, net of tax			0	0	0
Other comprehensive income (loss)			(350)	(1,567)	5,643
Reclassification, Tax Reform			(1,515)
Ending balance			(8,547)	(6,682)	(5,115)
Foreign Exchange
Increase (Decrease) in AOCI, Net of Tax
Beginning balance			4,967	3,627	17,386
Other comprehensive income (loss) before reclassifications, net of tax			8,452	1,340	(13,759)
Reclassifications, net of tax			0	0	0
Other comprehensive income (loss)			8,452	1,340	(13,759)
Reclassification, Tax Reform			2,883
Ending balance			16,302	4,967	3,627
Pension Adjustments
Increase (Decrease) in AOCI, Net of Tax
Beginning balance			(113,025)	(98,898)	(99,449)
Other comprehensive income (loss) before reclassifications, net of tax			(20,753)	(20,736)	(8,930)
Reclassifications, net of tax			8,083	6,609	9,481
Other comprehensive income (loss)			(12,670)	(14,127)	551
Reclassification, Tax Reform			(27,075)
Ending balance			$ (152,770)	$ (113,025)	$ (98,898)

[1] Income tax effects of certain items were reclassified from accumulated other comprehensive income to retained earnings to remove stranded tax effects as a result of early adoption of ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. See further discussion in Note 1—Significant Accounting Policies.