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	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	$ 6,700,398	$ 5,571,609	$ 5,415,177	$ 6,231,421
Other comprehensive income (loss) before reclassifications, net of tax	510,906	(128,498)	1,629,931	(947,929)
Reclassifications, net of tax	(7,032)	2,623	(6,282)	1,279
Other comprehensive income (loss)	503,874	(125,875)	1,623,649	(946,650)
Ending balance	7,312,531	5,538,905	7,312,531	5,538,905
Total [Member]
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	1,439,250	603,499	319,475	1,424,274
Ending balance	1,943,124	477,624	1,943,124	477,624
Available for Sale Assets
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	1,552,108	746,514	435,698	1,569,289
Other comprehensive income (loss) before reclassifications, net of tax	513,812	(129,296)	1,632,819	(944,758)
Reclassifications, net of tax	(8,706)	(362)	(11,303)	(7,675)
Other comprehensive income (loss)	505,106	(129,658)	1,621,516	(952,433)
Ending balance	2,057,214	616,856	2,057,214	616,856
Deferred Acquisition Costs
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	(6,556)	(6,579)	(4,163)	(8,547)
Other comprehensive income (loss) before reclassifications, net of tax	318	930	(2,075)	2,898
Reclassifications, net of tax	0	0	0	0
Other comprehensive income (loss)	318	930	(2,075)	2,898
Ending balance	(6,238)	(5,649)	(6,238)	(5,649)
Foreign Exchange
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	8,906	10,365	6,495	16,302
Other comprehensive income (loss) before reclassifications, net of tax	(3,224)	(132)	(813)	(6,069)
Reclassifications, net of tax	0	0	0	0
Other comprehensive income (loss)	(3,224)	(132)	(813)	(6,069)
Ending balance	5,682	10,233	5,682	10,233
Pension Adjustments
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	(115,208)	(146,801)	(118,555)	(152,770)
Other comprehensive income (loss) before reclassifications, net of tax	0	0	0	0
Reclassifications, net of tax	1,674	2,985	5,021	8,954
Other comprehensive income (loss)	1,674	2,985	5,021	8,954
Ending balance	$ (113,534)	$ (143,816)	$ (113,534)	$ (143,816)