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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	$ 6,043,426	$ 5,820,301	$ 5,415,177	$ 6,231,421
Other comprehensive income (loss) before reclassifications, net of tax	568,177	(332,921)	1,119,025	(819,431)
Reclassifications, net of tax	1,006	(3,779)	750	(1,344)
Other comprehensive income (loss)	569,183	(336,700)	1,119,775	(820,775)
Ending balance	6,700,398	5,571,609	6,700,398	5,571,609
Total [Member]
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	870,067	940,199	319,475	1,424,274
Ending balance	1,439,250	603,499	1,439,250	603,499
Available for Sale Assets
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	984,701	1,082,896	435,698	1,569,289
Other comprehensive income (loss) before reclassifications, net of tax	568,073	(329,619)	1,119,007	(815,462)
Reclassifications, net of tax	(666)	(6,763)	(2,597)	(7,313)
Other comprehensive income (loss)	567,407	(336,382)	1,116,410	(822,775)
Ending balance	1,552,108	746,514	1,552,108	746,514
Deferred Acquisition Costs
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	(6,691)	(8,191)	(4,163)	(8,547)
Other comprehensive income (loss) before reclassifications, net of tax	135	1,612	(2,393)	1,968
Reclassifications, net of tax	0	0	0	0
Other comprehensive income (loss)	135	1,612	(2,393)	1,968
Ending balance	(6,556)	(6,579)	(6,556)	(6,579)
Foreign Exchange
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	8,937	15,279	6,495	16,302
Other comprehensive income (loss) before reclassifications, net of tax	(31)	(4,914)	2,411	(5,937)
Reclassifications, net of tax	0	0	0	0
Other comprehensive income (loss)	(31)	(4,914)	2,411	(5,937)
Ending balance	8,906	10,365	8,906	10,365
Pension Adjustments
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	(116,880)	(149,785)	(118,555)	(152,770)
Other comprehensive income (loss) before reclassifications, net of tax	0	0	0	0
Reclassifications, net of tax	1,672	2,984	3,347	5,969
Other comprehensive income (loss)	1,672	2,984	3,347	5,969
Ending balance	$ (115,208)	$ (146,801)	$ (115,208)	$ (146,801)