Investments - Investments, Level 3 Rollforward (Detail) - Investments [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Beginning Balance	$ 15,282	$ 13,548
Net realized gains/(losses)	75	35
Net unrealized gains/(losses)	(1,284)	239
Purchases	348	217
Sales	(197)	(37)
Settlements	(325)	(232)
Transfers into level 3	6,480	967
Transfers out of level 3	(971)	(461)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	19,408	14,276
Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Beginning Balance	3,465	2,540
Net realized gains/(losses)	50	11
Net unrealized gains/(losses)	(465)	50
Purchases	69	86
Sales	(174)
Settlements	(166)	(42)
Transfers into level 3	4,394	260
Transfers out of level 3	(24)	(214)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	7,149	2,691
Securities backed by real estate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Beginning Balance	595	457
Net realized gains/(losses)	6	2
Net unrealized gains/(losses)	(72)	7
Purchases	52
Settlements	(12)	(33)
Transfers into level 3	206
Transfers out of level 3		(22)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	775	411
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Beginning Balance	10,903	10,335
Net realized gains/(losses)	17	22
Net unrealized gains/(losses)	(766)	182
Purchases	227	112
Sales	(23)	(36)
Settlements	(145)	(157)
Transfers into level 3	1,790	707
Transfers out of level 3	(947)	(225)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	11,056	10,940
Other debt obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Beginning Balance	319	216
Net realized gains/(losses)	2
Net unrealized gains/(losses)	19
Purchases		19
Sales		(1)
Settlements	(2)
Transfers into level 3	90
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	$ 428	$ 234