Investments - Investments, Level 3 Rollforward (Detail) - Investments [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 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	$ 17,916	$ 14,397	$ 15,282	$ 13,548
Net realized gains/(losses)	128	46	238	119
Net unrealized gains/(losses)	534	137	(808)	684
Purchases	405	399	1,250	1,058
Sales	(269)	(82)	(1,379)	(788)
Settlements	(686)	(389)	(1,180)	(799)
Transfers into level 3	230	1,001	5,373	2,477
Transfers out of level 3	(984)	(496)	(1,502)	(1,286)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	17,274	15,013	17,274	15,013
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	6,572	3,048	3,465	2,540
Net realized gains/(losses)	48	17	115	47
Net unrealized gains/(losses)	142	18	(263)	114
Purchases	137	156	401	425
Sales	(22)	(3)	(215)	(41)
Settlements	(386)	(69)	(553)	(177)
Transfers into level 3	121	440	3,234	725
Transfers out of level 3	(761)	(139)	(333)	(165)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	5,851	3,468	5,851	3,468
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	880	395	595	457
Net realized gains/(losses)	12	7	32	19
Net unrealized gains/(losses)	16	(2)	(97)
Purchases	35	6	124	14
Sales		(7)		(10)
Settlements	(11)	(17)	(43)	(70)
Transfers into level 3		59	323	59
Transfers out of level 3	(2)	(71)	(4)	(99)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	930	370	930	370
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	10,035	10,703	10,903	10,335
Net realized gains/(losses)	64	22	81	53
Net unrealized gains/(losses)	373	121	(448)	569
Purchases	206	177	680	522
Sales	(245)	(62)	(1,160)	(724)
Settlements	(283)	(303)	(575)	(552)
Transfers into level 3	109	502	1,722	1,693
Transfers out of level 3	(221)	(286)	(1,165)	(1,022)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	10,038	10,874	10,038	10,874
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	429	251	319	216
Net realized gains/(losses)	4		10
Net unrealized gains/(losses)	3			1
Purchases	27	60	45	97
Sales	(2)	(10)	(4)	(13)
Settlements	(6)		(9)
Transfers into level 3			94
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	$ 455	$ 301	$ 455	$ 301