Investments - Investments, Level 3 Rollforward (Detail) - Investments [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
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	$ 13,548	$ 12,208
Net realized gains/(losses)	252	237
Net unrealized gains/(losses)	1,295	834
Purchases	1,322	1,366
Sales	(986)	(1,512)
Settlements	(1,192)	(1,451)
Transfers into level 3	2,646	3,456
Transfers out of level 3	(1,603)	(1,590)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	15,282	13,548
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	2,540	1,722
Net realized gains/(losses)	64	62
Net unrealized gains/(losses)	198	66
Purchases	297	369
Sales	(43)	(231)
Settlements	(274)	(358)
Transfers into level 3	1,106	1,077
Transfers out of level 3	(423)	(167)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	3,465	2,540
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	457	662
Net realized gains/(losses)	27	20
Net unrealized gains/(losses)		(9)
Purchases	238	109
Sales	(82)	(97)
Settlements	(98)	(56)
Transfers into level 3	63	76
Transfers out of level 3	(10)	(248)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	595	457
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	10,335	9,626
Net realized gains/(losses)	160	155
Net unrealized gains/(losses)	1,096	775
Purchases	669	819
Sales	(852)	(1,161)
Settlements	(812)	(1,007)
Transfers into level 3	1,477	2,303
Transfers out of level 3	(1,170)	(1,175)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	10,903	10,335
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	216	198
Net realized gains/(losses)	1
Net unrealized gains/(losses)	1	2
Purchases	118	69
Sales	(9)	(23)
Settlements	(8)	(30)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value, Ending Balance	$ 319	$ 216