Gross Unrealized Losses and Fair Value of Investment Securities (Detail) $ in Millions	Dec. 31, 2018 USD ($) Securities	Dec. 31, 2017 USD ($) Securities
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 18,042	$ 6,090
Less than 12 months, Gross unrealized losses	$ (695)	$ (65)
Less than 12 months, Number of securities in a continuous loss position | Securities	2,585	1,039
12 months or more, Fair value	$ 6,678	$ 3,661
12 months or more, Gross unrealized losses	$ (401)	$ (143)
12 months or more, Number of securities in a continuous loss position | Securities	1,055	587
Fair value	$ 24,720	$ 9,751
Gross unrealized losses	$ (1,096)	$ (208)
Number of securities in a continuous loss position | Securities	3,640	1,626
Investment grade
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 16,726	$ 5,867
Less than 12 months, Gross unrealized losses	$ (615)	$ (55)
Less than 12 months, Number of securities in a continuous loss position | Securities	2,393	898
12 months or more, Fair value	$ 6,508	$ 3,488
12 months or more, Gross unrealized losses	$ (379)	$ (135)
12 months or more, Number of securities in a continuous loss position | Securities	1,024	528
Fair value	$ 23,234	$ 9,355
Gross unrealized losses	$ (994)	$ (190)
Number of securities in a continuous loss position | Securities	3,417	1,426
Below investment grade
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 1,316	$ 223
Less than 12 months, Gross unrealized losses	$ (80)	$ (10)
Less than 12 months, Number of securities in a continuous loss position | Securities	192	141
12 months or more, Fair value	$ 170	$ 173
12 months or more, Gross unrealized losses	$ (22)	$ (8)
12 months or more, Number of securities in a continuous loss position | Securities	31	59
Fair value	$ 1,486	$ 396
Gross unrealized losses	$ (102)	$ (18)
Number of securities in a continuous loss position | Securities	223	200
Fixed maturity securities
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 18,042	$ 6,016
Less than 12 months, Gross unrealized losses	$ (695)	$ (62)
Less than 12 months, Number of securities in a continuous loss position | Securities	2,585	905
12 months or more, Fair value	$ 6,678	$ 3,561
12 months or more, Gross unrealized losses	$ (401)	$ (138)
12 months or more, Number of securities in a continuous loss position | Securities	1,055	529
Fair value	$ 24,720	$ 9,577
Gross unrealized losses	$ (1,096)	$ (200)
Number of securities in a continuous loss position | Securities	3,640	1,434
Fixed maturity securities | Less Than 20 Percent Below Cost
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 18,008	$ 6,016
Less than 12 months, Gross unrealized losses	$ (685)	$ (62)
Less than 12 months, Number of securities in a continuous loss position | Securities	2,581	905
12 months or more, Fair value	$ 6,624	$ 3,555
12 months or more, Gross unrealized losses	$ (383)	$ (136)
12 months or more, Number of securities in a continuous loss position | Securities	1,045	526
Fair value	$ 24,632	$ 9,571
Gross unrealized losses	$ (1,068)	$ (198)
Number of securities in a continuous loss position | Securities	3,626	1,431
Fixed maturity securities | 20 To 50 percent below cost
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 34
Less than 12 months, Gross unrealized losses	$ (10)
Less than 12 months, Number of securities in a continuous loss position | Securities	4
12 months or more, Fair value	$ 54	$ 6
12 months or more, Gross unrealized losses	$ (18)	$ (2)
12 months or more, Number of securities in a continuous loss position | Securities	10	3
Fair value	$ 88	$ 6
Gross unrealized losses	$ (28)	$ (2)
Number of securities in a continuous loss position | Securities	14	3
Fixed maturity securities | U.S. government, agencies and government-sponsored enterprises
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 545	$ 78
Less than 12 months, Gross unrealized losses	$ (8)	$ (1)
Less than 12 months, Number of securities in a continuous loss position | Securities	17	21
12 months or more, Fair value	$ 161	$ 94
12 months or more, Gross unrealized losses	$ (9)	$ (2)
12 months or more, Number of securities in a continuous loss position | Securities	26	7
Fair value	$ 706	$ 172
Gross unrealized losses	$ (17)	$ (3)
Number of securities in a continuous loss position | Securities	43	28
Fixed maturity securities | State and Political Subdivisions
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 371	$ 125
Less than 12 months, Gross unrealized losses	$ (10)	$ (1)
Less than 12 months, Number of securities in a continuous loss position | Securities	63	35
12 months or more, Fair value	$ 233	$ 327
12 months or more, Gross unrealized losses	$ (12)	$ (21)
12 months or more, Number of securities in a continuous loss position | Securities	57	42
Fair value	$ 604	$ 452
Gross unrealized losses	$ (22)	$ (22)
Number of securities in a continuous loss position | Securities	120	77
Fixed maturity securities | Non-U.S. government
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 261	$ 583
Less than 12 months, Gross unrealized losses	$ (7)	$ (7)
Less than 12 months, Number of securities in a continuous loss position | Securities	51	26
12 months or more, Fair value	$ 508	$ 239
12 months or more, Gross unrealized losses	$ (17)	$ (13)
12 months or more, Number of securities in a continuous loss position | Securities	35	20
Fair value	$ 769	$ 822
Gross unrealized losses	$ (24)	$ (20)
Number of securities in a continuous loss position | Securities	86	46
Fixed maturity securities | U.S. corporate
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 9,975	$ 1,871
Less than 12 months, Gross unrealized losses	$ (472)	$ (26)
Less than 12 months, Number of securities in a continuous loss position | Securities	1,342	296
12 months or more, Fair value	$ 2,449	$ 1,347
12 months or more, Gross unrealized losses	$ (210)	$ (42)
12 months or more, Number of securities in a continuous loss position | Securities	365	190
Fair value	$ 12,424	$ 3,218
Gross unrealized losses	$ (682)	$ (68)
Number of securities in a continuous loss position | Securities	1,707	486
Fixed maturity securities | Non-U.S. corporate
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 4,172	$ 1,323
Less than 12 months, Gross unrealized losses	$ (150)	$ (12)
Less than 12 months, Number of securities in a continuous loss position | Securities	614	217
12 months or more, Fair value	$ 1,274	$ 548
12 months or more, Gross unrealized losses	$ (74)	$ (21)
12 months or more, Number of securities in a continuous loss position | Securities	209	77
Fair value	$ 5,446	$ 1,871
Gross unrealized losses	$ (224)	$ (33)
Number of securities in a continuous loss position | Securities	823	294
Fixed maturity securities | Residential mortgage-backed
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 363	$ 707
Less than 12 months, Gross unrealized losses	$ (6)	$ (7)
Less than 12 months, Number of securities in a continuous loss position | Securities	57	81
12 months or more, Fair value	$ 579	$ 130
12 months or more, Gross unrealized losses	$ (11)	$ (4)
12 months or more, Number of securities in a continuous loss position | Securities	96	46
Fair value	$ 942	$ 837
Gross unrealized losses	$ (17)	$ (11)
Number of securities in a continuous loss position | Securities	153	127
Fixed maturity securities | Commercial mortgage-backed
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 758	$ 476
Less than 12 months, Gross unrealized losses	$ (19)	$ (4)
Less than 12 months, Number of securities in a continuous loss position | Securities	115	69
12 months or more, Fair value	$ 870	$ 646
12 months or more, Gross unrealized losses	$ (62)	$ (33)
12 months or more, Number of securities in a continuous loss position | Securities	130	90
Fair value	$ 1,628	$ 1,122
Gross unrealized losses	$ (81)	$ (37)
Number of securities in a continuous loss position | Securities	245	159
Fixed maturity securities | Other asset-backed
Schedule of Investments [Line Items]
Less than 12 months, Fair value	$ 1,597	$ 853
Less than 12 months, Gross unrealized losses	$ (23)	$ (4)
Less than 12 months, Number of securities in a continuous loss position | Securities	326	160
12 months or more, Fair value	$ 604	$ 230
12 months or more, Gross unrealized losses	$ (6)	$ (2)
12 months or more, Number of securities in a continuous loss position | Securities	137	57
Fair value	$ 2,201	$ 1,083
Gross unrealized losses	$ (29)	$ (6)
Number of securities in a continuous loss position | Securities	463	217
Equity Securities
Schedule of Investments [Line Items]
Less than 12 months, Fair value		$ 74
Less than 12 months, Gross unrealized losses		$ (3)
Less than 12 months, Number of securities in a continuous loss position | Securities		134
12 months or more, Fair value		$ 100
12 months or more, Gross unrealized losses		$ (5)
12 months or more, Number of securities in a continuous loss position | Securities		58
Fair value		$ 174
Gross unrealized losses		$ (8)
Number of securities in a continuous loss position | Securities		192
Equity Securities | Less Than 20 Percent Below Cost
Schedule of Investments [Line Items]
Less than 12 months, Fair value		$ 74
Less than 12 months, Gross unrealized losses		$ (3)
Less than 12 months, Number of securities in a continuous loss position | Securities		134
12 months or more, Fair value		$ 100
12 months or more, Gross unrealized losses		$ (5)
12 months or more, Number of securities in a continuous loss position | Securities		58
Fair value		$ 174
Gross unrealized losses		$ (8)
Number of securities in a continuous loss position | Securities		192