INVESTMENTS - Fair Value and Gross Unrealized Losses for Securities That Have Been in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	$ 23,627	$ 25,402
Total Unrealized Losses	3,502	3,449
Less than 12 months Fair Value	4,673	4,239
Less than 12 months Unrealized Losses	471	364
12 months or Longer Fair Value	18,954	21,163
12 months or Longer Unrealized Losses	3,031	3,085
Dollar-denominated | U.S. government and agencies
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	141	123
Total Unrealized Losses	4	4
Less than 12 months Fair Value	59	53
Less than 12 months Unrealized Losses	1	1
12 months or Longer Fair Value	82	70
12 months or Longer Unrealized Losses	3	3
Dollar-denominated | Municipalities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	662	703
Total Unrealized Losses	49	38
Less than 12 months Fair Value	9	31
Less than 12 months Unrealized Losses	0	1
12 months or Longer Fair Value	653	672
12 months or Longer Unrealized Losses	49	37
Dollar-denominated | Mortgage- and asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	754	925
Total Unrealized Losses	48	56
Less than 12 months Fair Value	304	340
Less than 12 months Unrealized Losses	4	6
12 months or Longer Fair Value	450	585
12 months or Longer Unrealized Losses	44	50
Dollar-denominated | Public utilities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	1,383	1,120
Total Unrealized Losses	136	114
Less than 12 months Fair Value	391	228
Less than 12 months Unrealized Losses	9	4
12 months or Longer Fair Value	992	892
12 months or Longer Unrealized Losses	127	110
Dollar-denominated | Sovereign and supranational
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	14	35
Total Unrealized Losses	4	8
Less than 12 months Fair Value	0	0
Less than 12 months Unrealized Losses	0	0
12 months or Longer Fair Value	14	35
12 months or Longer Unrealized Losses	4	8
Dollar-denominated | Banks/financial institutions
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	628	655
Total Unrealized Losses	41	51
Less than 12 months Fair Value	228	159
Less than 12 months Unrealized Losses	2	4
12 months or Longer Fair Value	400	496
12 months or Longer Unrealized Losses	39	47
Dollar-denominated | Other corporate
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	6,196	6,380
Total Unrealized Losses	724	665
Less than 12 months Fair Value	1,254	799
Less than 12 months Unrealized Losses	36	19
12 months or Longer Fair Value	4,942	5,581
12 months or Longer Unrealized Losses	688	646
Yen-denominated | Japan government and agencies
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	7,828	8,393
Total Unrealized Losses	1,811	1,696
Less than 12 months Fair Value	1,628	1,657
Less than 12 months Unrealized Losses	395	303
12 months or Longer Fair Value	6,200	6,736
12 months or Longer Unrealized Losses	1,416	1,393
Yen-denominated | Municipalities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	277	301
Total Unrealized Losses	59	58
Less than 12 months Fair Value	31	34
Less than 12 months Unrealized Losses	1	0
12 months or Longer Fair Value	246	267
12 months or Longer Unrealized Losses	58	58
Yen-denominated | Mortgage- and asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	54	58
Total Unrealized Losses	12	11
Less than 12 months Fair Value	1	0
Less than 12 months Unrealized Losses	0	0
12 months or Longer Fair Value	53	58
12 months or Longer Unrealized Losses	12	11
Yen-denominated | Public utilities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	915	1,028
Total Unrealized Losses	77	82
Less than 12 months Fair Value	351	444
Less than 12 months Unrealized Losses	13	13
12 months or Longer Fair Value	564	584
12 months or Longer Unrealized Losses	64	69
Yen-denominated | Sovereign and supranational
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	55	60
Total Unrealized Losses	7	7
Less than 12 months Fair Value	0	0
Less than 12 months Unrealized Losses	0	0
12 months or Longer Fair Value	55	60
12 months or Longer Unrealized Losses	7	7
Yen-denominated | Banks/financial institutions
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	2,973	3,673
Total Unrealized Losses	285	365
Less than 12 months Fair Value	62	186
Less than 12 months Unrealized Losses	1	4
12 months or Longer Fair Value	2,911	3,487
12 months or Longer Unrealized Losses	284	361
Yen-denominated | Other corporate
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	1,747	1,948
Total Unrealized Losses	245	294
Less than 12 months Fair Value	355	308
Less than 12 months Unrealized Losses	9	9
12 months or Longer Fair Value	1,392	1,640
12 months or Longer Unrealized Losses	$ 236	$ 285