INVESTMENTS - Fair Value and Gross Unrealized Losses for Securities That Have Been in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	$ 28,071	$ 24,301
Total Unrealized Losses	5,385	4,128
Less than 12 months Fair Value	10,155	6,716
Less than 12 months Unrealized Losses	557	197
12 months or Longer Fair Value	17,916	17,585
12 months or Longer Unrealized Losses	4,828	3,931
U.S. government and agencies | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	65	106
Total Unrealized Losses	12	3
Less than 12 months Fair Value	20	59
Less than 12 months Unrealized Losses	6	1
12 months or Longer Fair Value	45	47
12 months or Longer Unrealized Losses	6	2
Japan government and agencies | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	12,544	8,136
Total Unrealized Losses	3,260	2,234
Less than 12 months Fair Value	7,003	2,070
Less than 12 months Unrealized Losses	447	57
12 months or Longer Fair Value	5,541	6,066
12 months or Longer Unrealized Losses	2,813	2,177
Municipalities | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	632	666
Total Unrealized Losses	60	53
Less than 12 months Fair Value	22	67
Less than 12 months Unrealized Losses	2	3
12 months or Longer Fair Value	610	599
12 months or Longer Unrealized Losses	58	50
Municipalities | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	418	341
Total Unrealized Losses	130	79
Less than 12 months Fair Value	118	96
Less than 12 months Unrealized Losses	3	2
12 months or Longer Fair Value	300	245
12 months or Longer Unrealized Losses	127	77
Mortgage- and asset-backed securities | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	1,028	567
Total Unrealized Losses	45	34
Less than 12 months Fair Value	659	173
Less than 12 months Unrealized Losses	19	2
12 months or Longer Fair Value	369	394
12 months or Longer Unrealized Losses	26	32
Mortgage- and asset-backed securities | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	199	196
Total Unrealized Losses	34	23
Less than 12 months Fair Value	22	12
Less than 12 months Unrealized Losses	1	0
12 months or Longer Fair Value	177	184
12 months or Longer Unrealized Losses	33	23
Public utilities | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	1,195	1,570
Total Unrealized Losses	111	151
Less than 12 months Fair Value	231	699
Less than 12 months Unrealized Losses	4	19
12 months or Longer Fair Value	964	871
12 months or Longer Unrealized Losses	107	132
Public utilities | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	999	1,020
Total Unrealized Losses	199	108
Less than 12 months Fair Value	76	368
Less than 12 months Unrealized Losses	4	11
12 months or Longer Fair Value	923	652
12 months or Longer Unrealized Losses	195	97
Sovereign and supranational | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	45	47
Total Unrealized Losses	12	8
Less than 12 months Fair Value	0	0
Less than 12 months Unrealized Losses	0	0
12 months or Longer Fair Value	45	47
12 months or Longer Unrealized Losses	12	8
Banks/financial institutions | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	229	625
Total Unrealized Losses	20	36
Less than 12 months Fair Value	27	376
Less than 12 months Unrealized Losses	0	7
12 months or Longer Fair Value	202	249
12 months or Longer Unrealized Losses	20	29
Banks/financial institutions | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	3,419	3,197
Total Unrealized Losses	441	342
Less than 12 months Fair Value	585	471
Less than 12 months Unrealized Losses	18	22
12 months or Longer Fair Value	2,834	2,726
12 months or Longer Unrealized Losses	423	320
Other corporate | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	4,843	6,097
Total Unrealized Losses	575	752
Less than 12 months Fair Value	614	2,036
Less than 12 months Unrealized Losses	12	59
12 months or Longer Fair Value	4,229	4,061
12 months or Longer Unrealized Losses	563	693
Other corporate | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	2,455	1,733
Total Unrealized Losses	486	305
Less than 12 months Fair Value	778	289
Less than 12 months Unrealized Losses	41	14
12 months or Longer Fair Value	1,677	1,444
12 months or Longer Unrealized Losses	$ 445	$ 291