INVESTMENTS - Fair Value and Gross Unrealized Losses for Securities That Have Been in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	$ 28,256	$ 24,301
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	5,924	4,128
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	10,656	6,716
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	770	197
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	17,600	17,585
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5,154	3,931
U.S. government and agencies | U.S. dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	37	106
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	2	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0	59
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	37	47
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2	2
Japan government and agencies | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	13,521	8,136
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	3,727	2,234
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	7,966	2,070
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	692	57
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	5,555	6,066
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,035	2,177
Municipalities | U.S. dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	630	666
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	54	53
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	8	67
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	622	599
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	54	50
Municipalities | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	520	341
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	145	79
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	234	96
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	6	2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	286	245
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	139	77
Mortgage- and asset-backed securities | U.S. dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	547	567
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	35	34
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	217	173
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5	2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	330	394
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	30	32
Mortgage- and asset-backed securities | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	183	196
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	38	23
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	9	12
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	174	184
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	37	23
Public utilities | U.S. dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	1,055	1,570
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	107	151
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	169	699
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	19
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	886	871
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	105	132
Public utilities | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	838	1,020
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	174	108
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0	368
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	11
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	838	652
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	174	97
Sovereign and supranational | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	276	47
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	13	8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	236	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	40	47
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	13	8
Banks/financial institutions | U.S. dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	252	625
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	21	36
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	62	376
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	190	249
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	21	29
Banks/financial institutions | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	3,467	3,197
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	477	342
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	495	471
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	26	22
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	2,972	2,726
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	451	320
Other corporate | U.S. dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	4,535	6,097
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	597	752
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	620	2,036
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5	59
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3,915	4,061
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	592	693
Other corporate | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Debt Securities, Available-for-Sale, Unrealized Loss Position, Total	2,395	1,733
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Total	534	305
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	640	289
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	33	14
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,755	1,444
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 501	$ 291