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INVESTMENTS - Available-for-Sale Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 65,456	$ 65,263
Allowance for Credit Losses	0	0	$ 0	$ 0
Gross Unrealized Gains	4,430	4,782
Gross Unrealized Losses	6,658	5,924
Fair Value	63,228	64,121
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,204	32,885
Allowance for Credit Losses	0	0
Gross Unrealized Gains	537	704
Gross Unrealized Losses	5,715	5,108
Fair Value	27,026	28,481
U.S. dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	33,134	32,257
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3,885	4,068
Gross Unrealized Losses	943	816
Fair Value	36,076	35,509
Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	118	121
Allowance for Credit Losses	0	0
Gross Unrealized Gains	8	10
Gross Unrealized Losses	0	0
Fair Value	126	131
Japan government and agencies \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,439	18,063
Allowance for Credit Losses	0	0
Gross Unrealized Gains	27	41
Gross Unrealized Losses	4,099	3,727
Fair Value	13,367	14,377
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,878	1,930
Municipalities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	838	856
Allowance for Credit Losses	0	0
Gross Unrealized Gains	2	5
Gross Unrealized Losses	163	145
Fair Value	677	716
Municipalities \| U.S. dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,180	1,185
Allowance for Credit Losses	0	0
Gross Unrealized Gains	82	83
Gross Unrealized Losses	61	54
Fair Value	1,201	1,214
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	5,037	4,366
Mortgage- and asset-backed securities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	417	297
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1	1
Gross Unrealized Losses	43	38
Fair Value	375	260
Mortgage- and asset-backed securities \| U.S. dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,410	3,854
Allowance for Credit Losses	0	0
Gross Unrealized Gains	245	239
Gross Unrealized Losses	40	35
Fair Value	4,615	4,058
Mortgage- and asset-backed securities \| Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	43	44
Allowance for Credit Losses	0	0
Gross Unrealized Gains	4	4
Gross Unrealized Losses	0	0
Fair Value	47	48
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	6,954	7,174
Public utilities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,467	2,519
Allowance for Credit Losses	0	0
Gross Unrealized Gains	85	123
Gross Unrealized Losses	204	174
Fair Value	2,348	2,468
Public utilities \| U.S. dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,241	4,292
Allowance for Credit Losses	0	0
Gross Unrealized Gains	438	465
Gross Unrealized Losses	127	107
Fair Value	4,552	4,650
Public utilities \| Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	51	52
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3	4
Gross Unrealized Losses	0	0
Fair Value	54	56
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	385	402
Sovereign and supranational \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	323	330
Allowance for Credit Losses	0	0
Gross Unrealized Gains	6	7
Gross Unrealized Losses	20	13
Fair Value	309	324
Sovereign and supranational \| U.S. dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	58	57
Allowance for Credit Losses	0	0
Gross Unrealized Gains	18	21
Gross Unrealized Losses	0	0
Fair Value	76	78
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	9,194	9,244
Banks/financial institutions \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,374	5,382
Allowance for Credit Losses	0	0
Gross Unrealized Gains	139	170
Gross Unrealized Losses	554	477
Fair Value	4,959	5,075
Banks/financial institutions \| U.S. dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,788	3,672
Allowance for Credit Losses	0	0
Gross Unrealized Gains	480	518
Gross Unrealized Losses	33	21
Fair Value	4,235	4,169
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	26,208	26,398
Other corporate \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,346	5,438
Allowance for Credit Losses	0	0
Gross Unrealized Gains	277	357
Gross Unrealized Losses	632	534
Fair Value	4,991	5,261
Other corporate \| U.S. dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,250	18,967
Allowance for Credit Losses	0	0
Gross Unrealized Gains	2,621	2,740
Gross Unrealized Losses	679	597
Fair Value	21,192	21,110
Other corporate \| Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	24	25
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1	2
Gross Unrealized Losses	0	0
Fair Value	25	27
U.S. government and agencies \| U.S. dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	207	230
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1	2
Gross Unrealized Losses	3	2
Fair Value	$ 205	$ 230