INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 75,306	$ 75,469
Allowance for Credit Losses	0	0
Gross Unrealized Gains	5,875	4,800
Gross Unrealized Losses	3,082	4,528
Fair Value	78,099	75,741
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	43,767	43,920
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3,119	2,595
Gross Unrealized Losses	1,931	2,849
Fair Value	44,955	43,666
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	31,539	31,549
Allowance for Credit Losses	0	0
Gross Unrealized Gains	2,756	2,205
Gross Unrealized Losses	1,151	1,679
Fair Value	33,144	32,075
Japan government and agencies \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,264	25,418
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,534	1,259
Gross Unrealized Losses	1,079	1,724
Fair Value	25,719	24,953
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,416	2,320
Municipalities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,028	1,034
Allowance for Credit Losses	0	0
Gross Unrealized Gains	152	124
Gross Unrealized Losses	42	61
Fair Value	1,138	1,097
Municipalities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,280	1,269
Allowance for Credit Losses	0	0
Gross Unrealized Gains	53	43
Gross Unrealized Losses	55	89
Fair Value	1,278	1,223
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,567	2,146
Mortgage- and asset-backed securities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	238	241
Allowance for Credit Losses	0	0
Gross Unrealized Gains	9	8
Gross Unrealized Losses	8	12
Fair Value	239	237
Mortgage- and asset-backed securities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,307	1,926
Allowance for Credit Losses	0	0
Gross Unrealized Gains	90	67
Gross Unrealized Losses	69	84
Fair Value	2,328	1,909
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	7,933	7,666
Public utilities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,979	3,932
Allowance for Credit Losses	0	0
Gross Unrealized Gains	351	301
Gross Unrealized Losses	63	108
Fair Value	4,267	4,125
Public utilities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,479	3,481
Allowance for Credit Losses	0	0
Gross Unrealized Gains	315	240
Gross Unrealized Losses	128	180
Fair Value	3,666	3,541
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	728	834
Sovereign and supranational \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	543	659
Allowance for Credit Losses	0	0
Gross Unrealized Gains	31	24
Gross Unrealized Losses	3	5
Fair Value	571	678
Sovereign and supranational \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	132	133
Allowance for Credit Losses	0	0
Gross Unrealized Gains	36	35
Gross Unrealized Losses	11	12
Fair Value	157	156
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	9,420	9,299
Banks/financial institutions \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,313	6,348
Allowance for Credit Losses	0	0
Gross Unrealized Gains	337	324
Gross Unrealized Losses	427	531
Fair Value	6,223	6,141
Banks/financial institutions \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,966	2,992
Allowance for Credit Losses	0	0
Gross Unrealized Gains	316	271
Gross Unrealized Losses	85	105
Fair Value	3,197	3,158
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	29,142	28,362
Other corporate \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,402	6,288
Allowance for Credit Losses	0	0
Gross Unrealized Gains	705	555
Gross Unrealized Losses	309	408
Fair Value	6,798	6,435
Other corporate \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	21,196	21,579
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,946	1,549
Gross Unrealized Losses	798	1,201
Fair Value	22,344	21,927
U.S. government and agencies \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	179	169
Allowance for Credit Losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	5	8
Fair Value	$ 174	$ 161

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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