INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 88,465	$ 91,630
Allowance for Credit Losses	27	38
Gross Unrealized Gains	12,715	14,771
Gross Unrealized Losses	409	481
Fair Value	100,744	105,882
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	53,727	56,049
Allowance for Credit Losses	0	0
Gross Unrealized Gains	7,441	8,420
Gross Unrealized Losses	230	198
Fair Value	60,938	64,271
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	34,738	35,581
Allowance for Credit Losses	27	38
Gross Unrealized Gains	5,274	6,351
Gross Unrealized Losses	179	283
Fair Value	39,806	41,611
Japan government and agencies \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,013	32,959
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3,614	4,182
Gross Unrealized Losses	80	52
Fair Value	35,547	37,089
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,901	3,018
Municipalities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,238	1,324
Allowance for Credit Losses	0	0
Gross Unrealized Gains	331	374
Gross Unrealized Losses	6	5
Fair Value	1,563	1,693
Municipalities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,188	1,154
Allowance for Credit Losses	0	0
Gross Unrealized Gains	154	173
Gross Unrealized Losses	4	2
Fair Value	1,338	1,325
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,184	1,038
Mortgage- and asset-backed securities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	318	342
Allowance for Credit Losses	0	0
Gross Unrealized Gains	23	27
Gross Unrealized Losses	1	1
Fair Value	340	368
Mortgage- and asset-backed securities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	816	667
Allowance for Credit Losses	0	0
Gross Unrealized Gains	29	8
Gross Unrealized Losses	1	5
Fair Value	844	670
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	10,199	10,817
Public utilities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,616	4,777
Allowance for Credit Losses	0	0
Gross Unrealized Gains	970	1,096
Gross Unrealized Losses	5	1
Fair Value	5,581	5,872
Public utilities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,867	4,013
Allowance for Credit Losses	0	0
Gross Unrealized Gains	760	947
Gross Unrealized Losses	9	15
Fair Value	4,618	4,945
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,275	1,382
Sovereign and supranational \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	902	981
Allowance for Credit Losses	0	0
Gross Unrealized Gains	89	108
Gross Unrealized Losses	0	0
Fair Value	991	1,089
Sovereign and supranational \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	226	232
Allowance for Credit Losses	0	0
Gross Unrealized Gains	61	64
Gross Unrealized Losses	3	3
Fair Value	284	293
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	11,575	12,060
Banks/financial institutions \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,188	7,552
Allowance for Credit Losses	0	0
Gross Unrealized Gains	837	886
Gross Unrealized Losses	103	102
Fair Value	7,922	8,336
Banks/financial institutions \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,991	2,973
Allowance for Credit Losses	0	0
Gross Unrealized Gains	668	758
Gross Unrealized Losses	6	7
Fair Value	3,653	3,724
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	37,819	40,217
Other corporate \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,452	8,114
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,577	1,747
Gross Unrealized Losses	35	37
Fair Value	8,994	9,824
Other corporate \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,416	26,297
Allowance for Credit Losses	27	38
Gross Unrealized Gains	3,592	4,385
Gross Unrealized Losses	156	251
Fair Value	28,825	30,393
U.S. government and agencies \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	234	245
Allowance for Credit Losses	0	0
Gross Unrealized Gains	10	16
Gross Unrealized Losses	0	0
Fair Value	$ 244	$ 261

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, before tax, of unrealized loss 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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