INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 88,488	$ 91,630
Allowance for Credit Losses	26	38
Gross Unrealized Gains	14,071	14,771
Gross Unrealized Losses	249	481
Fair Value	102,284	105,882
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	53,697	56,049
Allowance for Credit Losses	0	0
Gross Unrealized Gains	7,617	8,420
Gross Unrealized Losses	176	198
Fair Value	61,138	64,271
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	34,791	35,581
Allowance for Credit Losses	26	38
Gross Unrealized Gains	6,454	6,351
Gross Unrealized Losses	73	283
Fair Value	41,146	41,611
Japan government and agencies \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	31,948	32,959
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3,620	4,182
Gross Unrealized Losses	78	52
Fair Value	35,490	37,089
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,965	3,018
Municipalities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,240	1,324
Allowance for Credit Losses	0	0
Gross Unrealized Gains	341	374
Gross Unrealized Losses	7	5
Fair Value	1,574	1,693
Municipalities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,208	1,154
Allowance for Credit Losses	0	0
Gross Unrealized Gains	184	173
Gross Unrealized Losses	1	2
Fair Value	1,391	1,325
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,284	1,038
Mortgage- and asset-backed securities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	316	342
Allowance for Credit Losses	0	0
Gross Unrealized Gains	22	27
Gross Unrealized Losses	1	1
Fair Value	337	368
Mortgage- and asset-backed securities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	920	667
Allowance for Credit Losses	0	0
Gross Unrealized Gains	27	8
Gross Unrealized Losses	0	5
Fair Value	947	670
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	10,393	10,817
Public utilities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,619	4,777
Allowance for Credit Losses	0	0
Gross Unrealized Gains	981	1,096
Gross Unrealized Losses	2	1
Fair Value	5,598	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	932	947
Gross Unrealized Losses	4	15
Fair Value	4,795	4,945
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,258	1,382
Sovereign and supranational \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	881	981
Allowance for Credit Losses	0	0
Gross Unrealized Gains	91	108
Gross Unrealized Losses	0	0
Fair Value	972	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	60	64
Gross Unrealized Losses	0	3
Fair Value	286	293
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	11,904	12,060
Banks/financial institutions \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,230	7,552
Allowance for Credit Losses	0	0
Gross Unrealized Gains	897	886
Gross Unrealized Losses	65	102
Fair Value	8,062	8,336
Banks/financial institutions \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,083	2,973
Allowance for Credit Losses	0	0
Gross Unrealized Gains	761	758
Gross Unrealized Losses	2	7
Fair Value	3,842	3,724
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	38,798	40,217
Other corporate \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,463	8,114
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,665	1,747
Gross Unrealized Losses	23	37
Fair Value	9,105	9,824
Other corporate \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,306	26,297
Allowance for Credit Losses	26	38
Gross Unrealized Gains	4,479	4,385
Gross Unrealized Losses	66	251
Fair Value	29,693	30,393
U.S. government and agencies \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	181	245
Allowance for Credit Losses	0	0
Gross Unrealized Gains	11	16
Gross Unrealized Losses	0	0
Fair Value	$ 192	$ 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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