INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 87,228	$ 91,630
Allowance for Credit Losses	0	38
Gross Unrealized Gains	13,729	14,771
Gross Unrealized Losses	235	481
Fair Value	100,722	105,882
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	52,974	56,049
Allowance for Credit Losses	0	0
Gross Unrealized Gains	7,338	8,420
Gross Unrealized Losses	158	198
Fair Value	60,154	64,271
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	34,254	35,581
Allowance for Credit Losses	0	38
Gross Unrealized Gains	6,391	6,351
Gross Unrealized Losses	77	283
Fair Value	40,568	41,611
Japan government and agencies \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	31,549	32,959
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3,574	4,182
Gross Unrealized Losses	62	52
Fair Value	35,061	37,089
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,937	3,018
Municipalities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,224	1,324
Allowance for Credit Losses	0	0
Gross Unrealized Gains	323	374
Gross Unrealized Losses	5	5
Fair Value	1,542	1,693
Municipalities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,217	1,154
Allowance for Credit Losses	0	0
Gross Unrealized Gains	179	173
Gross Unrealized Losses	1	2
Fair Value	1,395	1,325
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,292	1,038
Mortgage- and asset-backed securities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	310	342
Allowance for Credit Losses	0	0
Gross Unrealized Gains	21	27
Gross Unrealized Losses	1	1
Fair Value	330	368
Mortgage- and asset-backed securities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	932	667
Allowance for Credit Losses	0	0
Gross Unrealized Gains	30	8
Gross Unrealized Losses	0	5
Fair Value	962	670
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	10,315	10,817
Public utilities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,584	4,777
Allowance for Credit Losses	0	0
Gross Unrealized Gains	930	1,096
Gross Unrealized Losses	2	1
Fair Value	5,512	5,872
Public utilities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,887	4,013
Allowance for Credit Losses	0	0
Gross Unrealized Gains	920	947
Gross Unrealized Losses	4	15
Fair Value	4,803	4,945
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,142	1,382
Sovereign and supranational \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	781	981
Allowance for Credit Losses	0	0
Gross Unrealized Gains	81	108
Gross Unrealized Losses	0	0
Fair Value	862	1,089
Sovereign and supranational \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	225	232
Allowance for Credit Losses	0	0
Gross Unrealized Gains	59	64
Gross Unrealized Losses	4	3
Fair Value	280	293
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	11,742	12,060
Banks/financial institutions \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,149	7,552
Allowance for Credit Losses	0	0
Gross Unrealized Gains	834	886
Gross Unrealized Losses	66	102
Fair Value	7,917	8,336
Banks/financial institutions \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,081	2,973
Allowance for Credit Losses	0	0
Gross Unrealized Gains	745	758
Gross Unrealized Losses	1	7
Fair Value	3,825	3,724
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	38,068	40,217
Other corporate \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,377	8,114
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,575	1,747
Gross Unrealized Losses	22	37
Fair Value	8,930	9,824
Other corporate \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	24,757	26,297
Allowance for Credit Losses	0	38
Gross Unrealized Gains	4,448	4,385
Gross Unrealized Losses	67	251
Fair Value	29,138	30,393
U.S. government and agencies \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	155	245
Allowance for Credit Losses	0	0
Gross Unrealized Gains	10	16
Gross Unrealized Losses	0	0
Fair Value	$ 165	$ 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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