INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 89,938	$ 79,371
Allowance for Credit Losses	38
Gross Unrealized Gains	13,866	12,266
Gross Unrealized Losses	638	375
Fair Value	103,128	91,262
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	55,074	45,684
Allowance for Credit Losses	0
Gross Unrealized Gains	7,839	7,410
Gross Unrealized Losses	317	113
Fair Value	62,596	52,981
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	34,864	33,687
Allowance for Credit Losses	38
Gross Unrealized Gains	6,027	4,856
Gross Unrealized Losses	321	262
Fair Value	40,532	38,281
Japan government and agencies \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,646	30,929
Allowance for Credit Losses	0
Gross Unrealized Gains	4,297	5,169
Gross Unrealized Losses	42	0
Fair Value	36,901	36,098
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,619	1,847
Municipalities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,013	516
Allowance for Credit Losses	0
Gross Unrealized Gains	305	116
Gross Unrealized Losses	3	3
Fair Value	1,315	629
Municipalities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,143	1,077
Allowance for Credit Losses	0
Gross Unrealized Gains	161	141
Gross Unrealized Losses	0	0
Fair Value	1,304	1,218
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	717	410
Mortgage- and asset-backed securities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	337	229
Allowance for Credit Losses	0
Gross Unrealized Gains	28	25
Gross Unrealized Losses	1	0
Fair Value	364	254
Mortgage- and asset-backed securities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	344	149
Allowance for Credit Losses	0
Gross Unrealized Gains	10	7
Gross Unrealized Losses	1	0
Fair Value	353	156
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	10,398	6,780
Public utilities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,585	1,855
Allowance for Credit Losses	0
Gross Unrealized Gains	960	406
Gross Unrealized Losses	1	0
Fair Value	5,544	2,261
Public utilities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,942	3,804
Allowance for Credit Losses	0
Gross Unrealized Gains	924	725
Gross Unrealized Losses	12	10
Fair Value	4,854	4,519
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,422	1,042
Sovereign and supranational \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,054	680
Allowance for Credit Losses	0
Gross Unrealized Gains	82	50
Gross Unrealized Losses	4	0
Fair Value	1,132	730
Sovereign and supranational \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	230	239
Allowance for Credit Losses	0
Gross Unrealized Gains	66	73
Gross Unrealized Losses	6	0
Fair Value	290	312
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	11,696	10,287
Banks/financial institutions \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,505	6,152
Allowance for Credit Losses	0
Gross Unrealized Gains	683	700
Gross Unrealized Losses	180	86
Fair Value	8,008	6,766
Banks/financial institutions \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,956	2,879
Allowance for Credit Losses	0
Gross Unrealized Gains	738	646
Gross Unrealized Losses	6	4
Fair Value	3,688	3,521
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	39,081	34,496
Other corporate \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,934	5,323
Allowance for Credit Losses	0
Gross Unrealized Gains	1,484	944
Gross Unrealized Losses	86	24
Fair Value	9,332	6,243
Other corporate \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,973	25,246
Allowance for Credit Losses	38
Gross Unrealized Gains	4,110	3,255
Gross Unrealized Losses	296	248
Fair Value	29,749	28,253
U.S. government and agencies \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	276	293
Allowance for Credit Losses	0
Gross Unrealized Gains	18	9
Gross Unrealized Losses	0	0
Fair Value	$ 294	$ 302

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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