INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 87,881	$ 79,371
Allowance for Credit Losses	63
Gross Unrealized Gains	10,611	12,266
Gross Unrealized Losses	2,141	375
Fair Value	96,288	91,262
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	53,831	45,684
Allowance for Credit Losses	0
Gross Unrealized Gains	6,645	7,410
Gross Unrealized Losses	1,081	113
Fair Value	59,395	52,981
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	34,050	33,687
Allowance for Credit Losses	63
Gross Unrealized Gains	3,966	4,856
Gross Unrealized Losses	1,060	262
Fair Value	36,893	38,281
Japan government and agencies \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	31,853	30,929
Allowance for Credit Losses	0
Gross Unrealized Gains	4,766	5,169
Gross Unrealized Losses	14	0
Fair Value	36,605	36,098
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,440	1,847
Municipalities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	984	516
Allowance for Credit Losses	0
Gross Unrealized Gains	228	116
Gross Unrealized Losses	1	3
Fair Value	1,211	629
Municipalities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,084	1,077
Allowance for Credit Losses	0
Gross Unrealized Gains	145	141
Gross Unrealized Losses	0	0
Fair Value	1,229	1,218
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	497	410
Mortgage- and asset-backed securities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	306	229
Allowance for Credit Losses	0
Gross Unrealized Gains	31	25
Gross Unrealized Losses	0	0
Fair Value	337	254
Mortgage- and asset-backed securities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	152	149
Allowance for Credit Losses	0
Gross Unrealized Gains	8	7
Gross Unrealized Losses	0	0
Fair Value	160	156
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	9,192	6,780
Public utilities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,412	1,855
Allowance for Credit Losses	0
Gross Unrealized Gains	430	406
Gross Unrealized Losses	93	0
Fair Value	4,749	2,261
Public utilities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,844	3,804
Allowance for Credit Losses	0
Gross Unrealized Gains	649	725
Gross Unrealized Losses	50	10
Fair Value	4,443	4,519
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,609	1,042
Sovereign and supranational \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,300	680
Allowance for Credit Losses	0
Gross Unrealized Gains	33	50
Gross Unrealized Losses	27	0
Fair Value	1,306	730
Sovereign and supranational \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	239	239
Allowance for Credit Losses	0
Gross Unrealized Gains	65	73
Gross Unrealized Losses	1	0
Fair Value	303	312
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	10,378	10,287
Banks/financial institutions \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,159	6,152
Allowance for Credit Losses	0
Gross Unrealized Gains	404	700
Gross Unrealized Losses	557	86
Fair Value	7,006	6,766
Banks/financial institutions \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,848	2,879
Allowance for Credit Losses	0
Gross Unrealized Gains	545	646
Gross Unrealized Losses	21	4
Fair Value	3,372	3,521
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	35,194	34,496
Other corporate \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,817	5,323
Allowance for Credit Losses	0
Gross Unrealized Gains	753	944
Gross Unrealized Losses	389	24
Fair Value	8,181	6,243
Other corporate \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,533	25,246
Allowance for Credit Losses	63
Gross Unrealized Gains	2,531	3,255
Gross Unrealized Losses	988	248
Fair Value	27,013	28,253
U.S. government and agencies \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	350	293
Allowance for Credit Losses	0
Gross Unrealized Gains	23	9
Gross Unrealized Losses	0	0
Fair Value	$ 373	$ 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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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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