INVESTMENTS - Available-for-Sale Debt Securities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	$ 91,630	$ 79,371
Allowance for Credit Losses	38
Gross Unrealized Gains	14,771	12,266
Gross Unrealized Losses	481	375
Fair Value	105,882	91,262
Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	56,049	45,684
Allowance for Credit Losses	0
Gross Unrealized Gains	8,420	7,410
Gross Unrealized Losses	198	113
Fair Value	64,271	52,981
Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	35,581	33,687
Allowance for Credit Losses	38
Gross Unrealized Gains	6,351	4,856
Gross Unrealized Losses	283	262
Fair Value	41,611	38,281
Japan government and agencies \| Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	32,959	30,929
Allowance for Credit Losses	0
Gross Unrealized Gains	4,182	5,169
Gross Unrealized Losses	52	0
Fair Value	37,089	36,098
U.S. government and agencies \| Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	245	293
Allowance for Credit Losses	0
Gross Unrealized Gains	16	9
Gross Unrealized Losses	0	0
Fair Value	261	302
Municipalities
Debt Securities, Available-for-Sale [Line Items]
Fair Value	3,018	1,847
Municipalities \| Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	1,324	516
Allowance for Credit Losses	0
Gross Unrealized Gains	374	116
Gross Unrealized Losses	5	3
Fair Value	1,693	629
Municipalities \| Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	1,154	1,077
Allowance for Credit Losses	0
Gross Unrealized Gains	173	141
Gross Unrealized Losses	2	0
Fair Value	1,325	1,218
Mortgage- and asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Fair Value	1,038	410
Mortgage- and asset-backed securities \| Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	342	229
Allowance for Credit Losses	0
Gross Unrealized Gains	27	25
Gross Unrealized Losses	1	0
Fair Value	368	254
Mortgage- and asset-backed securities \| Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	667	149
Allowance for Credit Losses	0
Gross Unrealized Gains	8	7
Gross Unrealized Losses	5	0
Fair Value	670	156
Public utilities
Debt Securities, Available-for-Sale [Line Items]
Fair Value	10,817	6,780
Public utilities \| Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	4,777	1,855
Allowance for Credit Losses	0
Gross Unrealized Gains	1,096	406
Gross Unrealized Losses	1	0
Fair Value	5,872	2,261
Public utilities \| Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	4,013	3,804
Allowance for Credit Losses	0
Gross Unrealized Gains	947	725
Gross Unrealized Losses	15	10
Fair Value	4,945	4,519
Sovereign and supranational
Debt Securities, Available-for-Sale [Line Items]
Fair Value	1,382	1,042
Sovereign and supranational \| Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	981	680
Allowance for Credit Losses	0
Gross Unrealized Gains	108	50
Gross Unrealized Losses	0	0
Fair Value	1,089	730
Sovereign and supranational \| Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	232	239
Allowance for Credit Losses	0
Gross Unrealized Gains	64	73
Gross Unrealized Losses	3	0
Fair Value	293	312
Banks/financial institutions
Debt Securities, Available-for-Sale [Line Items]
Fair Value	12,060	10,287
Banks/financial institutions \| Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	7,552	6,152
Allowance for Credit Losses	0
Gross Unrealized Gains	886	700
Gross Unrealized Losses	102	86
Fair Value	8,336	6,766
Banks/financial institutions \| Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	2,973	2,879
Allowance for Credit Losses	0
Gross Unrealized Gains	758	646
Gross Unrealized Losses	7	4
Fair Value	3,724	3,521
Other corporate
Debt Securities, Available-for-Sale [Line Items]
Fair Value	40,217	34,496
Other corporate \| Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	8,114	5,323
Allowance for Credit Losses	0
Gross Unrealized Gains	1,747	944
Gross Unrealized Losses	37	24
Fair Value	9,824	6,243
Other corporate \| Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	26,297	25,246
Allowance for Credit Losses	38
Gross Unrealized Gains	4,385	3,255
Gross Unrealized Losses	251	248
Fair Value	$ 30,393	$ 28,253

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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