INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 81,464	$ 85,369
Allowance for Credit Losses	0	0
Gross Unrealized Gains	9,438	13,566
Gross Unrealized Losses	937	239
Fair Value	89,965	98,696
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	48,041	51,160
Allowance for Credit Losses	0	0
Gross Unrealized Gains	4,988	6,994
Gross Unrealized Losses	625	167
Fair Value	52,404	57,987
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	33,423	34,209
Allowance for Credit Losses	0	0
Gross Unrealized Gains	4,450	6,572
Gross Unrealized Losses	312	72
Fair Value	37,561	40,709
Japan government and agencies \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	28,492	30,335
Allowance for Credit Losses	0	0
Gross Unrealized Gains	2,403	3,343
Gross Unrealized Losses	345	61
Fair Value	30,550	33,617
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,776	3,036
Municipalities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,120	1,192
Allowance for Credit Losses	0	0
Gross Unrealized Gains	259	322
Gross Unrealized Losses	15	5
Fair Value	1,364	1,509
Municipalities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,316	1,340
Allowance for Credit Losses	0	0
Gross Unrealized Gains	112	189
Gross Unrealized Losses	16	2
Fair Value	1,412	1,527
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,366	1,246
Mortgage- and asset-backed securities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	280	300
Allowance for Credit Losses	0	0
Gross Unrealized Gains	14	19
Gross Unrealized Losses	3	1
Fair Value	291	318
Mortgage- and asset-backed securities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,055	897
Allowance for Credit Losses	0	0
Gross Unrealized Gains	43	33
Gross Unrealized Losses	23	2
Fair Value	1,075	928
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	9,189	10,051
Public utilities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,240	4,462
Allowance for Credit Losses	0	0
Gross Unrealized Gains	629	906
Gross Unrealized Losses	17	2
Fair Value	4,852	5,366
Public utilities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,783	3,781
Allowance for Credit Losses	0	0
Gross Unrealized Gains	583	909
Gross Unrealized Losses	29	5
Fair Value	4,337	4,685
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,035	1,115
Sovereign and supranational \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	714	760
Allowance for Credit Losses	0	0
Gross Unrealized Gains	64	82
Gross Unrealized Losses	0	0
Fair Value	778	842
Sovereign and supranational \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	213	222
Allowance for Credit Losses	0	0
Gross Unrealized Gains	52	57
Gross Unrealized Losses	8	6
Fair Value	257	273
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	10,686	11,591
Banks/financial institutions \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,607	6,963
Allowance for Credit Losses	0	0
Gross Unrealized Gains	530	787
Gross Unrealized Losses	168	72
Fair Value	6,969	7,678
Banks/financial institutions \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,217	3,169
Allowance for Credit Losses	0	0
Gross Unrealized Gains	526	747
Gross Unrealized Losses	26	3
Fair Value	3,717	3,913
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	34,201	37,837
Other corporate \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,588	7,148
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,089	1,535
Gross Unrealized Losses	77	26
Fair Value	7,600	8,657
Other corporate \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	23,679	24,604
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3,129	4,629
Gross Unrealized Losses	207	53
Fair Value	26,601	29,180
U.S. government and agencies \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	160	196
Allowance for Credit Losses	0	0
Gross Unrealized Gains	5	8
Gross Unrealized Losses	3	1
Fair Value	$ 162	$ 203

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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Amount of 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 of allowance for credit loss 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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