INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 67,594	$ 70,689
Allowance for Credit Losses	0	0	$ 0	$ 0
Gross Unrealized Gains	6,038	6,050
Gross Unrealized Losses	3,502	3,449
Fair   Value	70,130	73,290
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	37,393	40,174
Allowance for Credit Losses	0	0
Gross Unrealized Gains	2,254	2,529
Gross Unrealized Losses	2,496	2,513
Fair   Value	37,151	40,190
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	30,201	30,515
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3,784	3,521
Gross Unrealized Losses	1,006	936
Fair   Value	32,979	33,100
Japan government and agencies | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	21,577	23,067
Allowance for Credit Losses	0	0
Gross Unrealized Gains	874	1,040
Gross Unrealized Losses	1,811	1,696
Fair   Value	20,640	22,411
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair   Value	2,210	2,298
Municipalities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	907	968
Allowance for Credit Losses	0	0
Gross Unrealized Gains	102	115
Gross Unrealized Losses	59	58
Fair   Value	950	1,025
Municipalities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,237	1,246
Allowance for Credit Losses	0	0
Gross Unrealized Gains	72	65
Gross Unrealized Losses	49	38
Fair   Value	1,260	1,273
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair   Value	3,385	3,086
Mortgage- and asset-backed securities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	201	215
Allowance for Credit Losses	0	0
Gross Unrealized Gains	6	6
Gross Unrealized Losses	12	11
Fair   Value	195	210
Mortgage- and asset-backed securities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,968	2,748
Allowance for Credit Losses	0	0
Gross Unrealized Gains	270	184
Gross Unrealized Losses	48	56
Fair   Value	3,190	2,876
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair   Value	7,313	7,592
Public utilities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,281	3,757
Allowance for Credit Losses	0	0
Gross Unrealized Gains	262	325
Gross Unrealized Losses	77	82
Fair   Value	3,466	4,000
Public utilities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,595	3,346
Allowance for Credit Losses	0	0
Gross Unrealized Gains	388	360
Gross Unrealized Losses	136	114
Fair   Value	3,847	3,592
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair   Value	490	537
Sovereign and supranational | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	349	373
Allowance for Credit Losses	0	0
Gross Unrealized Gains	24	24
Gross Unrealized Losses	7	7
Fair   Value	366	390
Sovereign and supranational | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	95	122
Allowance for Credit Losses	0	0
Gross Unrealized Gains	33	33
Gross Unrealized Losses	4	8
Fair   Value	124	147
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair   Value	8,899	8,835
Banks/financial institutions | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,546	5,896
Allowance for Credit Losses	0	0
Gross Unrealized Gains	323	320
Gross Unrealized Losses	285	365
Fair   Value	5,584	5,851
Banks/financial institutions | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,971	2,676
Allowance for Credit Losses	0	0
Gross Unrealized Gains	385	359
Gross Unrealized Losses	41	51
Fair   Value	3,315	2,984
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair   Value	27,019	28,342
Other corporate | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,532	5,898
Allowance for Credit Losses	0	0
Gross Unrealized Gains	663	699
Gross Unrealized Losses	245	294
Fair   Value	5,950	6,303
Other corporate | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,158	20,186
Allowance for Credit Losses	0	0
Gross Unrealized Gains	2,635	2,518
Gross Unrealized Losses	724	665
Fair   Value	21,069	22,039
U.S. government and agencies | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	177	191
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1	2
Gross Unrealized Losses	4	4
Fair   Value	$ 174	$ 189