INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 64,603		$ 70,689
Allowance for Credit Losses	0	$ 0	0	$ 0	$ 0	$ 0
Gross Unrealized Gains	5,971		6,050
Gross Unrealized Losses	4,315		3,449
Fair   Value	66,259		73,290
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	34,744		40,174
Allowance for Credit Losses	0		0
Gross Unrealized Gains	1,741		2,529
Gross Unrealized Losses	3,192		2,513
Fair   Value	33,293		40,190
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	29,859		30,515
Allowance for Credit Losses	0		0
Gross Unrealized Gains	4,230		3,521
Gross Unrealized Losses	1,123		936
Fair   Value	32,966		33,100
Japan government and agencies | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,952		23,067
Allowance for Credit Losses	0		0
Gross Unrealized Gains	524		1,040
Gross Unrealized Losses	2,376		1,696
Fair   Value	18,100		22,411
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair   Value	2,083		2,298
Municipalities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	853		968
Allowance for Credit Losses	0		0
Gross Unrealized Gains	71		115
Gross Unrealized Losses	74		58
Fair   Value	850		1,025
Municipalities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,206		1,246
Allowance for Credit Losses	0		0
Gross Unrealized Gains	76		65
Gross Unrealized Losses	49		38
Fair   Value	1,233		1,273
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair   Value	3,703		3,086
Mortgage- and asset-backed securities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	345		215
Allowance for Credit Losses	0		0
Gross Unrealized Gains	4		6
Gross Unrealized Losses	22		11
Fair   Value	327		210
Mortgage- and asset-backed securities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,064		2,748
Allowance for Credit Losses	0		0
Gross Unrealized Gains	356		184
Gross Unrealized Losses	44		56
Fair   Value	3,376		2,876
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair   Value	6,826		7,592
Public utilities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,821		3,757
Allowance for Credit Losses	0		0
Gross Unrealized Gains	220		325
Gross Unrealized Losses	94		82
Fair   Value	2,947		4,000
Public utilities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,603		3,346
Allowance for Credit Losses	0		0
Gross Unrealized Gains	426		360
Gross Unrealized Losses	150		114
Fair   Value	3,879		3,592
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair   Value	460		537
Sovereign and supranational | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	326		373
Allowance for Credit Losses	0		0
Gross Unrealized Gains	17		24
Gross Unrealized Losses	7		7
Fair   Value	336		390
Sovereign and supranational | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	93		122
Allowance for Credit Losses	0		0
Gross Unrealized Gains	34		33
Gross Unrealized Losses	3		8
Fair   Value	124		147
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair   Value	8,699		8,835
Banks/financial institutions | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,258		5,896
Allowance for Credit Losses	0		0
Gross Unrealized Gains	294		320
Gross Unrealized Losses	332		365
Fair   Value	5,220		5,851
Banks/financial institutions | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,090		2,676
Allowance for Credit Losses	0		0
Gross Unrealized Gains	439		359
Gross Unrealized Losses	50		51
Fair   Value	3,479		2,984
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair   Value	26,207		28,342
Other corporate | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,189		5,898
Allowance for Credit Losses	0		0
Gross Unrealized Gains	611		699
Gross Unrealized Losses	287		294
Fair   Value	5,513		6,303
Other corporate | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	18,619		20,186
Allowance for Credit Losses	0		0
Gross Unrealized Gains	2,898		2,518
Gross Unrealized Losses	823		665
Fair   Value	20,694		22,039
U.S. government and agencies | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	184		191
Allowance for Credit Losses	0		0
Gross Unrealized Gains	1		2
Gross Unrealized Losses	4		4
Fair   Value	$ 181		$ 189