INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 70,419		$ 70,689
Allowance for Credit Losses	0	$ 0	0	$ 0	$ 0	$ 0
Gross Unrealized Gains	5,748		6,050
Gross Unrealized Losses	3,875		3,449
Fair Value	72,292		73,290
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	38,889		40,174
Allowance for Credit Losses	0		0
Gross Unrealized Gains	2,040		2,529
Gross Unrealized Losses	3,177		2,513
Fair Value	37,752		40,190
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	31,503		30,515
Allowance for Credit Losses	0		0
Gross Unrealized Gains	3,707		3,521
Gross Unrealized Losses	698		936
Fair Value	34,512		33,100
Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27
Allowance for Credit Losses	0
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Fair Value	28
Japan government and agencies | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,304		23,067
Allowance for Credit Losses	0		0
Gross Unrealized Gains	758		1,040
Gross Unrealized Losses	2,322		1,696
Fair Value	20,740		22,411
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,230		2,298
Municipalities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	963		968
Allowance for Credit Losses	0		0
Gross Unrealized Gains	85		115
Gross Unrealized Losses	77		58
Fair Value	971		1,025
Municipalities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,218		1,246
Allowance for Credit Losses	0		0
Gross Unrealized Gains	73		65
Gross Unrealized Losses	32		38
Fair Value	1,259		1,273
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	3,730		3,086
Mortgage- and asset-backed securities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	384		215
Allowance for Credit Losses	0		0
Gross Unrealized Gains	5		6
Gross Unrealized Losses	22		11
Fair Value	367		210
Mortgage- and asset-backed securities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,211		2,748
Allowance for Credit Losses	0		0
Gross Unrealized Gains	199		184
Gross Unrealized Losses	47		56
Fair Value	3,363		2,876
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	7,474		7,592
Public utilities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,043		3,757
Allowance for Credit Losses	0		0
Gross Unrealized Gains	232		325
Gross Unrealized Losses	116		82
Fair Value	3,159		4,000
Public utilities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,982		3,346
Allowance for Credit Losses	0		0
Gross Unrealized Gains	431		360
Gross Unrealized Losses	98		114
Fair Value	4,315		3,592
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	494		537
Sovereign and supranational | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	368		373
Allowance for Credit Losses	0		0
Gross Unrealized Gains	23		24
Gross Unrealized Losses	8		7
Fair Value	383		390
Sovereign and supranational | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	79		122
Allowance for Credit Losses	0		0
Gross Unrealized Gains	32		33
Gross Unrealized Losses	0		8
Fair Value	111		147
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	9,760		8,835
Banks/financial institutions | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,929		5,896
Allowance for Credit Losses	0		0
Gross Unrealized Gains	309		320
Gross Unrealized Losses	332		365
Fair Value	5,906		5,851
Banks/financial institutions | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,462		2,676
Allowance for Credit Losses	0		0
Gross Unrealized Gains	415		359
Gross Unrealized Losses	23		51
Fair Value	3,854		2,984
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	27,663		28,342
Other corporate | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,898		5,898
Allowance for Credit Losses	0		0
Gross Unrealized Gains	628		699
Gross Unrealized Losses	300		294
Fair Value	6,226		6,303
Other corporate | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,352		20,186
Allowance for Credit Losses	0		0
Gross Unrealized Gains	2,554		2,518
Gross Unrealized Losses	497		665
Fair Value	21,409		22,039
Other corporate | Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27
Allowance for Credit Losses	0
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Fair Value	28
U.S. government and agencies | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	199		191
Allowance for Credit Losses	0		0
Gross Unrealized Gains	3		2
Gross Unrealized Losses	1		4
Fair Value	$ 201		$ 189