INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 68,271		$ 64,089
Allowance for Credit Losses	0	$ 0	0	$ 0	$ 0	$ 0
Gross Unrealized Gains	4,390		5,308
Gross Unrealized Losses	5,385		4,128
Fair Value	67,276		65,269
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	35,343		34,386
Allowance for Credit Losses	0		0
Gross Unrealized Gains	829		1,587
Gross Unrealized Losses	4,562		3,099
Fair Value	31,610		32,874
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,800		29,678
Allowance for Credit Losses	0		0
Gross Unrealized Gains	3,557		3,720
Gross Unrealized Losses	823		1,029
Fair Value	35,534		32,369
Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	128		25
Allowance for Credit Losses	0		0
Gross Unrealized Gains	4		1
Gross Unrealized Losses	0		0
Fair Value	132		26
Japan government and agencies | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,541		19,409
Allowance for Credit Losses	0		0
Gross Unrealized Gains	83		465
Gross Unrealized Losses	3,260		2,234
Fair Value	16,364		17,640
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,986		2,034
Municipalities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	900		869
Allowance for Credit Losses	0		0
Gross Unrealized Gains	13		65
Gross Unrealized Losses	130		79
Fair Value	783		855
Municipalities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,187		1,167
Allowance for Credit Losses	0		0
Gross Unrealized Gains	76		65
Gross Unrealized Losses	60		53
Fair Value	1,203		1,179
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	4,362		3,563
Mortgage- and asset-backed securities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	322		327
Allowance for Credit Losses	0		0
Gross Unrealized Gains	2		4
Gross Unrealized Losses	34		23
Fair Value	290		308
Mortgage- and asset-backed securities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,903		2,987
Allowance for Credit Losses	0		0
Gross Unrealized Gains	166		302
Gross Unrealized Losses	45		34
Fair Value	4,024		3,255
Mortgage- and asset-backed securities | Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	46
Allowance for Credit Losses	0
Gross Unrealized Gains	2
Gross Unrealized Losses	0
Fair Value	48
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	7,430		7,045
Public utilities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,783		2,746
Allowance for Credit Losses	0		0
Gross Unrealized Gains	138		202
Gross Unrealized Losses	199		108
Fair Value	2,722		2,840
Public utilities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,333		3,938
Allowance for Credit Losses	0		0
Gross Unrealized Gains	428		418
Gross Unrealized Losses	111		151
Fair Value	4,650		4,205
Public utilities | Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	56
Allowance for Credit Losses	0
Gross Unrealized Gains	2
Gross Unrealized Losses	0
Fair Value	58
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	425		416
Sovereign and supranational | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	349		330
Allowance for Credit Losses	0		0
Gross Unrealized Gains	9		16
Gross Unrealized Losses	12		8
Fair Value	346		338
Sovereign and supranational | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	58		57
Allowance for Credit Losses	0		0
Gross Unrealized Gains	21		21
Gross Unrealized Losses	0		0
Fair Value	79		78
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	9,719		8,956
Banks/financial institutions | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,715		5,376
Allowance for Credit Losses	0		0
Gross Unrealized Gains	201		267
Gross Unrealized Losses	441		342
Fair Value	5,475		5,301
Banks/financial institutions | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,814		3,271
Allowance for Credit Losses	0		0
Gross Unrealized Gains	450		420
Gross Unrealized Losses	20		36
Fair Value	4,244		3,655
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	26,771		25,409
Other corporate | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,733		5,329
Allowance for Credit Losses	0		0
Gross Unrealized Gains	383		568
Gross Unrealized Losses	486		305
Fair Value	5,630		5,592
Other corporate | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,275		18,050
Allowance for Credit Losses	0		0
Gross Unrealized Gains	2,415		2,493
Gross Unrealized Losses	575		752
Fair Value	21,115		19,791
Other corporate | Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	26		25
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		1
Gross Unrealized Losses	0		0
Fair Value	26		26
U.S. government and agencies | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	230		208
Allowance for Credit Losses	0		0
Gross Unrealized Gains	1		1
Gross Unrealized Losses	12		3
Fair Value	$ 219		$ 206