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INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 70,030		$ 64,089
Allowance for Credit Losses	0	$ 0	0	$ 0	$ 0	$ 0
Gross Unrealized Gains	4,072		5,308
Gross Unrealized Losses	5,223		4,128
Fair Value	68,879		65,269
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	36,323		34,386
Allowance for Credit Losses	0		0
Gross Unrealized Gains	1,059		1,587
Gross Unrealized Losses	4,135		3,099
Fair Value	33,247		32,874
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	33,577		29,678
Allowance for Credit Losses	0		0
Gross Unrealized Gains	3,010		3,720
Gross Unrealized Losses	1,088		1,029
Fair Value	35,499		32,369
Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	130		25
Allowance for Credit Losses	0		0
Gross Unrealized Gains	3		1
Gross Unrealized Losses	0		0
Fair Value	133		26
Japan government and agencies \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,289		19,409
Allowance for Credit Losses	0		0
Gross Unrealized Gains	174		465
Gross Unrealized Losses	2,935		2,234
Fair Value	17,528		17,640
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,074		2,034
Municipalities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	950		869
Allowance for Credit Losses	0		0
Gross Unrealized Gains	49		65
Gross Unrealized Losses	118		79
Fair Value	881		855
Municipalities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,212		1,167
Allowance for Credit Losses	0		0
Gross Unrealized Gains	66		65
Gross Unrealized Losses	85		53
Fair Value	1,193		1,179
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	4,526		3,563
Mortgage- and asset-backed securities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	338		327
Allowance for Credit Losses	0		0
Gross Unrealized Gains	3		4
Gross Unrealized Losses	30		23
Fair Value	311		308
Mortgage- and asset-backed securities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,065		2,987
Allowance for Credit Losses	0		0
Gross Unrealized Gains	171		302
Gross Unrealized Losses	69		34
Fair Value	4,167		3,255
Mortgage- and asset-backed securities \| Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	47
Allowance for Credit Losses	0
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Fair Value	48
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	7,474		7,045
Public utilities \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,861		2,746
Allowance for Credit Losses	0		0
Gross Unrealized Gains	165		202
Gross Unrealized Losses	183		108
Fair Value	2,843		2,840
Public utilities \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,378		3,938
Allowance for Credit Losses	0		0
Gross Unrealized Gains	350		418
Gross Unrealized Losses	155		151
Fair Value	4,573		4,205
Public utilities \| Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	57
Allowance for Credit Losses	0
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Fair Value	58
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	435		416
Sovereign and supranational \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	359		330
Allowance for Credit Losses	0		0
Gross Unrealized Gains	9		16
Gross Unrealized Losses	11		8
Fair Value	357		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	20		21
Gross Unrealized Losses	0		0
Fair Value	78		78
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	9,827		8,956
Banks/financial institutions \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,771		5,376
Allowance for Credit Losses	0		0
Gross Unrealized Gains	214		267
Gross Unrealized Losses	420		342
Fair Value	5,565		5,301
Banks/financial institutions \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,930		3,271
Allowance for Credit Losses	0		0
Gross Unrealized Gains	371		420
Gross Unrealized Losses	39		36
Fair Value	4,262		3,655
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	26,790		25,409
Other corporate \| Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,755		5,329
Allowance for Credit Losses	0		0
Gross Unrealized Gains	445		568
Gross Unrealized Losses	438		305
Fair Value	5,762		5,592
Other corporate \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,708		18,050
Allowance for Credit Losses	0		0
Gross Unrealized Gains	2,031		2,493
Gross Unrealized Losses	738		752
Fair Value	21,001		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	1		1
Gross Unrealized Losses	0		0
Fair Value	27		26
U.S. government and agencies \| Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	226		208
Allowance for Credit Losses	0		0
Gross Unrealized Gains	1		1
Gross Unrealized Losses	2		3
Fair Value	$ 225		$ 206