INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 67,545	$ 64,089
Allowance for Credit Losses	0	0	$ 0	$ 0
Gross Unrealized Gains	4,345	5,308
Gross Unrealized Losses	4,746	4,128
Fair Value	67,144	65,269
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	35,594	34,386
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,061	1,587
Gross Unrealized Losses	3,768	3,099
Fair Value	32,887	32,874
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	31,870	29,678
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3,284	3,720
Gross Unrealized Losses	977	1,029
Fair Value	34,177	32,369
Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	81	25
Allowance for Credit Losses	0	0
Gross Unrealized Gains	0	1
Gross Unrealized Losses	1	0
Fair Value	80	26
Japan government and agencies | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,005	19,409
Allowance for Credit Losses	0	0
Gross Unrealized Gains	145	465
Gross Unrealized Losses	2,608	2,234
Fair Value	17,542	17,640
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,055	2,034
Municipalities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	919	869
Allowance for Credit Losses	0	0
Gross Unrealized Gains	50	65
Gross Unrealized Losses	105	79
Fair Value	864	855
Municipalities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,190	1,167
Allowance for Credit Losses	0	0
Gross Unrealized Gains	69	65
Gross Unrealized Losses	68	53
Fair Value	1,191	1,179
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	4,048	3,563
Mortgage- and asset-backed securities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	342	327
Allowance for Credit Losses	0	0
Gross Unrealized Gains	2	4
Gross Unrealized Losses	29	23
Fair Value	315	308
Mortgage- and asset-backed securities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,574	2,987
Allowance for Credit Losses	0	0
Gross Unrealized Gains	205	302
Gross Unrealized Losses	46	34
Fair Value	3,733	3,255
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	7,221	7,045
Public utilities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,771	2,746
Allowance for Credit Losses	0	0
Gross Unrealized Gains	155	202
Gross Unrealized Losses	155	108
Fair Value	2,771	2,840
Public utilities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,162	3,938
Allowance for Credit Losses	0	0
Gross Unrealized Gains	376	418
Gross Unrealized Losses	142	151
Fair Value	4,396	4,205
Public utilities | Other currencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	55
Allowance for Credit Losses	0
Gross Unrealized Gains	0
Gross Unrealized Losses	1
Fair Value	54
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	426	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	10	8
Fair Value	348	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	9,361	8,956
Banks/financial institutions | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,553	5,376
Allowance for Credit Losses	0	0
Gross Unrealized Gains	225	267
Gross Unrealized Losses	448	342
Fair Value	5,330	5,301
Banks/financial institutions | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,673	3,271
Allowance for Credit Losses	0	0
Gross Unrealized Gains	388	420
Gross Unrealized Losses	30	36
Fair Value	4,031	3,655
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	26,216	25,409
Other corporate | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,655	5,329
Allowance for Credit Losses	0	0
Gross Unrealized Gains	475	568
Gross Unrealized Losses	413	305
Fair Value	5,717	5,592
Other corporate | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	18,938	18,050
Allowance for Credit Losses	0	0
Gross Unrealized Gains	2,224	2,493
Gross Unrealized Losses	689	752
Fair Value	20,473	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	275	208
Allowance for Credit Losses	0	0
Gross Unrealized Gains	2	1
Gross Unrealized Losses	2	3
Fair Value	$ 275	$ 206