INVESTMENTS - Available-for-Sale Debt Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 64,089	$ 70,689
Allowance for Credit Losses	0	0	$ 0	$ 0
Gross Unrealized Gains	5,308	6,050
Gross Unrealized Losses	4,128	3,449
Fair Value	65,269	73,290
Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	34,386	40,174
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,587	2,529
Gross Unrealized Losses	3,099	2,513
Fair Value	32,874	40,190
U.S. dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	29,678	30,515
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3,720	3,521
Gross Unrealized Losses	1,029	936
Fair Value	32,369	33,100
Other currencies
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	25
Allowance for Credit Losses	0
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Fair Value	26
Japan government and agencies | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	19,409	23,067
Allowance for Credit Losses	0	0
Gross Unrealized Gains	465	1,040
Gross Unrealized Losses	2,234	1,696
Fair Value	17,640	22,411
Municipalities
Debt Securities, Available-for-Sale [Line Items]
Fair Value	2,034	2,298
Municipalities | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	869	968
Allowance for Credit Losses	0	0
Gross Unrealized Gains	65	115
Gross Unrealized Losses	79	58
Fair Value	855	1,025
Municipalities | U.S. dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1,167	1,246
Allowance for Credit Losses	0	0
Gross Unrealized Gains	65	65
Gross Unrealized Losses	53	38
Fair Value	1,179	1,273
Mortgage- and asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Fair Value	3,563	3,086
Mortgage- and asset-backed securities | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	327	215
Allowance for Credit Losses	0	0
Gross Unrealized Gains	4	6
Gross Unrealized Losses	23	11
Fair Value	308	210
Mortgage- and asset-backed securities | U.S. dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	2,987	2,748
Allowance for Credit Losses	0	0
Gross Unrealized Gains	302	184
Gross Unrealized Losses	34	56
Fair Value	3,255	2,876
Public utilities
Debt Securities, Available-for-Sale [Line Items]
Fair Value	7,045	7,592
Public utilities | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	2,746	3,757
Allowance for Credit Losses	0	0
Gross Unrealized Gains	202	325
Gross Unrealized Losses	108	82
Fair Value	2,840	4,000
Public utilities | U.S. dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	3,938	3,346
Allowance for Credit Losses	0	0
Gross Unrealized Gains	418	360
Gross Unrealized Losses	151	114
Fair Value	4,205	3,592
Sovereign and supranational
Debt Securities, Available-for-Sale [Line Items]
Fair Value	416	537
Sovereign and supranational | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	330	373
Allowance for Credit Losses	0	0
Gross Unrealized Gains	16	24
Gross Unrealized Losses	8	7
Fair Value	338	390
Sovereign and supranational | U.S. dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	57	122
Allowance for Credit Losses	0	0
Gross Unrealized Gains	21	33
Gross Unrealized Losses	0	8
Fair Value	78	147
Banks/financial institutions
Debt Securities, Available-for-Sale [Line Items]
Fair Value	8,956	8,835
Banks/financial institutions | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	5,376	5,896
Allowance for Credit Losses	0	0
Gross Unrealized Gains	267	320
Gross Unrealized Losses	342	365
Fair Value	5,301	5,851
Banks/financial institutions | U.S. dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	3,271	2,676
Allowance for Credit Losses	0	0
Gross Unrealized Gains	420	359
Gross Unrealized Losses	36	51
Fair Value	3,655	2,984
Other corporate
Debt Securities, Available-for-Sale [Line Items]
Fair Value	25,409	28,342
Other corporate | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	5,329	5,898
Allowance for Credit Losses	0	0
Gross Unrealized Gains	568	699
Gross Unrealized Losses	305	294
Fair Value	5,592	6,303
Other corporate | U.S. dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	18,050	20,186
Allowance for Credit Losses	0	0
Gross Unrealized Gains	2,493	2,518
Gross Unrealized Losses	752	665
Fair Value	19,791	22,039
Other corporate | Other currencies
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	25
Allowance for Credit Losses	0
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Fair Value	26
U.S. government and agencies | U.S. dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	208	191
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1	2
Gross Unrealized Losses	3	4
Fair Value	$ 206	$ 189