INVESTMENTS - Fair Value and Gross Unrealized Losses for Securities That Have Been in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	$ 30,364	$ 24,301
Total Unrealized Losses	4,746	4,128
Less than 12 months Fair Value	12,653	6,716
Less than 12 months Unrealized Losses	383	197
12 months or Longer Fair Value	17,711	17,585
12 months or Longer Unrealized Losses	4,363	3,931
U.S. government and agencies | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	68	106
Total Unrealized Losses	2	3
Less than 12 months Fair Value	33	59
Less than 12 months Unrealized Losses	0	1
12 months or Longer Fair Value	35	47
12 months or Longer Unrealized Losses	2	2
Japan government and agencies | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	11,951	8,136
Total Unrealized Losses	2,608	2,234
Less than 12 months Fair Value	6,090	2,070
Less than 12 months Unrealized Losses	179	57
12 months or Longer Fair Value	5,861	6,066
12 months or Longer Unrealized Losses	2,429	2,177
Municipalities | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	656	666
Total Unrealized Losses	68	53
Less than 12 months Fair Value	72	67
Less than 12 months Unrealized Losses	3	3
12 months or Longer Fair Value	584	599
12 months or Longer Unrealized Losses	65	50
Municipalities | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	339	341
Total Unrealized Losses	105	79
Less than 12 months Fair Value	96	96
Less than 12 months Unrealized Losses	8	2
12 months or Longer Fair Value	243	245
12 months or Longer Unrealized Losses	97	77
Mortgage- and asset-backed securities | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	1,091	567
Total Unrealized Losses	46	34
Less than 12 months Fair Value	697	173
Less than 12 months Unrealized Losses	20	2
12 months or Longer Fair Value	394	394
12 months or Longer Unrealized Losses	26	32
Mortgage- and asset-backed securities | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	208	196
Total Unrealized Losses	29	23
Less than 12 months Fair Value	22	12
Less than 12 months Unrealized Losses	0	0
12 months or Longer Fair Value	186	184
12 months or Longer Unrealized Losses	29	23
Public utilities | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	1,689	1,570
Total Unrealized Losses	142	151
Less than 12 months Fair Value	816	699
Less than 12 months Unrealized Losses	17	19
12 months or Longer Fair Value	873	871
12 months or Longer Unrealized Losses	125	132
Public utilities | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	1,037	1,020
Total Unrealized Losses	155	108
Less than 12 months Fair Value	258	368
Less than 12 months Unrealized Losses	23	11
12 months or Longer Fair Value	779	652
12 months or Longer Unrealized Losses	132	97
Public utilities | Other currencies
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	53
Total Unrealized Losses	1
Less than 12 months Fair Value	53
Less than 12 months Unrealized Losses	1
12 months or Longer Fair Value	0
12 months or Longer Unrealized Losses	0
Sovereign and supranational | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	0
Total Unrealized Losses	0
Less than 12 months Fair Value	0
Less than 12 months Unrealized Losses	0
12 months or Longer Fair Value	0
12 months or Longer Unrealized Losses	0
Sovereign and supranational | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	49	47
Total Unrealized Losses	10	8
Less than 12 months Fair Value	0	0
Less than 12 months Unrealized Losses	0	0
12 months or Longer Fair Value	49	47
12 months or Longer Unrealized Losses	10	8
Banks/financial institutions | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	749	625
Total Unrealized Losses	30	36
Less than 12 months Fair Value	544	376
Less than 12 months Unrealized Losses	7	7
12 months or Longer Fair Value	205	249
12 months or Longer Unrealized Losses	23	29
Banks/financial institutions | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	3,576	3,197
Total Unrealized Losses	448	342
Less than 12 months Fair Value	472	471
Less than 12 months Unrealized Losses	14	22
12 months or Longer Fair Value	3,104	2,726
12 months or Longer Unrealized Losses	434	320
Other corporate | Dollar-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	6,596	6,097
Total Unrealized Losses	689	752
Less than 12 months Fair Value	2,690	2,036
Less than 12 months Unrealized Losses	58	59
12 months or Longer Fair Value	3,906	4,061
12 months or Longer Unrealized Losses	631	693
Other corporate | Yen-denominated
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	2,302	1,733
Total Unrealized Losses	413	305
Less than 12 months Fair Value	810	289
Less than 12 months Unrealized Losses	53	14
12 months or Longer Fair Value	1,492	1,444
12 months or Longer Unrealized Losses	$ 360	$ 291