INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 81,474	$ 76,856
Gross Unrealized Gains	13,002	7,733
Gross Unrealized Losses	536	1,694
Fair Value	93,940	82,895
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	47,060	44,027
Gross Unrealized Gains	8,109	5,308
Gross Unrealized Losses	119	435
Fair Value	55,050	48,900
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	34,414	32,829
Gross Unrealized Gains	4,893	2,425
Gross Unrealized Losses	417	1,259
Fair Value	38,890	33,995
Japan government and agencies | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	32,076	30,637
Gross Unrealized Gains	5,858	3,700
Gross Unrealized Losses	0	140
Fair Value	37,934	34,197
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,898	1,863
Municipalities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	524	385
Gross Unrealized Gains	125	32
Gross Unrealized Losses	1	9
Fair Value	648	408
Municipalities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	1,095	1,343
Gross Unrealized Gains	155	120
Gross Unrealized Losses	0	8
Fair Value	1,250	1,455
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	409	339
Mortgage- and asset-backed securities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	235	155
Gross Unrealized Gains	28	22
Gross Unrealized Losses	0	0
Fair Value	263	177
Mortgage- and asset-backed securities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	139	155
Gross Unrealized Gains	7	8
Gross Unrealized Losses	0	1
Fair Value	146	162
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	6,950	7,171
Public utilities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	1,864	1,732
Gross Unrealized Gains	395	280
Gross Unrealized Losses	0	4
Fair Value	2,259	2,008
Public utilities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	3,918	4,772
Gross Unrealized Gains	785	496
Gross Unrealized Losses	12	105
Fair Value	4,691	5,163
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,084	1,260
Sovereign and supranational | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	717	826
Gross Unrealized Gains	53	123
Gross Unrealized Losses	0	0
Fair Value	770	949
Sovereign and supranational | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	240	251
Gross Unrealized Gains	74	60
Gross Unrealized Losses	0	0
Fair Value	314	311
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	10,644	8,918
Banks/financial institutions | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	6,384	5,440
Gross Unrealized Gains	694	502
Gross Unrealized Losses	98	238
Fair Value	6,980	5,704
Banks/financial institutions | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	3,020	2,860
Gross Unrealized Gains	652	389
Gross Unrealized Losses	8	35
Fair Value	3,664	3,214
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	34,659	29,002
Other corporate | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	5,260	4,852
Gross Unrealized Gains	956	649
Gross Unrealized Losses	20	44
Fair Value	6,196	5,457
Other corporate | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	25,655	23,311
Gross Unrealized Gains	3,205	1,343
Gross Unrealized Losses	397	1,109
Fair Value	28,463	23,545
U.S. government and agencies | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	347	137
Gross Unrealized Gains	15	9
Gross Unrealized Losses	0	1
Fair Value	$ 362	$ 145