INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 81,195	$ 76,856
Gross Unrealized Gains	11,949	7,733
Gross Unrealized Losses	709	1,694
Fair Value	92,435	82,895
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	47,002	44,027
Gross Unrealized Gains	8,033	5,308
Gross Unrealized Losses	129	435
Fair Value	54,906	48,900
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	34,193	32,829
Gross Unrealized Gains	3,916	2,425
Gross Unrealized Losses	580	1,259
Fair Value	37,529	33,995
Japan government and agencies | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	32,154	30,637
Gross Unrealized Gains	5,800	3,700
Gross Unrealized Losses	0	140
Fair Value	37,954	34,197
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,975	1,863
Municipalities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	525	385
Gross Unrealized Gains	120	32
Gross Unrealized Losses	1	9
Fair Value	644	408
Municipalities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	1,198	1,343
Gross Unrealized Gains	133	120
Gross Unrealized Losses	0	8
Fair Value	1,331	1,455
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	416	339
Mortgage- and asset-backed securities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	237	155
Gross Unrealized Gains	29	22
Gross Unrealized Losses	0	0
Fair Value	266	177
Mortgage- and asset-backed securities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	143	155
Gross Unrealized Gains	7	8
Gross Unrealized Losses	0	1
Fair Value	150	162
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	6,805	7,171
Public utilities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	1,865	1,732
Gross Unrealized Gains	392	280
Gross Unrealized Losses	0	4
Fair Value	2,257	2,008
Public utilities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	3,941	4,772
Gross Unrealized Gains	634	496
Gross Unrealized Losses	27	105
Fair Value	4,548	5,163
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,152	1,260
Sovereign and supranational | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	762	826
Gross Unrealized Gains	60	123
Gross Unrealized Losses	0	0
Fair Value	822	949
Sovereign and supranational | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	257	251
Gross Unrealized Gains	73	60
Gross Unrealized Losses	0	0
Fair Value	330	311
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	10,152	8,918
Banks/financial institutions | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	6,126	5,440
Gross Unrealized Gains	700	502
Gross Unrealized Losses	106	238
Fair Value	6,720	5,704
Banks/financial institutions | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	2,880	2,860
Gross Unrealized Gains	566	389
Gross Unrealized Losses	14	35
Fair Value	3,432	3,214
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	33,775	29,002
Other corporate | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	5,333	4,852
Gross Unrealized Gains	932	649
Gross Unrealized Losses	22	44
Fair Value	6,243	5,457
Other corporate | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	25,579	23,311
Gross Unrealized Gains	2,492	1,343
Gross Unrealized Losses	539	1,109
Fair Value	27,532	23,545
U.S. government and agencies | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	195	137
Gross Unrealized Gains	11	9
Gross Unrealized Losses	0	1
Fair Value	$ 206	$ 145