INVESTMENTS - Available-for-sale Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 78,706	$ 76,856
Gross Unrealized Gains	10,026	7,733
Gross Unrealized Losses	808	1,694
Fair Value	87,924	82,895
Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	45,249	44,027
Gross Unrealized Gains	6,763	5,308
Gross Unrealized Losses	164	435
Fair Value	51,848	48,900
Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	33,457	32,829
Gross Unrealized Gains	3,263	2,425
Gross Unrealized Losses	644	1,259
Fair Value	36,076	33,995
Japan government and agencies | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	31,238	30,637
Gross Unrealized Gains	4,813	3,700
Gross Unrealized Losses	2	140
Fair Value	36,049	34,197
Municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,908	1,863
Municipalities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	509	385
Gross Unrealized Gains	93	32
Gross Unrealized Losses	3	9
Fair Value	599	408
Municipalities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	1,204	1,343
Gross Unrealized Gains	105	120
Gross Unrealized Losses	0	8
Fair Value	1,309	1,455
Mortgage- and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	414	339
Mortgage- and asset-backed securities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	233	155
Gross Unrealized Gains	27	22
Gross Unrealized Losses	0	0
Fair Value	260	177
Mortgage- and asset-backed securities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	147	155
Gross Unrealized Gains	7	8
Gross Unrealized Losses	0	1
Fair Value	154	162
Public utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value	6,497	7,171
Public utilities | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	1,822	1,732
Gross Unrealized Gains	344	280
Gross Unrealized Losses	0	4
Fair Value	2,166	2,008
Public utilities | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	3,841	4,772
Gross Unrealized Gains	522	496
Gross Unrealized Losses	32	105
Fair Value	4,331	5,163
Sovereign and supranational
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,154	1,260
Sovereign and supranational | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	740	826
Gross Unrealized Gains	65	123
Gross Unrealized Losses	0	0
Fair Value	805	949
Sovereign and supranational | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	276	251
Gross Unrealized Gains	73	60
Gross Unrealized Losses	0	0
Fair Value	349	311
Banks/financial institutions
Debt Securities, Available-for-sale [Line Items]
Fair Value	9,391	8,918
Banks/financial institutions | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	5,664	5,440
Gross Unrealized Gains	597	502
Gross Unrealized Losses	139	238
Fair Value	6,122	5,704
Banks/financial institutions | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	2,798	2,860
Gross Unrealized Gains	484	389
Gross Unrealized Losses	13	35
Fair Value	3,269	3,214
Other corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value	32,311	29,002
Other corporate | Yen-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	5,043	4,852
Gross Unrealized Gains	824	649
Gross Unrealized Losses	20	44
Fair Value	5,847	5,457
Other corporate | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	25,001	23,311
Gross Unrealized Gains	2,061	1,343
Gross Unrealized Losses	598	1,109
Fair Value	26,464	23,545
U.S. government and agencies | Dollar-denominated
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	190	137
Gross Unrealized Gains	11	9
Gross Unrealized Losses	1	1
Fair Value	$ 200	$ 145