INVESTMENTS - Available-for-Sale Debt Securities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	$ 79,371	$ 76,856
Gross Unrealized Gains	12,266	7,733
Gross Unrealized Losses	375	1,694
Fair Value	91,262	82,895
Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	45,684	44,027
Gross Unrealized Gains	7,410	5,308
Gross Unrealized Losses	113	435
Fair Value	52,981	48,900
Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	33,687	32,829
Gross Unrealized Gains	4,856	2,425
Gross Unrealized Losses	262	1,259
Fair Value	38,281	33,995
Japan government and agencies | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	30,929	30,637
Gross Unrealized Gains	5,169	3,700
Gross Unrealized Losses	0	140
Fair Value	36,098	34,197
U.S. government and agencies | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	293	137
Gross Unrealized Gains	9	9
Gross Unrealized Losses	0	1
Fair Value	302	145
Municipalities
Debt Securities, Available-for-Sale [Line Items]
Fair Value	1,847	1,863
Municipalities | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	516	385
Gross Unrealized Gains	116	32
Gross Unrealized Losses	3	9
Fair Value	629	408
Municipalities | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	1,077	1,343
Gross Unrealized Gains	141	120
Gross Unrealized Losses	0	8
Fair Value	1,218	1,455
Mortgage- and asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Fair Value	410	339
Mortgage- and asset-backed securities | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	229	155
Gross Unrealized Gains	25	22
Gross Unrealized Losses	0	0
Fair Value	254	177
Mortgage- and asset-backed securities | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	149	155
Gross Unrealized Gains	7	8
Gross Unrealized Losses	0	1
Fair Value	156	162
Public utilities
Debt Securities, Available-for-Sale [Line Items]
Fair Value	6,780	7,171
Public utilities | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	1,855	1,732
Gross Unrealized Gains	406	280
Gross Unrealized Losses	0	4
Fair Value	2,261	2,008
Public utilities | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	3,804	4,772
Gross Unrealized Gains	725	496
Gross Unrealized Losses	10	105
Fair Value	4,519	5,163
Sovereign and supranational
Debt Securities, Available-for-Sale [Line Items]
Fair Value	1,042	1,260
Sovereign and supranational | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	680	826
Gross Unrealized Gains	50	123
Gross Unrealized Losses	0	0
Fair Value	730	949
Sovereign and supranational | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	239	251
Gross Unrealized Gains	73	60
Gross Unrealized Losses	0	0
Fair Value	312	311
Banks/financial institutions
Debt Securities, Available-for-Sale [Line Items]
Fair Value	10,287	8,918
Banks/financial institutions | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	6,152	5,440
Gross Unrealized Gains	700	502
Gross Unrealized Losses	86	238
Fair Value	6,766	5,704
Banks/financial institutions | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	2,879	2,860
Gross Unrealized Gains	646	389
Gross Unrealized Losses	4	35
Fair Value	3,521	3,214
Other corporate
Debt Securities, Available-for-Sale [Line Items]
Fair Value	34,496	29,002
Other corporate | Yen-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	5,323	4,852
Gross Unrealized Gains	944	649
Gross Unrealized Losses	24	44
Fair Value	6,243	5,457
Other corporate | Dollar-denominated
Debt Securities, Available-for-Sale [Line Items]
Cost or Amortized Cost	25,246	23,311
Gross Unrealized Gains	3,255	1,343
Gross Unrealized Losses	248	1,109
Fair Value	$ 28,253	$ 23,545