INVESTMENTS - Fair Value of AFS Securities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair value
Less than 12 months	$ 55,954	$ 69,236
12 months or longer	111,575	118,258
Total	167,529	187,494
Gross unrealized losses
Less than 12 months	582	752
12 months or longer	1,524	2,468
Total	2,106	3,220
Mortgage-backed securities - U.S. agency-sponsored
Fair value
Less than 12 months	9,176	11,160
12 months or longer	8,102	13,143
Total	17,278	24,303
Gross unrealized losses
Less than 12 months	268	286
12 months or longer	166	439
Total	434	725
Mortgage-backed securities - Non-U.S. residential
Fair value
Less than 12 months	387	284
12 months or longer	1	2
Total	388	286
Gross unrealized losses
Less than 12 months	1	2
12 months or longer	0	0
Total	1	2
Commercial and other
Fair value
Less than 12 months	14	79
12 months or longer	104	82
Total	118	161
Gross unrealized losses
Less than 12 months	0	1
12 months or longer	1	1
Total	1	2
Mortgage-backed securities
Fair value
Less than 12 months	9,577	11,523
12 months or longer	8,207	13,227
Total	17,784	24,750
Gross unrealized losses
Less than 12 months	269	289
12 months or longer	167	440
Total	436	729
U.S. Treasury
Fair value
Less than 12 months	8,496	8,389
12 months or longer	80,174	77,883
Total	88,670	86,272
Gross unrealized losses
Less than 12 months	48	42
12 months or longer	955	1,297
Total	1,003	1,339
Agency obligations
Fair value
Less than 12 months	126	277
12 months or longer	8,098	8,660
Total	8,224	8,937
Gross unrealized losses
Less than 12 months	2	2
12 months or longer	88	130
Total	90	132
U.S. Treasury and federal agency securities
Fair value
Less than 12 months	8,622	8,666
12 months or longer	88,272	86,543
Total	96,894	95,209
Gross unrealized losses
Less than 12 months	50	44
12 months or longer	1,043	1,427
Total	1,093	1,471
State and municipal securities
Fair value
Less than 12 months	928	1,614
12 months or longer	968	1,303
Total	1,896	2,917
Gross unrealized losses
Less than 12 months	6	34
12 months or longer	59	228
Total	65	262
Foreign government
Fair value
Less than 12 months	32,453	40,655
12 months or longer	11,945	15,053
Total	44,398	55,708
Gross unrealized losses
Less than 12 months	159	265
12 months or longer	236	331
Total	395	596
Corporate
Fair value
Less than 12 months	3,252	4,547
12 months or longer	2,127	2,077
Total	5,379	6,624
Gross unrealized losses
Less than 12 months	96	115
12 months or longer	19	42
Total	115	157
Asset-backed securities
Fair value
Less than 12 months	306	441
12 months or longer	56	55
Total	362	496
Gross unrealized losses
Less than 12 months	2	4
12 months or longer	0	0
Total	2	4
Other debt securities
Fair value
Less than 12 months	816	1,790
12 months or longer	0	0
Total	816	1,790
Gross unrealized losses
Less than 12 months	0	1
12 months or longer	0	0
Total	$ 0	$ 1