INVESTMENTS - Fair Value of AFS Securities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair value
Less than 12 months	$ 147,358
Less than 12 months		$ 181,216
12 months or longer	50,575
12 months or longer		21,920
Total	197,933
Total		203,136
Gross unrealized losses
Less than 12 months	2,235
Less than 12 months		1,854
12 months or longer	1,834
12 months or longer		673
Total	4,069
Total		2,527
Mortgage-backed securities - U.S. agency-sponsored
Fair value
Less than 12 months	13,676
Less than 12 months		30,994
12 months or longer	13,363
12 months or longer		2,206
Total	27,039
Total		33,200
Gross unrealized losses
Less than 12 months	328
Less than 12 months		438
12 months or longer	666
12 months or longer		62
Total	994
Total		500
Mortgage-backed securities - Non-U.S. residential
Fair value
Less than 12 months	203
Less than 12 months		753
12 months or longer	1
12 months or longer		0
Total	204
Total		753
Gross unrealized losses
Less than 12 months	1
Less than 12 months		6
12 months or longer	0
12 months or longer		0
Total	1
Total		6
Mortgage-backed securities - Commercial
Fair value
Less than 12 months	234
Less than 12 months		150
12 months or longer	27
12 months or longer		57
Total	261
Total		207
Gross unrealized losses
Less than 12 months	2
Less than 12 months		1
12 months or longer	1
12 months or longer		1
Total	3
Total		2
Mortgage-backed securities
Fair value
Less than 12 months	14,113
Less than 12 months		31,897
12 months or longer	13,391
12 months or longer		2,263
Total	27,504
Total		34,160
Gross unrealized losses
Less than 12 months	331
Less than 12 months		445
12 months or longer	667
12 months or longer		63
Total	998
Total		508
U.S. Treasury
Fair value
Less than 12 months	68,095
Less than 12 months		79,050
12 months or longer	20,384
12 months or longer		7,404
Total	88,479
Total		86,454
Gross unrealized losses
Less than 12 months	1,208
Less than 12 months		856
12 months or longer	564
12 months or longer		115
Total	1,772
Total		971
Agency obligations
Fair value
Less than 12 months	4,900
Less than 12 months		8,857
12 months or longer	4,619
12 months or longer		1,163
Total	9,519
Total		10,020
Gross unrealized losses
Less than 12 months	78
Less than 12 months		110
12 months or longer	112
12 months or longer		14
Total	190
Total		124
U.S. Treasury and federal agency securities
Fair value
Less than 12 months	72,995
Less than 12 months		87,907
12 months or longer	25,003
12 months or longer		8,567
Total	97,998
Total		96,474
Gross unrealized losses
Less than 12 months	1,286
Less than 12 months		966
12 months or longer	676
12 months or longer		129
Total	1,962
Total		1,095
State and municipal securities
Fair value
Less than 12 months	2,043
Less than 12 months		1,009
12 months or longer	1,161
12 months or longer		1,155
Total	3,204
Total		2,164
Gross unrealized losses
Less than 12 months	25
Less than 12 months		11
12 months or longer	219
12 months or longer		234
Total	244
Total		245
Foreign government
Fair value
Less than 12 months	50,160
Less than 12 months		53,206
12 months or longer	10,488
12 months or longer		9,051
Total	60,648
Total		62,257
Gross unrealized losses
Less than 12 months	470
Less than 12 months		356
12 months or longer	262
12 months or longer		234
Total	732
Total		590
Corporate
Fair value
Less than 12 months	6,362
Less than 12 months		6,737
12 months or longer	521
12 months or longer		859
Total	6,883
Total		7,596
Gross unrealized losses
Less than 12 months	120
Less than 12 months		74
12 months or longer	10
12 months or longer		12
Total	130
Total		86
Asset-backed securities
Fair value
Less than 12 months	511
Less than 12 months		449
12 months or longer	11
12 months or longer		25
Total	522
Total		474
Gross unrealized losses
Less than 12 months	3
Less than 12 months		1
12 months or longer	0
12 months or longer		1
Total	3
Total		2
Other debt securities
Fair value
Less than 12 months	1,174
Less than 12 months		0
12 months or longer	0
12 months or longer		0
Total	1,174
Total		0
Gross unrealized losses
Less than 12 months	0
Less than 12 months		0
12 months or longer	0
12 months or longer		0
Total	$ 0
Total		0
Equity securities
Fair value
Less than 12 months		11
12 months or longer		0
Total		11
Gross unrealized losses
Less than 12 months		1
12 months or longer		0
Total		$ 1