INVESTMENTS - Fair Value of AFS Securities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair value
Less than 12 months	$ 178,408
Less than 12 months		$ 181,216
12 months or longer	23,005
12 months or longer		21,920
Total	201,413
Total		203,136
Gross unrealized losses
Less than 12 months	3,019
Less than 12 months		1,854
12 months or longer	797
12 months or longer		673
Total	3,816
Total		2,527
Mortgage-backed securities - U.S. agency-sponsored
Fair value
Less than 12 months	32,313
Less than 12 months		30,994
12 months or longer	1,873
12 months or longer		2,206
Total	34,186
Total		33,200
Gross unrealized losses
Less than 12 months	900
Less than 12 months		438
12 months or longer	93
12 months or longer		62
Total	993
Total		500
Mortgage-backed securities - Non-U.S. residential
Fair value
Less than 12 months	891
Less than 12 months		753
12 months or longer	0
12 months or longer		0
Total	891
Total		753
Gross unrealized losses
Less than 12 months	2
Less than 12 months		6
12 months or longer	0
12 months or longer		0
Total	2
Total		6
Mortgage-backed securities - Commercial
Fair value
Less than 12 months	241
Less than 12 months		150
12 months or longer	41
12 months or longer		57
Total	282
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	33,445
Less than 12 months		31,897
12 months or longer	1,914
12 months or longer		2,263
Total	35,359
Total		34,160
Gross unrealized losses
Less than 12 months	904
Less than 12 months		445
12 months or longer	94
12 months or longer		63
Total	998
Total		508
U.S. Treasury
Fair value
Less than 12 months	81,738
Less than 12 months		79,050
12 months or longer	7,526
12 months or longer		7,404
Total	89,264
Total		86,454
Gross unrealized losses
Less than 12 months	1,417
Less than 12 months		856
12 months or longer	194
12 months or longer		115
Total	1,611
Total		971
Agency obligations
Fair value
Less than 12 months	7,964
Less than 12 months		8,857
12 months or longer	1,567
12 months or longer		1,163
Total	9,531
Total		10,020
Gross unrealized losses
Less than 12 months	144
Less than 12 months		110
12 months or longer	34
12 months or longer		14
Total	178
Total		124
U.S. Treasury and federal agency securities
Fair value
Less than 12 months	89,702
Less than 12 months		87,907
12 months or longer	9,093
12 months or longer		8,567
Total	98,795
Total		96,474
Gross unrealized losses
Less than 12 months	1,561
Less than 12 months		966
12 months or longer	228
12 months or longer		129
Total	1,789
Total		1,095
State and municipal securities
Fair value
Less than 12 months	2,250
Less than 12 months		1,009
12 months or longer	1,116
12 months or longer		1,155
Total	3,366
Total		2,164
Gross unrealized losses
Less than 12 months	35
Less than 12 months		11
12 months or longer	234
12 months or longer		234
Total	269
Total		245
Foreign government
Fair value
Less than 12 months	46,459
Less than 12 months		53,206
12 months or longer	9,972
12 months or longer		9,051
Total	56,431
Total		62,257
Gross unrealized losses
Less than 12 months	371
Less than 12 months		356
12 months or longer	227
12 months or longer		234
Total	598
Total		590
Corporate
Fair value
Less than 12 months	5,831
Less than 12 months		6,737
12 months or longer	890
12 months or longer		859
Total	6,721
Total		7,596
Gross unrealized losses
Less than 12 months	101
Less than 12 months		74
12 months or longer	14
12 months or longer		12
Total	115
Total		86
Asset-backed securities
Fair value
Less than 12 months	699
Less than 12 months		449
12 months or longer	20
12 months or longer		25
Total	719
Total		474
Gross unrealized losses
Less than 12 months	47
Less than 12 months		1
12 months or longer	0
12 months or longer		1
Total	47
Total		2
Other debt securities
Fair value
Less than 12 months	22
Less than 12 months		0
12 months or longer	0
12 months or longer		0
Total	22
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