INVESTMENTS - Fair Value of AFS Debt Securities by Contractual Maturity Date (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Amortized cost
Amortized cost	$ 294,262	$ 292,220
Fair value
Total fair value	291,523	290,725
Mortgage-backed securities
Amortized cost
Due within 1 year, amortized cost	92	45
After 1 but within 5 years, amortized cost	1,331	1,306
After 5 but within 10 years, amortized cost	1,320	1,376
After 10 years, amortized cost	42,374	42,504
Amortized cost	45,117	45,231
Fair value
Fair value, due within 1 year	91	45
Fair value, after 1 but within 5 years	1,327	1,304
Fair value, after 5 but within 10 years	1,298	1,369
Fair value, after 10 years	41,639	42,239
Total fair value	44,355	44,957
U.S. Treasury and federal agency securities
Amortized cost
Due within 1 year, amortized cost	12,897	4,913
After 1 but within 5 years, amortized cost	104,061	111,236
After 5 but within 10 years, amortized cost	2,066	3,008
After 10 years, amortized cost	0	0
Amortized cost	119,024	119,157
Fair value
Fair value, due within 1 year	12,906	4,907
Fair value, after 1 but within 5 years	102,375	110,238
Fair value, after 5 but within 10 years	2,033	3,001
Fair value, after 10 years	0	0
Total fair value	117,314	118,146
State and municipal securities
Amortized cost
Due within 1 year, amortized cost	1,874	1,792
After 1 but within 5 years, amortized cost	2,496	2,579
After 5 but within 10 years, amortized cost	591	514
After 10 years, amortized cost	5,019	3,985
Amortized cost	9,980	8,870
Fair value
Fair value, due within 1 year	1,873	1,792
Fair value, after 1 but within 5 years	2,494	2,576
Fair value, after 5 but within 10 years	607	528
Fair value, after 10 years	4,863	3,869
Total fair value	9,837	8,765
Foreign government
Amortized cost
Due within 1 year, amortized cost	32,899	32,130
After 1 but within 5 years, amortized cost	55,910	53,034
After 5 but within 10 years, amortized cost	12,454	12,949
After 10 years, amortized cost	2,570	2,502
Amortized cost	103,833	100,615
Fair value
Fair value, due within 1 year	32,909	32,100
Fair value, after 1 but within 5 years	55,571	53,165
Fair value, after 5 but within 10 years	12,639	12,680
Fair value, after 10 years	2,656	2,588
Total fair value	103,775	100,533
All other
Amortized cost
Due within 1 year, amortized cost	2,821	3,998
After 1 but within 5 years, amortized cost	9,147	9,047
After 5 but within 10 years, amortized cost	2,905	3,415
After 10 years, amortized cost	1,435	1,887
Amortized cost	16,308	18,347
Fair value
Fair value, due within 1 year	2,818	3,991
Fair value, after 1 but within 5 years	9,092	9,027
Fair value, after 5 but within 10 years	2,907	3,431
Fair value, after 10 years	1,425	1,875
Total fair value	$ 16,242	$ 18,324