INVESTMENTS - Fair Value of AFS Debt Securities by Contractual Maturity Date (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	$ 290,423	$ 292,220
Total fair value	288,038	290,725
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	14	45
After 1 but within 5 years, amortized cost	662	1,306
After 5 but within 10 years, amortized cost	2,779	1,376
After 10 years, amortized cost	41,533	42,504
Total amortized cost	44,988	45,231
Due within 1 year, fair value	14	45
After 1 but within 5 years, fair value	661	1,304
After 5 but within 10 years, fair value	2,828	1,369
After 10 Years,, fair value	41,002	42,239
Total fair value	44,505	44,957
U.S. Treasury and federal agency securities
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	41,941	4,913
After 1 but within 5 years, amortized cost	76,139	111,236
After 5 but within 10 years, amortized cost	489	3,008
After 10 years, amortized cost	90	0
Total amortized cost	118,659	119,157
Due within 1 year, fair value	41,867	4,907
After 1 but within 5 years, fair value	74,800	110,238
After 5 but within 10 years, fair value	462	3,001
After 10 Years,, fair value	93	0
Total fair value	117,222	118,146
State and municipal securities
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	2,586	1,792
After 1 but within 5 years, amortized cost	1,676	2,579
After 5 but within 10 years, amortized cost	585	514
After 10 years, amortized cost	4,525	3,985
Total amortized cost	9,372	8,870
Due within 1 year, fair value	2,586	1,792
After 1 but within 5 years, fair value	1,675	2,576
After 5 but within 10 years, fair value	602	528
After 10 Years,, fair value	4,343	3,869
Total fair value	9,206	8,765
Foreign government
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	39,078	32,130
After 1 but within 5 years, amortized cost	50,125	53,034
After 5 but within 10 years, amortized cost	10,153	12,949
After 10 years, amortized cost	1,516	2,502
Total amortized cost	100,872	100,615
Due within 1 year, fair value	39,028	32,100
After 1 but within 5 years, fair value	49,962	53,165
After 5 but within 10 years, fair value	10,149	12,680
After 10 Years,, fair value	1,552	2,588
Total fair value	100,691	100,533
All other
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	6,166	3,998
After 1 but within 5 years, amortized cost	8,459	9,047
After 5 but within 10 years, amortized cost	1,474	3,415
After 10 years, amortized cost	433	1,887
Total amortized cost	16,532	18,347
Due within 1 year, fair value	6,166	3,991
After 1 but within 5 years, fair value	8,416	9,027
After 5 but within 10 years, fair value	1,427	3,431
After 10 Years,, fair value	405	1,875
Total fair value	$ 16,414	$ 18,324