INVESTMENTS - Fair Value of AFS Debt Securities by Contractual Maturity Date (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 280,093	$ 290,423
Total fair value	280,265	288,038
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	20	14
After 1 but within 5 years, amortized cost	573	662
After 5 but within 10 years, amortized cost	594	2,779
After 10 years, amortized cost	34,640	41,533
Amortized cost	35,827	44,988
Due within 1 year, fair value	20	14
After 1 but within 5 years, fair value	574	661
After 5 but within 10 years, fair value	626	2,828
After 10 Years,, fair value	34,877	41,002
Total fair value	36,097	44,505
U.S. Treasury and federal agency securities
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	40,757	41,941
After 1 but within 5 years, amortized cost	70,128	76,139
After 5 but within 10 years, amortized cost	854	489
After 10 years, amortized cost	26	90
Amortized cost	111,765	118,659
Due within 1 year, fair value	40,688	41,867
After 1 but within 5 years, fair value	69,850	74,800
After 5 but within 10 years, fair value	851	462
After 10 Years,, fair value	29	93
Total fair value	111,418	117,222
State and municipal securities
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	932	2,586
After 1 but within 5 years, amortized cost	714	1,676
After 5 but within 10 years, amortized cost	195	585
After 10 years, amortized cost	3,183	4,525
Amortized cost	5,024	9,372
Due within 1 year, fair value	932	2,586
After 1 but within 5 years, fair value	723	1,675
After 5 but within 10 years, fair value	215	602
After 10 Years,, fair value	3,108	4,343
Total fair value	4,978	9,206
Foreign government
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	42,611	39,078
After 1 but within 5 years, amortized cost	58,820	50,125
After 5 but within 10 years, amortized cost	8,192	10,153
After 10 years, amortized cost	1,335	1,516
Amortized cost	110,958	100,872
Due within 1 year, fair value	42,666	39,028
After 1 but within 5 years, fair value	59,071	49,962
After 5 but within 10 years, fair value	8,198	10,149
After 10 Years,, fair value	1,368	1,552
Total fair value	111,303	100,691
All other
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	7,306	6,166
After 1 but within 5 years, amortized cost	8,279	8,459
After 5 but within 10 years, amortized cost	818	1,474
After 10 years, amortized cost	116	433
Amortized cost	16,519	16,532
Due within 1 year, fair value	7,311	6,166
After 1 but within 5 years, fair value	8,275	8,416
After 5 but within 10 years, fair value	797	1,427
After 10 Years,, fair value	86	405
Total fair value	$ 16,469	$ 16,414