INVESTMENTS - Fair Value of AFS Debt Securities by Contractual Maturity Date (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 330,409	$ 280,093
Total fair value	335,084	280,265	$ 4,000
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	27	20
After 1 but within 5 years, amortized cost	567	573
After 5 but within 10 years, amortized cost	688	594
After 10 years, amortized cost	42,171	34,640
Amortized cost	43,453	35,827
Due within 1 year, fair value	27	20
After 1 but within 5 years, fair value	571	574
After 5 but within 10 years, fair value	757	626
After 10 Years,, fair value	43,184	34,877
Total fair value	44,539	36,097
U.S. Treasury and federal agency securities
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	34,834	40,757
After 1 but within 5 years, amortized cost	108,160	70,128
After 5 but within 10 years, amortized cost	1,150	854
After 10 years, amortized cost	0	26
Amortized cost	144,144	111,765
Due within 1 year, fair value	34,951	40,688
After 1 but within 5 years, fair value	110,091	69,850
After 5 but within 10 years, fair value	1,162	851
After 10 Years,, fair value	0	29
Total fair value	146,204	111,418
State and municipal
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	427	932
After 1 but within 5 years, amortized cost	189	714
After 5 but within 10 years, amortized cost	276	195
After 10 years, amortized cost	2,861	3,183
Amortized cost	3,753	5,024
Due within 1 year, fair value	428	932
After 1 but within 5 years, fair value	198	723
After 5 but within 10 years, fair value	267	215
After 10 Years,, fair value	2,826	3,108
Total fair value	3,719	4,978
Foreign government
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	48,133	42,611
After 1 but within 5 years, amortized cost	67,365	58,820
After 5 but within 10 years, amortized cost	5,908	8,192
After 10 years, amortized cost	2,061	1,335
Amortized cost	123,467	110,958
Due within 1 year, fair value	48,258	42,666
After 1 but within 5 years, fair value	68,586	59,071
After 5 but within 10 years, fair value	6,011	8,198
After 10 Years,, fair value	2,113	1,368
Total fair value	124,968	111,303
All other
Debt Securities, Available-for-sale [Line Items]
Due within 1 year, amortized cost	6,661	7,306
After 1 but within 5 years, amortized cost	7,814	8,279
After 5 but within 10 years, amortized cost	1,018	818
After 10 years, amortized cost	99	116
Amortized cost	15,592	16,519
Due within 1 year, fair value	6,665	7,311
After 1 but within 5 years, fair value	7,891	8,275
After 5 but within 10 years, fair value	1,034	797
After 10 Years,, fair value	64	86
Total fair value	$ 15,654	$ 16,469