INVESTMENTS - Carrying Value and Fair Value of HTM Debt Securities by Contractual Maturity Dates (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Carrying value	$ 104,943	$ 80,775
Held-to-maturity Securities, Debt Maturities, Fair Value;
Fair value	107,790	82,223
Allowance for credit losses on HTM debt securities	86	0
Mortgage-backed securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	81	17
After 1 but within 5 years, carrying value	463	458
After 5 but within 10 years, carrying value	1,699	1,662
After 10 years, carrying value	48,710	46,121
Carrying value	50,953	48,258
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	81	17
After 1 but within 5 years, fair value	477	463
After 5 but within 10 years, fair value	1,873	1,729
After 10 years, fair value	50,671	47,081
Fair value	53,102	49,290
US Treasury securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	0	0
After 1 but within 5 years, carrying value	18,955	0
After 5 but within 10 years, carrying value	2,338	0
After 10 years, carrying value	0	0
Carrying value	21,293	0
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	0	0
After 1 but within 5 years, fair value	19,127	0
After 5 but within 10 years, fair value	2,115	0
After 10 years, fair value	0	0
Fair value	21,242	0
State and municipal
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	6	2
After 1 but within 5 years, carrying value	139	123
After 5 but within 10 years, carrying value	818	597
After 10 years, carrying value	8,222	8,382
Carrying value	9,185	9,104
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	6	26
After 1 but within 5 years, fair value	142	160
After 5 but within 10 years, fair value	869	590
After 10 years, fair value	8,912	8,755
Fair value	9,929	9,531
Foreign government
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	361	650
After 1 but within 5 years, carrying value	1,570	1,284
After 5 but within 10 years, carrying value	0	0
After 10 years, carrying value	0	0
Carrying value	1,931	1,934
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	360	652
After 1 but within 5 years, fair value	1,662	1,318
After 5 but within 10 years, fair value	0	0
After 10 years, fair value	0	0
Fair value	2,022	1,970
All other
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, carrying value	0	0
After 1 but within 5 years, carrying value	0	0
After 5 but within 10 years, carrying value	11,795	8,545
After 10 years, carrying value	9,786	12,934
Carrying value	21,581	21,479
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	0	0
After 1 but within 5 years, fair value	0	0
After 5 but within 10 years, fair value	15,020	8,543
After 10 years, fair value	6,475	12,889
Fair value	$ 21,495	$ 21,432

Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity).

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