INVESTMENTS - Carrying Value and Fair Value of HTM Debt Securities by Contractual Maturity Dates (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Amortized cost	$ 161,742	$ 104,943
Held-to-maturity Securities, Debt Maturities, Fair Value;
Fair value	162,375	107,790
Allowance for credit losses on HTM debt securities	78	86	$ 76	$ 0
Mortgage-backed securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	244	81
After 1 but within 5 years, amortized cost	596	463
After 5 but within 10 years, amortized cost	1,641	1,699
After 10 years, amortized cost	63,296	48,710
Amortized cost	65,777	50,953
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	399	81
After 1 but within 5 years, fair value	626	477
After 5 but within 10 years, fair value	1,749	1,873
After 10 years, fair value	63,931	50,671
Fair value	66,705	53,102
U.S. treasury securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	0	0
After 1 but within 5 years, amortized cost	28,176	18,955
After 5 but within 10 years, amortized cost	30,204	2,338
After 10 years, amortized cost	0	0
Amortized cost	58,380	21,293
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	0	0
After 1 but within 5 years, fair value	27,697	19,127
After 5 but within 10 years, fair value	29,758	2,115
After 10 years, fair value	0	0
Fair value	57,455	21,242
State and municipal
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	8	6
After 1 but within 5 years, amortized cost	172	139
After 5 but within 10 years, amortized cost	848	818
After 10 years, amortized cost	8,418	8,222
Amortized cost	9,446	9,185
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	7	6
After 1 but within 5 years, fair value	176	142
After 5 but within 10 years, fair value	887	869
After 10 years, fair value	8,990	8,912
Fair value	10,060	9,929
Foreign government
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	352	361
After 1 but within 5 years, amortized cost	1,525	1,570
After 5 but within 10 years, amortized cost	0	0
After 10 years, amortized cost	0	0
Amortized cost	1,877	1,931
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	349	360
After 1 but within 5 years, fair value	1,565	1,662
After 5 but within 10 years, fair value	0	0
After 10 years, fair value	0	0
Fair value	1,914	2,022
All other
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	0	0
After 1 but within 5 years, amortized cost	0	0
After 5 but within 10 years, amortized cost	13,973	11,795
After 10 years, amortized cost	12,289	9,786
Amortized cost	26,262	21,581
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	13,956	15,020
After 10 years, fair value	12,285	6,475
Fair value	$ 26,241	$ 21,495

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

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