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INVESTMENTS - Carrying Value and Fair Value of HTM Debt Securities by Contractual Maturity Dates (Details) - USD ($) $ in Millions	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Amortized cost	$ 176,742		$ 104,943
Held-to-maturity Securities, Debt Maturities, Fair Value;
Fair value	178,102		107,790
Allowance for credit losses on HTM debt securities	83	$ 78	86	$ 107	$ 76	$ 0
Mortgage-backed securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	307		81
After 1 but within 5 years, amortized cost	855		463
After 5 but within 10 years, amortized cost	1,571		1,699
After 10 years, amortized cost	64,302		48,710
Amortized cost	67,035		50,953
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	304		81
After 1 but within 5 years, fair value	919		477
After 5 but within 10 years, fair value	1,676		1,873
After 10 years, fair value	65,190		50,671
Fair value	68,089		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	32,125		18,955
After 5 but within 10 years, amortized cost	40,217		2,338
After 10 years, amortized cost	0		0
Amortized cost	72,342		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	31,742		19,127
After 5 but within 10 years, fair value	40,268		2,115
After 10 years, fair value	0		0
Fair value	72,010		21,242
State and municipal
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	11		6
After 1 but within 5 years, amortized cost	193		139
After 5 but within 10 years, amortized cost	742		818
After 10 years, amortized cost	8,084		8,222
Amortized cost	9,030		9,185
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	11		6
After 1 but within 5 years, fair value	197		142
After 5 but within 10 years, fair value	785		869
After 10 years, fair value	8,683		8,912
Fair value	9,676		9,929
Foreign government
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	344		361
After 1 but within 5 years, amortized cost	1,411		1,570
After 5 but within 10 years, amortized cost	0		0
After 10 years, amortized cost	0		0
Amortized cost	1,755		1,931
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	347		360
After 1 but within 5 years, fair value	1,419		1,662
After 5 but within 10 years, fair value	0		0
After 10 years, fair value	0		0
Fair value	1,766		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	10,452		11,795
After 10 years, amortized cost	16,128		9,786
Amortized cost	26,580		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	10,449		15,020
After 10 years, fair value	16,112		6,475
Fair value	$ 26,561		$ 21,495