INVESTMENTS - Carrying Value and Fair Value of HTM Debt Securities by Contractual Maturity Dates (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Amortized cost	$ 198,056		$ 104,943
Held-to-maturity Securities, Debt Maturities, Fair Value;
Fair value	198,724		107,790
Allowance for credit losses on HTM debt securities	73	$ 83	86	$ 98	$ 107	$ 0
Mortgage-backed securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	160		81
After 1 but within 5 years, amortized cost	737		463
After 5 but within 10 years, amortized cost	1,637		1,699
After 10 years, amortized cost	66,679		48,710
Amortized cost	69,213		50,953
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	160		81
After 1 but within 5 years, fair value	792		477
After 5 but within 10 years, fair value	1,736		1,873
After 10 years, fair value	67,484		50,671
Fair value	70,172		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	42,783		18,955
After 5 but within 10 years, amortized cost	45,487		2,338
After 10 years, amortized cost	0		0
Amortized cost	88,270		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	42,287		19,127
After 5 but within 10 years, fair value	45,159		2,115
After 10 years, fair value	0		0
Fair value	87,446		21,242
State and municipal
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	54		6
After 1 but within 5 years, amortized cost	168		139
After 5 but within 10 years, amortized cost	838		818
After 10 years, amortized cost	7,892		8,222
Amortized cost	8,952		9,185
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	54		6
After 1 but within 5 years, fair value	172		142
After 5 but within 10 years, fair value	881		869
After 10 years, fair value	8,402		8,912
Fair value	9,509		9,929
Foreign government
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	334		361
After 1 but within 5 years, amortized cost	1,360		1,570
After 5 but within 10 years, amortized cost	0		0
After 10 years, amortized cost	0		0
Amortized cost	1,694		1,931
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	335		360
After 1 but within 5 years, fair value	1,353		1,662
After 5 but within 10 years, fair value	0		0
After 10 years, fair value	0		0
Fair value	1,688		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	11,299		11,795
After 10 years, amortized cost	18,628		9,786
Amortized cost	29,927		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	11,297		15,020
After 10 years, fair value	18,612		6,475
Fair value	$ 29,909		$ 21,495