INVESTMENTS - Carrying Value and Fair Value of HTM Debt Securities by Contractual Maturity Dates (Details) - USD ($)	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Amortized cost	$ 96,065,000,000		$ 80,775,000,000
Held-to-maturity Securities, Debt Maturities, Fair Value;
Fair value	99,014,000,000		82,223,000,000
Allowance for credit losses on HTM debt securities	98,000,000	$ 107,000,000	0
Mortgage-backed securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	113,000,000		17,000,000
After 1 but within 5 years, amortized cost	367,000,000		458,000,000
After 5 but within 10 years, amortized cost	1,640,000,000		1,662,000,000
After 10 years, amortized cost	46,586,000,000		46,121,000,000
Amortized cost	48,706,000,000		48,258,000,000
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	116,000,000		17,000,000
After 1 but within 5 years, fair value	382,000,000		463,000,000
After 5 but within 10 years, fair value	1,823,000,000		1,729,000,000
After 10 years, fair value	48,796,000,000		47,081,000,000
Fair value	51,117,000,000		49,290,000,000
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	14,917,000,000		0
After 5 but within 10 years, amortized cost	0		0
After 10 years, amortized cost	0		0
Amortized cost	14,917,000,000		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	14,893,000,000		0
After 5 but within 10 years, fair value	0		0
After 10 years, fair value	0		0
Fair value	14,893,000,000		0
State and municipal
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	7,000,000		2,000,000
After 1 but within 5 years, amortized cost	90,000,000		123,000,000
After 5 but within 10 years, amortized cost	660,000,000		597,000,000
After 10 years, amortized cost	8,440,000,000		8,382,000,000
Amortized cost	9,197,000,000		9,104,000,000
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	7,000,000		26,000,000
After 1 but within 5 years, fair value	93,000,000		160,000,000
After 5 but within 10 years, fair value	699,000,000		590,000,000
After 10 years, fair value	9,074,000,000		8,755,000,000
Fair value	9,873,000,000		9,531,000,000
Foreign government
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	284,000,000		650,000,000
After 1 but within 5 years, amortized cost	1,455,000,000		1,284,000,000
After 5 but within 10 years, amortized cost	0		0
After 10 years, amortized cost	0		0
Amortized cost	1,739,000,000		1,934,000,000
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	291,000,000		652,000,000
After 1 but within 5 years, fair value	1,522,000,000		1,318,000,000
After 5 but within 10 years, fair value	0		0
After 10 years, fair value	0		0
Fair value	1,813,000,000		1,970,000,000
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	10,312,000,000		8,545,000,000
After 10 years, amortized cost	11,194,000,000		12,934,000,000
Amortized cost	21,506,000,000		21,479,000,000
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	10,243,000,000		8,543,000,000
After 10 years, fair value	11,075,000,000		12,889,000,000
Fair value	$ 21,318,000,000		$ 21,432,000,000