INVESTMENTS - Carrying Value and Fair Value of HTM Debt Securities by Contractual Maturity Dates (Details) - USD ($)	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Amortized cost	$ 83,332,000,000		$ 80,775,000,000
Held-to-maturity Securities, Debt Maturities, Fair Value;
Fair value	86,008,000,000		82,223,000,000
Allowance for credit losses on HTM debt securities	107,000,000	$ 76,000,000	0
Mortgage-backed securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	75,000,000		17,000,000
After 1 but within 5 years, amortized cost	432,000,000		458,000,000
After 5 but within 10 years, amortized cost	1,585,000,000		1,662,000,000
After 10 years, amortized cost	49,313,000,000		46,121,000,000
Amortized cost	51,405,000,000		48,258,000,000
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	76,000,000		17,000,000
After 1 but within 5 years, fair value	449,000,000		463,000,000
After 5 but within 10 years, fair value	1,748,000,000		1,729,000,000
After 10 years, fair value	51,562,000,000		47,081,000,000
Fair value	53,835,000,000		49,290,000,000
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	81,000,000		123,000,000
After 5 but within 10 years, amortized cost	632,000,000		597,000,000
After 10 years, amortized cost	8,432,000,000		8,382,000,000
Amortized cost	9,152,000,000		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	84,000,000		160,000,000
After 5 but within 10 years, fair value	666,000,000		590,000,000
After 10 years, fair value	9,029,000,000		8,755,000,000
Fair value	9,786,000,000		9,531,000,000
Foreign government
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;
Due within 1 year, amortized cost	273,000,000		650,000,000
After 1 but within 5 years, amortized cost	964,000,000		1,284,000,000
After 5 but within 10 years, amortized cost	0		0
After 10 years, amortized cost	0		0
Amortized cost	1,237,000,000		1,934,000,000
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value	272,000,000		652,000,000
After 1 but within 5 years, fair value	1,043,000,000		1,318,000,000
After 5 but within 10 years, fair value	0		0
After 10 years, fair value	0		0
Fair value	1,315,000,000		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	7,262,000,000		8,545,000,000
After 10 years, amortized cost	14,276,000,000		12,934,000,000
Amortized cost	21,538,000,000		21,479,000,000
Held-to-maturity Securities, Debt Maturities, Fair Value;
Due within 1 year, fair value			0
After 1 but within 5 years, fair value			0
After 5 but within 10 years, fair value	7,123,000,000		8,543,000,000
After 10 years, fair value	13,949,000,000		12,889,000,000
Fair value	$ 21,072,000,000		$ 21,432,000,000