SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Mortgage Servicing Rights (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Mortgage servicing rights
Classification of Securitizations
Fair value of capitalized mortgage servicing rights	$ 419,000,000	$ 345,000,000	$ 419,000,000	$ 345,000,000
Principal amount of loans and other financial instruments	50,000,000,000	57,000,000,000	50,000,000,000	57,000,000,000
Capitalized MSRs
Balance, at beginning of period	433,000,000	367,000,000	336,000,000	495,000,000
Originations	25,000,000	24,000,000	68,000,000	56,000,000
Changes in fair value of MSRs due to changes in inputs and assumptions	(21,000,000)	(26,000,000)	52,000,000	(169,000,000)
Other changes	(18,000,000)	(20,000,000)	(37,000,000)	(37,000,000)
Sale of MSRs	0	0	0	0
Balance, as of June 30	419,000,000	345,000,000	419,000,000	345,000,000
MSR fees
Servicing fees	37,000,000	34,000,000	68,000,000	73,000,000
Late fees	0	1,000,000	1,000,000	3,000,000
Ancillary fees	0	0	0	0
Total MSR fees	37,000,000	35,000,000	69,000,000	76,000,000
Mortgage securitizations - Non-agency-sponsored
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	0		0		$ 0
Mortgage securitizations - U.S. agency-sponsored
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	1,200,000,000		1,200,000,000		1,600,000,000
Securities transferred to re-securitization entities	11,400,000,000	$ 12,000,000,000	24,500,000,000	$ 19,400,000,000
Market value of retained interest related to re-securitization transaction	410,000,000		410,000,000		916,000,000
Original fair value of re-securitizations deals in which the entity holds a retained interest	$ 76,000,000,000		$ 76,000,000,000		$ 83,600,000,000