SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Mortgage Servicing Rights (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Mortgage servicing rights
Classification of Securitizations
Fair value of capitalized mortgage servicing rights	$ 409,000,000	$ 334,000,000	$ 409,000,000	$ 334,000,000
Principal amount of loans and other financial instruments	48,000,000,000	56,000,000,000	48,000,000,000	56,000,000,000
Capitalized MSRs
Balance, at beginning of period	419,000,000	345,000,000	336,000,000	495,000,000
Originations	8,000,000	31,000,000	76,000,000	87,000,000
Changes in fair value of MSRs due to changes in inputs and assumptions	(3,000,000)	(22,000,000)	49,000,000	(191,000,000)
Other changes	(15,000,000)	(20,000,000)	(52,000,000)	(57,000,000)
Sale of MSRs	0	0	0	0
Balance, as of September 30	409,000,000	334,000,000	409,000,000	334,000,000
MSR fees
Servicing fees	32,000,000	29,000,000	100,000,000	102,000,000
Late fees	1,000,000	1,000,000	2,000,000	4,000,000
Ancillary fees	0	0	0	0
Total MSR fees	33,000,000	30,000,000	102,000,000	106,000,000
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	12,600,000,000	$ 11,500,000,000	37,100,000,000	$ 31,800,000,000
Market value of retained interest related to re-securitization transaction	526,000,000		526,000,000		916,000,000
Original fair value of re-securitizations deals in which the entity holds a retained interest	$ 78,000,000,000		$ 78,000,000,000		$ 83,600,000,000