SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Mortgage Servicing Rights (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Mortgage servicing rights
Classification of Securitizations
Fair value of capitalized mortgage servicing rights	$ 334,000,000	$ 472,000,000	$ 334,000,000	$ 472,000,000
Principal amount of loans and other financial instruments	56,000,000,000	59,000,000,000	56,000,000,000	59,000,000,000
Capitalized MSRs
Balance, at beginning of period	345,000,000	508,000,000	495,000,000	584,000,000
Originations	31,000,000	19,000,000	87,000,000	47,000,000
Changes in fair value of MSRs due to changes in inputs and assumptions	(22,000,000)	(35,000,000)	(191,000,000)	(99,000,000)
Other changes	(20,000,000)	(20,000,000)	(57,000,000)	(60,000,000)
Sale of MSRs	0	0	0	0
Balance, as of September 30	334,000,000	472,000,000	334,000,000	472,000,000
MSR fees
Servicing fees	29,000,000	36,000,000	102,000,000	112,000,000
Late fees	1,000,000	2,000,000	4,000,000	6,000,000
Ancillary fees	0	0	0	1,000,000
Total MSR fees	30,000,000	38,000,000	106,000,000	119,000,000
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	1,800,000,000		1,800,000,000		2,200,000,000
Securities transferred to re-securitization entities	10,700,000,000	$ 8,900,000,000	30,100,000,000	$ 23,400,000,000
Market value of retained interest related to re-securitization transaction	1,000,000,000.0		1,000,000,000.0		1,300,000,000
Original fair value of re-securitizations deals in which the entity holds a retained interest	$ 76,900,000,000		$ 76,900,000,000		$ 73,500,000,000