SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Mortgage Servicing Rights (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Mortgage servicing rights
Classification of Securitizations
Fair value of capitalized mortgage servicing rights	$ 345,000,000		$ 345,000,000		$ 508,000,000
Principal amount of loans and other financial instruments	57,000,000,000		57,000,000,000		60,000,000,000
Capitalized MSRs
Balance, at beginning of period	367,000,000	$ 551,000,000	495,000,000	$ 584,000,000
Originations	24,000,000	16,000,000	56,000,000	28,000,000
Changes in fair value of MSRs due to changes in inputs and assumptions	(26,000,000)	(37,000,000)	(169,000,000)	(64,000,000)
Other changes	(20,000,000)	(22,000,000)	(37,000,000)	(40,000,000)
Sale of MSRs	0	0	0	0
Balance, as of June 30	345,000,000	508,000,000	345,000,000	508,000,000
MSR fees
Servicing fees	34,000,000	35,000,000	73,000,000	76,000,000
Late fees	1,000,000	2,000,000	3,000,000	4,000,000
Ancillary fees	0	0	0	1,000,000
Total MSR fees	35,000,000	37,000,000	76,000,000	81,000,000
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-backed securities - U.S. government-sponsored agency guaranteed
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	12,000,000,000	$ 6,900,000,000	19,400,000,000	$ 14,500,000,000
Market value of retained interest related to re-securitization transaction	858,700,000		858,700,000		1,300,000,000
Original fair value of re-securitizations deals in which the entity holds a retained interest	$ 71,800,000,000		$ 71,800,000,000		$ 73,500,000,000