SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Mortgage Servicing Rights (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Mortgage servicing rights
Classification of Securitizations
Fair value of capitalized mortgage servicing rights	$ 472,000,000	$ 618,000,000	$ 472,000,000	$ 618,000,000
Principal amount of loans and other financial instruments	59,000,000,000	62,000,000,000	59,000,000,000	62,000,000,000
Capitalized MSRs
Balance, at beginning of period	508,000,000	596,000,000	584,000,000	558,000,000
Originations	19,000,000	14,000,000	47,000,000	46,000,000
Changes in fair value of MSRs due to changes in inputs and assumptions	(35,000,000)	25,000,000	(99,000,000)	82,000,000
Other changes	(20,000,000)	(17,000,000)	(60,000,000)	(50,000,000)
Sale of MSRs	0	0	0	(18,000,000)
Balance, as of September 30	472,000,000	618,000,000	472,000,000	618,000,000
MSR fees
Servicing fees	36,000,000	41,000,000	112,000,000	130,000,000
Late fees	2,000,000	1,000,000	6,000,000	3,000,000
Ancillary fees	0	1,000,000	1,000,000	7,000,000
Total MSR fees	38,000,000	43,000,000	119,000,000	140,000,000
Mortgage securitizations - Non-agency-sponsored
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	0		0		$ 16,000,000
Original par value of re-securitizations deals in which the entity holds a retained interest					271,000,000
U.S. government-sponsored agency guaranteed
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	2,500,000,000		2,500,000,000		2,500,000,000
Securities transferred to re-securitization entities	8,900,000,000	$ 6,800,000,000	23,400,000,000	$ 20,400,000,000
Market value of retained interest related to re-securitization transaction	1,300,000,000		1,300,000,000		1,400,000,000
Original fair value of re-securitizations deals in which the entity holds a retained interest	$ 72,900,000,000		$ 72,900,000,000		$ 70,900,000,000