SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Mortgage Servicing Rights (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Capitalized MSRs
Balance, at beginning of period			$ 558
Balance, as of September 30	$ 618		618
Mortgage servicing rights
Classification of Securitizations
Fair value of capitalized mortgage servicing rights	618	$ 553	618	$ 553
Principal amount of loans and other financial instruments	62,000	68,000	62,000	68,000
Capitalized MSRs
Balance, at beginning of period	596	560	558	1,564
Originations	14	19	46	75
Changes in fair value of MSRs due to changes in inputs and assumptions	25	(6)	82	50
Other changes	(17)	(20)	(50)	(90)
Sale of MSRs	0	0	(18)	(1,046)
Balance, as of September 30	618	553	618	553
MSR fees
Servicing fees	41	65	130	236
Late fees	1	2	3	8
Ancillary fees	1	3	7	11
Total MSR fees	43	70	140	255
Mortgage securitizations - Non-agency-sponsored
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	33		33		$ 79
Original par value of re-securitizations deals in which the entity holds a retained interest	316		316		887
U.S. government-sponsored agency guaranteed
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	2,400		2,400		2,100
Securities transferred to re-securitization entities	6,800	$ 9,900	20,400	$ 20,000
Market value of retained interest related to re-securitization transaction	1,300		1,300		854
Original fair value of re-securitizations deals in which the entity holds a retained interest	$ 67,200		$ 67,200		$ 68,300