SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Mortgage Servicing Rights (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Capitalized MSRs
Balance, at beginning of period			$ 558
Balance, as of June 30	$ 596		596
Mortgage servicing rights
Classification of Securitizations
Fair value of capitalized mortgage servicing rights	596	$ 560	596	$ 560
Principal amount of loans and other financial instruments	63,000	71,000	63,000	71,000
Capitalized MSRs
Balance, at beginning of period	587	567	558	1,564
Originations	15	21	32	56
Changes in fair value of MSRs due to changes in inputs and assumptions	11	(11)	57	56
Other changes	(16)	(17)	(33)	(70)
Sale of MSRs	(1)	0	(18)	(1,046)
Balance, as of June 30	596	560	596	560
MSR fees
Servicing fees	43	65	89	171
Late fees	1	3	2	6
Ancillary fees	3	4	6	8
Total MSR fees	47	72	97	185
Mortgage securitizations - Non-agency-sponsored
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	90		90		$ 79
Original par value of re-securitizations deals in which the entity holds a retained interest	548		548		887
Senior interests
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	33		33
Subordinated interests
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	57		57
U.S. government-sponsored agency guaranteed
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	2,500		2,500		2,100
Securities transferred to re-securitization entities	6,600	$ 5,600	13,600	$ 10,100
Market value of retained interest related to re-securitization transaction	1,200		1,200		854
Original fair value of re-securitizations deals in which the entity holds a retained interest	$ 64,200		$ 64,200		$ 68,300