SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Mortgage Servicing Rights (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Capitalized MSRs
Balance at beginning of period	$ 1,564
Balance at end of period	558	$ 1,564
Mortgage servicing rights
Classification of Securitizations
Fair value of capitalized mortgage servicing rights	558	1,600
Principal amount of loans and other financial instruments	66,000	168,000
Capitalized MSRs
Balance at beginning of period	1,564	1,781
Originations	96	152
Changes in fair value of MSRs due to changes in inputs and assumptions	65	(36)
Other changes	(110)	(313)
Sale of MSRs	(1,057)	(20)
Balance at end of period	558	1,564	$ 1,781
MSR fees
Servicing fees	276	484	552
Late fees	10	14	16
Ancillary fees	13	17	31
Total MSR fees	299	515	$ 599
Mortgage securitizations - Non-agency-sponsored
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	79	126
Original par value of re-securitizations deals in which the entity holds a retained interest	887	1,300
U.S. government-sponsored agency guaranteed
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	2,100	2,300
Securities transferred to re-securitization entities	26,600	26,500
Market value of retained interest related to re-securitization transaction	854	741
Original fair value of re-securitizations deals in which the entity holds a retained interest	$ 68,300	$ 71,800