FAIR VALUE ELECTIONS - Changes in Fair Value Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Federal funds sold and securities borrowed or purchased under agreements to resell—selected portfolios
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	$ (17)	$ (54)	$ (108)	$ (7)
Trading account assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	581	571	1,243	509
Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	0	(4)	(3)	(25)
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	(61)	5	(42)	65
Consumer loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	1	1	3	0
Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	(60)	6	(39)	65
MSRs
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	(6)	13	50	(349)
Certain mortgage loans held for sale
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	34	100	115	271
Other assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	0	6	0	376
Total other assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	28	119	165	298
Total assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	532	638	1,258	840
Interest-bearing deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	(16)	(16)	(60)	(84)
Federal funds purchased and securities loaned or sold under agreements to repurchase—selected portfolios
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	97	32	183	24
Trading account liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	19	4	70	101
Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	(30)	(173)	(110)	(207)
Long-term debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	(198)	(305)	(669)	(845)
Total liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	$ (128)	$ (458)	$ (586)	$ (1,011)