FAIR VALUE ELECTIONS - Changes in Fair Value Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Federal funds sold and securities borrowed and purchased under agreements to resell
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	$ (17)	$ (17)	$ (14)	$ (108)
Trading account assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	3	581	(98)	1,243
Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	0	0	0	(3)
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	11	(61)	(115)	(42)
Consumer loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	0	1	0	3
Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	11	(60)	(115)	(39)
MSRs
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	25	(6)	82	50
Certain mortgage loans held for sale
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	9	34	21	115
Total other assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	34	28	103	165
Total assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	31	532	(124)	1,258
Interest-bearing deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	(20)	(16)	18	(60)
Federal funds purchased and securities loaned and sold under agreements to repurchase
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	230	97	104	183
Trading account liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	25	19	4	70
Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	20	(30)	138	(110)
Long-term debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	(270)	(510)	1,269	(981)
Total liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Fair value elections, changes in fair value gains (losses)	$ (15)	$ (440)	$ 1,533	$ (898)