FAIR VALUE ELECTIONS - Changes in Fair Value Gains (Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Securities borrowed and purchased under agreements to resell
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	$ 0	$ 6
Trading account assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	(136)	77
Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	0	0
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	2,486	(222)
Consumer loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	1	0
Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	2,487	(222)
Mortgage servicing rights
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	(204)	(84)
Certain mortgage loans (HFS)
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	299	91
Total other assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	95	7
Total assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	2,446	(132)
Interest-bearing deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	(154)	(205)
Securities loaned and sold under agreements to repurchase
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	(559)	386
Trading account liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	(1)	27
Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	802	(78)
Long-term debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	(2,700)	(5,174)
Total liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gains (losses) from changes in fair value	$ (2,612)	$ (5,044)