FAIR VALUE ELECTIONS - Changes in Fair Value Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Securities borrowed and purchased under agreements to resell
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	$ (28)	$ 92
Trading account assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	101	(834)
Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	0	0
Certain corporate loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	129	(863)
Certain consumer loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	0	1
Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	129	(862)
MSRs
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	73	(143)
Certain mortgage loans HFS
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	(3)	62
Total other assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	70	(81)
Total assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	272	(1,685)
Interest-bearing deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	37	112
Securities loaned and sold under agreements to repurchase
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	13	(288)
Trading account liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	2	(61)
Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	(135)	1,256
Long-term debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	2,008	7,365
Total liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	$ 1,925	$ 8,384