FAIR VALUE ELECTIONS - Changes in Fair Value Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Securities borrowed and purchased under agreements to resell
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	$ (8)	$ (48)	$ (36)	$ 44
Trading account assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	52	373	153	(461)
Certain corporate loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	539	(154)	668	(1,017)
Certain consumer loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	0	(1)	0	0
Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	539	(155)	668	(1,017)
MSRs
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	(21)	(26)	52	(169)
Certain mortgage loans HFS
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	47	72	44	134
Total other assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	26	46	96	(35)
Total assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	609	216	881	(1,469)
Interest-bearing deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	(130)	(164)	(93)	(52)
Securities loaned and sold under agreements to repurchase
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	5	196	18	(92)
Trading account liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	8	44	10	(17)
Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	327	(259)	192	997
Long-term debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	(2,441)	(5,402)	(433)	1,963
Total liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions.
Gain (loss) from changes in fair value	$ (2,231)	$ (5,585)	$ (306)	$ 2,799