ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Summary of Changes in Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Provision charged to income			$ 1,492	$ 283
Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	$ 2,448	$ 1,227	1,252	1,242
Charge-offs	(257)	(159)	(611)	(453)
Recoveries	38	46	108	145
Net charge-offs	(219)	(113)	(503)	(308)
Provision charged to income	313	149	1,340	329
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	2,542	1,263	2,542	1,263
Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	79	93	44	91
Provision for unfunded lending commitments	115	(48)	152	(46)
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	194	45	194	45
Commercial \| Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	1,235	680	674	690
Charge-offs	(171)	(35)	(292)	(106)
Recoveries	1	3	7	17
Net charge-offs	(170)	(32)	(285)	(89)
Provision charged to income	224	64	1,076	111
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,289	712	1,289	712
Commercial \| Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	69	93	44	91
Provision for unfunded lending commitments	83	(48)	111	(46)
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	152	45	152	45
Retail \| Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	1,213	547	578	552
Charge-offs	(86)	(124)	(319)	(347)
Recoveries	37	43	101	128
Net charge-offs	(49)	(81)	(218)	(219)
Provision charged to income	89	85	264	218
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,253	551	1,253	551
Retail \| Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	10	0	0	0
Provision for unfunded lending commitments	32	0	41	0
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	42	$ 0	42	$ 0
Cumulative Effect, Period Of Adoption, Adjustment \| Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	0		453
Cumulative Effect, Period Of Adoption, Adjustment \| Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	0		(2)
Cumulative Effect, Period Of Adoption, Adjustment \| Commercial \| Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	0		(176)
Cumulative Effect, Period Of Adoption, Adjustment \| Commercial \| Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	0		(3)
Cumulative Effect, Period Of Adoption, Adjustment \| Retail \| Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	0		629
Cumulative Effect, Period Of Adoption, Adjustment \| Retail \| Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	0		1
Cumulative Effect, Period Of Adoption, Adjusted Balance \| Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	2,448		1,705
Cumulative Effect, Period Of Adoption, Adjusted Balance \| Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	79		42
Cumulative Effect, Period Of Adoption, Adjusted Balance \| Commercial \| Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	1,235		498
Cumulative Effect, Period Of Adoption, Adjusted Balance \| Commercial \| Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	69		41
Cumulative Effect, Period Of Adoption, Adjusted Balance \| Retail \| Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	1,213		1,207
Cumulative Effect, Period Of Adoption, Adjusted Balance \| Retail \| Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	$ 10		$ 1

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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Amount, after recoveries, of loans and leases that have been written-off.

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Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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Amount of expense related to estimated loss from loan and lease transactions.

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Amount of expense related to credit loss from transactions other than loan and lease transactions.

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