ALLOWANCE FOR CREDIT LOSSES, NONACCRUAL LOANS AND LEASES, 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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance			$ 2,443
Charge-offs			(280)
Provision charged to income	$ (33)	$ 428	(386)	$ 1,492
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,855		1,855
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	1,947	2,448	2,443	1,252
Charge-offs	(87)	(257)	(439)	(611)
Recoveries	43	38	159	108
Net charge-offs	(44)	(219)	(280)	(503)
Provision charged to income	(48)	313	(308)	1,340
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,855	2,542	1,855	2,542
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	134	79	227	44
Provision for unfunded lending commitments	15	115	(78)	152
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	149	194	149	194
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	953	1,235	1,233	674
Charge-offs	(17)	(171)	(196)	(292)
Recoveries	3	1	37	7
Net charge-offs	(14)	(170)	(159)	(285)
Provision charged to income	(72)	224	(207)	1,076
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	867	1,289	867	1,289
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	121	69	186	44
Provision for unfunded lending commitments	9	83	(56)	111
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	130	152	130	152
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	994	1,213	1,210	578
Charge-offs	(70)	(86)	(243)	(319)
Recoveries	40	37	122	101
Net charge-offs	(30)	(49)	(121)	(218)
Provision charged to income	24	89	(101)	264
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	988	1,253	988	1,253
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	13	10	41	0
Provision for unfunded lending commitments	6	32	(22)	41
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	$ 19	42	$ 19	42
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

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Amount, after recovery, of writeoff of financing receivable, charged against allowance for credit loss.

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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 increase in allowance for credit loss on financing receivable from recovery.

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Amount of credit loss expense (reversal of expense) for financing receivable.

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