ALLOWANCE FOR CREDIT LOSSES, NONACCRUING LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Summary of Changes in Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 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	(158)
Provision charged to income	(140)	$ 600
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	2,194
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	2,443	1,252
Charge-offs	(227)	(174)
Recoveries	69	37
Net charge-offs	(158)	(137)
Provision charged to income	(91)	603
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	2,194	2,171
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	227	44
Provision for unfunded lending commitments	(49)	(3)
Allowance, Credit Loss
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	178	39
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Charge-offs	(60)
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	1,233	674
Charge-offs	(134)	(47)
Recoveries	30	3
Net charge-offs	(104)	(44)
Provision charged to income	17	298
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,146	752
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	186	44
Provision for unfunded lending commitments	(21)	(3)
Commercial \| Allowance, Credit Loss
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	165	38
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	1,210	578
Charge-offs	(93)	(127)
Recoveries	39	34
Net charge-offs	(54)	(93)
Provision charged to income	(108)	305
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,048	1,419
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	41	0
Provision for unfunded lending commitments	(28)	0
Retail \| Allowance, Credit Loss
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	13	1
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, Ending balance		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, Ending balance	(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, Ending balance		(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, Ending balance	(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, Ending balance		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, Ending balance	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, Ending balance		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, Ending balance	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, Ending balance		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, Ending balance	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, Ending balance		$ 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, Ending balance	$ 1

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- Definition

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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