ALLOWANCE FOR CREDIT LOSSES, NONACCRUAL LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructuring Activity (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) contract	Mar. 31, 2021 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2,198	1,642
Post-modification	$ 302	$ 43
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	27	9
Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	38	14
Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 237	$ 20
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	10	7
Post-modification	$ 31	$ 3
Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	0
Commercial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	24	3
Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 7	$ 0
Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	10	7
Post-modification	$ 31	$ 3
Commercial | Commercial and industrial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	0
Commercial | Commercial and industrial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	24	3
Commercial | Commercial and industrial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 7	$ 0
Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2,188	1,635
Post-modification	$ 271	$ 40
Retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	27	9
Retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	14	11
Retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 230	$ 20
Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1,181	42
Post-modification	$ 250	$ 13
Retail | Residential mortgages | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	22	4
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	14	6
Retail | Residential mortgages | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 214	$ 3
Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	178	147
Post-modification	$ 11	$ 11
Retail | Home equity | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	2	2
Retail | Home equity | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	5
Retail | Home equity | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 9	$ 4
Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	165	669
Post-modification	$ 2	$ 8
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	1	0
Retail | Automobile | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	0
Retail | Automobile | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 1	$ 8
Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	143	147
Post-modification	$ 6	$ 4
Retail | Education | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	0
Retail | Education | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	0
Retail | Education | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 6	$ 4
Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	521	630
Post-modification	$ 2	$ 4
Retail | Other retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	2	3
Retail | Other retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	0
Retail | Other retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 0	$ 1