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
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2,198
Amortized Cost Basis	$ 302
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	27
Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	38
Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	$ 237
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	10
Amortized Cost Basis	$ 31
Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	0
Commercial | Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	24
Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	$ 7
Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	10
Amortized Cost Basis	$ 31
Commercial | Commercial and industrial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	0
Commercial | Commercial and industrial | Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	24
Commercial | Commercial and industrial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	$ 7
Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2,188
Amortized Cost Basis	$ 271
Retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	27
Retail | Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	14
Retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	$ 230
Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1,181
Amortized Cost Basis	$ 250
Retail | Residential mortgages | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	22
Retail | Residential mortgages | Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	14
Retail | Residential mortgages | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	$ 214
Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	178
Amortized Cost Basis	$ 11
Retail | Home equity | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	2
Retail | Home equity | Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	0
Retail | Home equity | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	$ 9
Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	165
Amortized Cost Basis	$ 2
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	1
Retail | Automobile | Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	0
Retail | Automobile | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	$ 1
Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	143
Amortized Cost Basis	$ 6
Retail | Education | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	0
Retail | Education | Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	0
Retail | Education | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	$ 6
Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	521
Amortized Cost Basis	$ 2
Retail | Other retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	2
Retail | Other retail | Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	0
Retail | Other retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost Basis	$ 0