ALLOWANCE FOR CREDIT LOSSES, NONACCRUAL LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructurings (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR's that defaulted within 12 months of modification date	$ 15	$ 37
Commercial | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR's that defaulted within 12 months of modification date	0	22
Retail | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR's that defaulted within 12 months of modification date	15	15
Retail | Loans Insured or Guaranteed by US Government Authorities
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR's that defaulted within 12 months of modification date	$ 10	$ 2