ALLOWANCE FOR CREDIT LOSSES, NONACCRUING LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructurings (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs from modification of loans	$ 2	$ 4	$ 4	$ 6
TDR commitment to lend	45		45		$ 49
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR's that defaulted within 12 months of modification date	1	26	23	39
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR's that defaulted within 12 months of modification date	$ 14	$ 14	$ 29	$ 25