ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Nonperforming Loans and Leases by Class (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Nonperforming loans	$ 703	$ 766
Loans accruing and 90 days or more past due	25	25
Commercial
Financing Receivable, Past Due [Line Items]
Nonperforming loans	245	201
Loans accruing and 90 days or more past due	2	1
Commercial | Commercial
Financing Receivable, Past Due [Line Items]
Nonperforming loans	240	194
Loans accruing and 90 days or more past due	2	1
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Nonperforming loans	2	7
Loans accruing and 90 days or more past due	0	0
Commercial | Leases
Financing Receivable, Past Due [Line Items]
Nonperforming loans	3	0
Loans accruing and 90 days or more past due	0	0
Retail
Financing Receivable, Past Due [Line Items]
Nonperforming loans	458	565
Loans accruing and 90 days or more past due	23	24
Retail | Residential mortgages
Financing Receivable, Past Due [Line Items]
Nonperforming loans	93	105
Loans accruing and 90 days or more past due	13	15
Retail | Home equity loans
Financing Receivable, Past Due [Line Items]
Nonperforming loans	33	50
Loans accruing and 90 days or more past due	0	0
Retail | Home equity lines of credit
Financing Receivable, Past Due [Line Items]
Nonperforming loans	187	231
Loans accruing and 90 days or more past due	0	0
Retail | Home equity loans serviced by others
Financing Receivable, Past Due [Line Items]
Nonperforming loans	14	17
Loans accruing and 90 days or more past due	0	0
Retail | Home equity lines of credit serviced by others
Financing Receivable, Past Due [Line Items]
Nonperforming loans	12	15
Loans accruing and 90 days or more past due	0	0
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Nonperforming loans	67	81
Loans accruing and 90 days or more past due	0	0
Retail | Education
Financing Receivable, Past Due [Line Items]
Nonperforming loans	18	38
Loans accruing and 90 days or more past due	2	2
Retail | Credit cards
Financing Receivable, Past Due [Line Items]
Nonperforming loans	22	20
Loans accruing and 90 days or more past due	0	0
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Nonperforming loans	12	8
Loans accruing and 90 days or more past due	$ 8	$ 7