ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Nonperforming Loans and Leases by Class (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	$ 832	$ 871
Loans accruing and 90 days or more past due	25	32
GNMA
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans sold with right to repurchase	116	30
Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans accruing and 90 days or more past due	13	15
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	228	238
Loans accruing and 90 days or more past due	0	5
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	30	27
Loans accruing and 90 days or more past due	0	3
Commercial | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	0	0
Loans accruing and 90 days or more past due	0	0
Commercial | Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	258	265
Loans accruing and 90 days or more past due	0	8
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	136	128
Loans accruing and 90 days or more past due	16	16
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	53	72
Loans accruing and 90 days or more past due	0	0
Retail | Home equity lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	221	233
Loans accruing and 90 days or more past due	0	0
Retail | Home equity loans serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	19	25
Loans accruing and 90 days or more past due	0	0
Retail | Home equity lines of credit serviced by others
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	16	18
Loans accruing and 90 days or more past due	0	0
Retail | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	67	70
Loans accruing and 90 days or more past due	0	0
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	38	38
Loans accruing and 90 days or more past due	3	3
Retail | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	17	17
Loans accruing and 90 days or more past due	0	0
Retail | Other retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	7	5
Loans accruing and 90 days or more past due	6	5
Retail | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	574	606
Loans accruing and 90 days or more past due	$ 25	$ 24