ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Nonperforming Loans and Leases by Class (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	$ 868	$ 871
Loans accruing and 90 days or more past due	26	32
GNMA
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans sold with right to repurchase	24	30
Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans accruing and 90 days or more past due	14	15
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	244	238
Loans accruing and 90 days or more past due	1	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	2	0
Commercial | Commercial Banking
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	274	265
Loans accruing and 90 days or more past due	3	8
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	128	128
Loans accruing and 90 days or more past due	14	16
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	66	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	235	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	23	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	17	18
Loans accruing and 90 days or more past due	0	0
Retail | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	60	70
Loans accruing and 90 days or more past due	0	0
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	41	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	18	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	6	5
Loans accruing and 90 days or more past due	6	5
Retail | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	594	606
Loans accruing and 90 days or more past due	$ 23	$ 24