ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Nonperforming Loans and Leases by Class (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	$ 780	$ 797
Loans accruing and 90 days or more past due	32	25
GNMA
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans sold with right to repurchase	144	133
Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans accruing and 90 days or more past due	13	12
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	208	194
Loans accruing and 90 days or more past due	1	1
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	4	7
Loans accruing and 90 days or more past due	0	0
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	212	201
Loans accruing and 90 days or more past due	1	1
Retail | Residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	138	136
Loans accruing and 90 days or more past due	20	15
Retail | Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	42	50
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	217	231
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	15	17
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	14	15
Loans accruing and 90 days or more past due	0	0
Retail | Automobile
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	70	81
Loans accruing and 90 days or more past due	0	0
Retail | Education
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	43	38
Loans accruing and 90 days or more past due	2	2
Retail | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	22	20
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	8
Loans accruing and 90 days or more past due	9	7
Retail | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonperforming loans	568	596
Loans accruing and 90 days or more past due	$ 31	$ 24