ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Loans with Indicators of High Credit Risk (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total	$ 110,617	$ 107,669
High loan-to-value
Financing Receivable, Recorded Investment [Line Items]
Total	796	1,592
High loan-to-value | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	366	566
High loan-to-value | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	166	550
High loan-to-value | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	264	476
High loan-to-value | Credit Cards
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
High loan-to-value | Education
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Interest only/negative amortization
Financing Receivable, Recorded Investment [Line Items]
Total	1,764	1,583
Interest only/negative amortization | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	1,763	1,582
Interest only/negative amortization | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Interest only/negative amortization | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Interest only/negative amortization | Credit Cards
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Interest only/negative amortization | Education
Financing Receivable, Recorded Investment [Line Items]
Total	1	1
Low introductory rate
Financing Receivable, Recorded Investment [Line Items]
Total	197	112
Low introductory rate | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Low introductory rate | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Low introductory rate | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Low introductory rate | Credit Cards
Financing Receivable, Recorded Investment [Line Items]
Total	197	112
Low introductory rate | Education
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Multiple characteristics and other
Financing Receivable, Recorded Investment [Line Items]
Total	1	3
Multiple characteristics and other | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	1	3
Multiple characteristics and other | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Multiple characteristics and other | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Multiple characteristics and other | Credit Cards
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Multiple characteristics and other | Education
Financing Receivable, Recorded Investment [Line Items]
Total	0	0
Credit risk, loans with increased credit exposure
Financing Receivable, Recorded Investment [Line Items]
Total	2,758	3,290
Credit risk, loans with increased credit exposure | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	2,130	2,151
Credit risk, loans with increased credit exposure | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	166	550
Credit risk, loans with increased credit exposure | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	264	476
Credit risk, loans with increased credit exposure | Credit Cards
Financing Receivable, Recorded Investment [Line Items]
Total	197	112
Credit risk, loans with increased credit exposure | Education
Financing Receivable, Recorded Investment [Line Items]
Total	$ 1	$ 1