ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Loans with Indicators of High Credit Risk (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total	$ 113,407	$ 110,617
High loan-to-value
Financing Receivable, Recorded Investment [Line Items]
Total	780	796
High loan-to-value | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	430	366
High loan-to-value | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	145	166
High loan-to-value | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	205	264
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	1,770	1,764
Interest-only/negative amortization | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	1,770	1,763
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	0	1
Low introductory rate
Financing Receivable, Recorded Investment [Line Items]
Total	197	197
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	197
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	1
Multiple characteristics and other | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	1	1
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	2,748	2,758
Credit risk, loans with increased credit exposure | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	2,201	2,130
Credit risk, loans with increased credit exposure | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	145	166
Credit risk, loans with increased credit exposure | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	205	264
Credit risk, loans with increased credit exposure | Credit Cards
Financing Receivable, Recorded Investment [Line Items]
Total	197	197
Credit risk, loans with increased credit exposure | Education
Financing Receivable, Recorded Investment [Line Items]
Total	$ 0	$ 1