ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Loans with Indicators of High Credit Risk (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Total	$ 117,615	$ 116,660
High loan-to-value
Financing Receivable, Recorded Investment [Line Items]
Total	555	553
High loan-to-value | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	342	318
High loan-to-value | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	79	87
High loan-to-value | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	134	148
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
Interest-only/negative amortization
Financing Receivable, Recorded Investment [Line Items]
Total	1,724	1,795
Interest-only/negative amortization | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	1,724	1,794
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
Low introductory rate
Financing Receivable, Recorded Investment [Line Items]
Total	219	217
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	219	217
Low introductory rate | Education
Financing Receivable, Recorded Investment [Line Items]
Total		0
Multiple characteristics and other
Financing Receivable, Recorded Investment [Line Items]
Total	2	1
Multiple characteristics and other | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	2	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
Credit risk, loans with increased credit exposure
Financing Receivable, Recorded Investment [Line Items]
Total	2,500	2,566
Credit risk, loans with increased credit exposure | Residential Mortgages
Financing Receivable, Recorded Investment [Line Items]
Total	2,068	2,113
Credit risk, loans with increased credit exposure | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment [Line Items]
Total	79	87
Credit risk, loans with increased credit exposure | Home Equity Products Serviced by Others
Financing Receivable, Recorded Investment [Line Items]
Total	134	148
Credit risk, loans with increased credit exposure | Credit Cards
Financing Receivable, Recorded Investment [Line Items]
Total	$ 219	217
Credit risk, loans with increased credit exposure | Education
Financing Receivable, Recorded Investment [Line Items]
Total		$ 1