ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Loans with Indicators of High Credit Risk (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 119,088	$ 116,660
High loan-to-value
Financing Receivable, Credit Quality Indicator [Line Items]
Total	553	553
High loan-to-value | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total	402	318
High loan-to-value | Home Equity Loans and Lines of Credit
Financing Receivable, Credit Quality Indicator [Line Items]
Total	61	87
High loan-to-value | Home Equity Products Serviced by Others
Financing Receivable, Credit Quality Indicator [Line Items]
Total	90	148
High loan-to-value | Credit Cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
High loan-to-value | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Interest only/negative amortization
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2,043	1,795
Interest only/negative amortization | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2,043	1,794
Interest only/negative amortization | Home Equity Loans and Lines of Credit
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Interest only/negative amortization | Home Equity Products Serviced by Others
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Interest only/negative amortization | Credit Cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Interest only/negative amortization | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Total		1
Low introductory rate
Financing Receivable, Credit Quality Indicator [Line Items]
Total	235	217
Low introductory rate | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Low introductory rate | Home Equity Loans and Lines of Credit
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Low introductory rate | Home Equity Products Serviced by Others
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Low introductory rate | Credit Cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total	235	217
Low introductory rate | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Multiple characteristics and other
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	1
Multiple characteristics and other | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	1
Multiple characteristics and other | Home Equity Loans and Lines of Credit
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Multiple characteristics and other | Home Equity Products Serviced by Others
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Multiple characteristics and other | Credit Cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Multiple characteristics and other | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Credit risk, loans with increased credit exposure
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2,831	2,566
Credit risk, loans with increased credit exposure | Residential mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2,445	2,113
Credit risk, loans with increased credit exposure | Home Equity Loans and Lines of Credit
Financing Receivable, Credit Quality Indicator [Line Items]
Total	61	87
Credit risk, loans with increased credit exposure | Home Equity Products Serviced by Others
Financing Receivable, Credit Quality Indicator [Line Items]
Total	90	148
Credit risk, loans with increased credit exposure | Credit Cards
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 235	217
Credit risk, loans with increased credit exposure | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Total		$ 1