ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Loans with Indicators of High Credit Risk (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
High loan-to-value
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 543	$ 553
High loan-to-value | Residential Mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total	412	402
High loan-to-value | Home Equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total	131	151
High loan-to-value | Other Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Interest-only/negative amortization
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2,126	2,043
Interest-only/negative amortization | Residential Mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2,126	2,043
Interest-only/negative amortization | Home Equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Interest-only/negative amortization | Other Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Low introductory rate
Financing Receivable, Credit Quality Indicator [Line Items]
Total	245	235
Low introductory rate | Residential Mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Low introductory rate | Home Equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Low introductory rate | Other Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Total	245	235
Credit risk, loans with increased credit exposure
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2,914	2,831
Credit risk, loans with increased credit exposure | Residential Mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total	2,538	2,445
Credit risk, loans with increased credit exposure | Home Equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total	131	151
Credit risk, loans with increased credit exposure | Other Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 245	$ 235