ALLOWANCE FOR CREDIT LOSSES, NONACCRUING LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Loans with Indicators of High Credit Risk (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
High loan to value criteria (exceeds)	90.00%
High loan-to-value
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 262	$ 353
High loan-to-value | Residential Mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total	235	289
High loan-to-value | Home Equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total	27	64
High loan-to-value | Other Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
High loan-to-value | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Interest-only
Financing Receivable, Credit Quality Indicator [Line Items]
Total	3,144	2,801
Interest-only | Residential Mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total	3,143	2,801
Interest-only | Home Equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Interest-only | Other Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Interest-only | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Total	1	0
Low introductory rate
Financing Receivable, Credit Quality Indicator [Line Items]
Total	135	170
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	135	170
Low introductory rate | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0	0
Credit risk, loans with increased credit exposure
Financing Receivable, Credit Quality Indicator [Line Items]
Total	3,541	3,324
Credit risk, loans with increased credit exposure | Residential Mortgages
Financing Receivable, Credit Quality Indicator [Line Items]
Total	3,378	3,090
Credit risk, loans with increased credit exposure | Home Equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total	27	64
Credit risk, loans with increased credit exposure | Other Retail
Financing Receivable, Credit Quality Indicator [Line Items]
Total	135	170
Credit risk, loans with increased credit exposure | Education
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 1	$ 1