ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructuring Activity (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) contract	Mar. 31, 2019 USD ($) contract
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	992	681
Amortized Cost | $	$ 15	$ 10
Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	45	56
Amortized Cost | $	$ 7	$ 9
Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	495	537
Amortized Cost | $	$ 54	$ 59
Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0
Amortized Cost | $	$ 0	$ 0
Commercial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2	5
Amortized Cost | $	$ 0	$ 1
Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	17	12
Amortized Cost | $	$ 41	$ 40
Commercial | Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0
Amortized Cost | $	$ 0	$ 0
Commercial | Commercial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2	5
Amortized Cost | $	$ 0	$ 1
Commercial | Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	17	12
Amortized Cost | $	$ 41	$ 40
Commercial | Commercial real estate | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0
Amortized Cost | $	$ 0	$ 0
Commercial | Commercial real estate | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0
Amortized Cost | $	$ 0	$ 0
Commercial | Commercial real estate | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0
Amortized Cost | $	$ 0	$ 0
Retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	992	681
Amortized Cost | $	$ 15	$ 10
Retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	43	51
Amortized Cost | $	$ 7	$ 8
Retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	478	525
Amortized Cost | $	$ 13	$ 19
Retail | Residential mortgages | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	38	4
Amortized Cost | $	$ 6	$ 2
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	37	11
Amortized Cost | $	$ 7	$ 2
Retail | Residential mortgages | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	21	30
Amortized Cost | $	$ 4	$ 4
Retail | Home equity | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	46	36
Amortized Cost | $	$ 4	$ 4
Retail | Home equity | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	6	35
Amortized Cost | $	$ 0	$ 6
Retail | Home equity | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	71	138
Amortized Cost | $	$ 4	$ 9
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	47	25
Amortized Cost | $	$ 1	$ 0
Retail | Automobile | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	5
Amortized Cost | $	$ 0	$ 0
Retail | Automobile | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	183	289
Amortized Cost | $	$ 2	$ 4
Retail | Education | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0
Amortized Cost | $	$ 0	$ 0
Retail | Education | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0
Amortized Cost | $	$ 0	$ 0
Retail | Education | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	91	67
Amortized Cost | $	$ 2	$ 2
Retail | Other retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	861	616
Amortized Cost | $	$ 4	$ 4
Retail | Other retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0
Amortized Cost | $	$ 0	$ 0
Retail | Other retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	112	1
Amortized Cost | $	$ 1	$ 0