ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructuring (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) contract	Mar. 31, 2018 USD ($) contract
Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	681	668
Recorded Investment | $	$ 10	$ 7
Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	56	73
Recorded Investment | $	$ 9	$ 8
Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	537	591
Recorded Investment | $	$ 59	$ 95
Commercial | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	1
Recorded Investment | $	$ 0	$ 0
Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	5	7
Recorded Investment | $	$ 1	$ 1
Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	12	18
Recorded Investment | $	$ 40	$ 75
Commercial | Commercial | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	1
Recorded Investment | $	$ 0	$ 0
Commercial | Commercial | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	5	6
Recorded Investment | $	$ 1	$ 1
Commercial | Commercial | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	12	18
Recorded Investment | $	$ 40	$ 75
Commercial | Commercial real estate | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment | $	$ 0	$ 0
Commercial | Commercial real estate | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	1
Recorded Investment | $	$ 0	$ 0
Commercial | Commercial real estate | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment | $	$ 0	$ 0
Retail | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	681	667
Recorded Investment | $	$ 10	$ 7
Retail | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	51	66
Recorded Investment | $	$ 8	$ 7
Retail | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	525	573
Recorded Investment | $	$ 19	$ 20
Retail | Residential mortgages | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	4	7
Recorded Investment | $	$ 2	$ 1
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	11	7
Recorded Investment | $	$ 2	$ 1
Retail | Residential mortgages | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	30	53
Recorded Investment | $	$ 4	$ 6
Retail | Home equity loans | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	7	11
Recorded Investment | $	$ 0	$ 1
Retail | Home equity loans | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment | $	$ 0	$ 0
Retail | Home equity loans | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	27	32
Recorded Investment | $	$ 1	$ 2
Retail | Home equity lines of credit | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	29	15
Recorded Investment | $	$ 4	$ 1
Retail | Home equity lines of credit | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	35	42
Recorded Investment | $	$ 6	$ 5
Retail | Home equity lines of credit | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	105	93
Recorded Investment | $	$ 8	$ 7
Retail | Home equity loans serviced by others | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	1
Recorded Investment | $	$ 0	$ 0
Retail | Home equity loans serviced by others | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment | $	$ 0	$ 0
Retail | Home equity loans serviced by others | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	4	7
Recorded Investment | $	$ 0	$ 0
Retail | Home equity lines of credit serviced by others | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	2
Recorded Investment | $	$ 0	$ 0
Retail | Home equity lines of credit serviced by others | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment | $	$ 0	$ 0
Retail | Home equity lines of credit serviced by others | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	2	3
Recorded Investment | $	$ 0	$ 0
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	25	36
Recorded Investment | $	$ 0	$ 1
Retail | Automobile | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	5	17
Recorded Investment | $	$ 0	$ 1
Retail | Automobile | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	289	269
Recorded Investment | $	$ 4	$ 4
Retail | Education | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment | $	$ 0	$ 0
Retail | Education | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment | $	$ 0	$ 0
Retail | Education | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	67	112
Recorded Investment | $	$ 2	$ 1
Retail | Credit cards | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	616	594
Recorded Investment | $	$ 4	$ 3
Retail | Credit cards | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment | $	$ 0	$ 0
Retail | Credit cards | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment | $	$ 0	$ 0
Retail | Other retail | Interest Rate Reduction
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	1
Recorded Investment | $	$ 0	$ 0
Retail | Other retail | Maturity Extension
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment | $	$ 0	$ 0
Retail | Other retail | Other
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1	4
Recorded Investment | $	$ 0	$ 0