ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructuring Activity (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) contract	Jun. 30, 2019 USD ($) contract	Jun. 30, 2020 USD ($) contract	Jun. 30, 2019 USD ($) contract
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	762	1,040	1,754	1,721
Amortized Cost | $	$ 16	$ 13	$ 31	$ 23
Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	125	40	170	96
Amortized Cost | $	$ 21	$ 4	$ 28	$ 13
Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1,342	629	1,837	1,166
Amortized Cost | $	$ 87	$ 64	$ 141	$ 123
Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	1	0	1
Amortized Cost | $	$ 0	$ 0	$ 0	$ 0
Commercial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	4	8	6	13
Amortized Cost | $	$ 3	$ 0	$ 3	$ 1
Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	15	6	32	18
Amortized Cost | $	$ 53	$ 47	$ 94	$ 87
Commercial | Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	1	0	1
Amortized Cost | $	$ 0	$ 0	$ 0	$ 0
Commercial | Commercial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	4	7	6	12
Amortized Cost | $	$ 3	$ 0	$ 3	$ 1
Commercial | Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	15	6	32	18
Amortized Cost | $	$ 53	$ 47	$ 94	$ 87
Commercial | Commercial real estate | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	0
Amortized Cost | $	$ 0	$ 0	$ 0	$ 0
Commercial | Commercial real estate | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	1	0	1
Amortized Cost | $	$ 0	$ 0	$ 0	$ 0
Commercial | Commercial real estate | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	0
Amortized Cost | $	$ 0	$ 0	$ 0	$ 0
Retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	762	1,039	1,754	1,720
Amortized Cost | $	$ 16	$ 13	$ 31	$ 23
Retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	121	32	164	83
Amortized Cost | $	$ 18	$ 4	$ 25	$ 12
Retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1,327	623	1,805	1,148
Amortized Cost | $	$ 34	$ 17	$ 47	$ 36
Retail | Residential mortgages | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	54	9	92	13
Amortized Cost | $	$ 11	$ 2	$ 17	$ 4
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	71	10	108	21
Amortized Cost | $	$ 14	$ 1	$ 21	$ 3
Retail | Residential mortgages | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	20	32	41	62
Amortized Cost | $	$ 3	$ 5	$ 7	$ 9
Retail | Home equity | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	27	49	73	85
Amortized Cost | $	$ 2	$ 5	$ 6	$ 9
Retail | Home equity | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	49	15	55	50
Amortized Cost | $	$ 4	$ 3	$ 4	$ 9
Retail | Home equity | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	190	100	261	238
Amortized Cost | $	$ 11	$ 6	$ 15	$ 15
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	36	40	83	65
Amortized Cost | $	$ 0	$ 1	$ 1	$ 1
Retail | Automobile | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1	7	1	12
Amortized Cost | $	$ 0	$ 0	$ 0	$ 0
Retail | Automobile | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	910	335	1,093	624
Amortized Cost | $	$ 15	$ 5	$ 17	$ 9
Retail | Education | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	0
Amortized Cost | $	$ 0	$ 0	$ 0	$ 0
Retail | Education | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	0
Amortized Cost | $	$ 0	$ 0	$ 0	$ 0
Retail | Education | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	142	13	233	80
Amortized Cost | $	$ 4	$ 1	$ 6	$ 3
Retail | Other retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	645	941	1,506	1,557
Amortized Cost | $	$ 3	$ 5	$ 7	$ 9
Retail | Other retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	0	0	0
Amortized Cost | $	$ 0	$ 0	$ 0	$ 0
Retail | Other retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	65	143	177	144
Amortized Cost | $	$ 1	$ 0	$ 2	$ 0