ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructuring Activity (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) contract	Sep. 30, 2019 USD ($) contract	Sep. 30, 2020 USD ($) contract	Sep. 30, 2019 USD ($) contract
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	505	928	2,259	2,649
Amortized Cost | $	$ 13	$ 14	$ 44	$ 37
Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	152	34	322	130
Amortized Cost | $	$ 113	$ 4	$ 141	$ 17
Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1,458	809	3,295	1,975
Amortized Cost | $	$ 32	$ 31	$ 173	$ 154
Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	2	0	3
Amortized Cost | $	$ 0	$ 0	$ 0	$ 0
Commercial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	12	6	18	19
Amortized Cost | $	$ 103	$ 1	$ 106	$ 2
Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2	6	34	24
Amortized Cost | $	$ 1	$ 15	$ 95	$ 102
Commercial | Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	0	2	0	3
Amortized Cost | $	$ 0	$ 0	$ 0	$ 0
Commercial | Commercial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	12	6	18	18
Amortized Cost | $	$ 103	$ 1	$ 106	$ 2
Commercial | Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2	6	34	24
Amortized Cost | $	$ 1	$ 15	$ 95	$ 102
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	0	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	505	926	2,259	2,646
Amortized Cost | $	$ 13	$ 14	$ 44	$ 37
Retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	140	28	304	111
Amortized Cost | $	$ 10	$ 3	$ 35	$ 15
Retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1,456	803	3,261	1,951
Amortized Cost | $	$ 31	$ 16	$ 78	$ 52
Retail | Residential mortgages | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	47	12	139	25
Amortized Cost | $	$ 9	$ 2	$ 26	$ 6
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	41	8	149	29
Amortized Cost | $	$ 6	$ 2	$ 27	$ 5
Retail | Residential mortgages | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	19	25	60	87
Amortized Cost | $	$ 4	$ 4	$ 11	$ 13
Retail | Home equity | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	23	63	96	148
Amortized Cost | $	$ 2	$ 6	$ 8	$ 15
Retail | Home equity | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	52	16	107	66
Amortized Cost | $	$ 4	$ 1	$ 8	$ 10
Retail | Home equity | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	104	120	365	358
Amortized Cost | $	$ 6	$ 6	$ 21	$ 21
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	25	46	108	111
Amortized Cost | $	$ 1	$ 1	$ 2	$ 2
Retail | Automobile | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	47	4	48	16
Amortized Cost | $	$ 0	$ 0	$ 0	$ 0
Retail | Automobile | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1,119	309	2,212	933
Amortized Cost | $	$ 18	$ 4	$ 35	$ 13
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	140	131	373	211
Amortized Cost | $	$ 3	$ 2	$ 9	$ 5
Retail | Other retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	410	805	1,916	2,362
Amortized Cost | $	$ 1	$ 5	$ 8	$ 14
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	74	218	251	362
Amortized Cost | $	$ 0	$ 0	$ 2	$ 0