ALLOWANCE FOR CREDIT LOSSES, NONACCRUAL LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructuring Activity (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) contract	Dec. 31, 2020 USD ($) contract	Dec. 31, 2019 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	5,939	7,559	6,492
Amortized Cost	$ 468	$ 644	$ 358
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	37	63	53
Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	192	161	26
Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 239	$ 420	$ 279
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	44	71	86
Amortized Cost	$ 167	$ 439	$ 215
Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	0	0	0
Commercial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	44	114	5
Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 123	$ 325	$ 210
Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	44	70	85
Amortized Cost	$ 167	$ 432	$ 215
Commercial | Commercial and industrial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	0	0	0
Commercial | Commercial and industrial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	44	107	5
Commercial | Commercial and industrial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 123	$ 325	$ 210
Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract		1	1
Amortized Cost		$ 7	$ 0
Commercial | Commercial real estate | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost		0	0
Commercial | Commercial real estate | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost		7	0
Commercial | Commercial real estate | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost		$ 0	$ 0
Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	5,895	7,488	6,406
Amortized Cost	$ 301	$ 205	$ 143
Retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	37	63	53
Retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	148	47	21
Retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 116	$ 95	$ 69
Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	922	473	242
Amortized Cost	$ 218	$ 86	$ 39
Retail | Residential mortgages | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	21	39	12
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	137	34	10
Retail | Residential mortgages | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 60	$ 13	$ 17
Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	412	723	722
Amortized Cost	$ 29	$ 47	$ 57
Retail | Home equity | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	5	12	20
Retail | Home equity | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	11	12	11
Retail | Home equity | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 13	$ 23	$ 26
Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1,463	3,236	1,431
Amortized Cost	$ 17	$ 50	$ 20
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	2	2	3
Retail | Automobile | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	0	1	0
Retail | Automobile | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 15	$ 47	$ 17
Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	807	465	272
Amortized Cost	$ 26	$ 10	$ 7
Retail | Education | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	0	0	0
Retail | Education | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	0	0	0
Retail | Education | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 26	$ 10	$ 7
Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	2,291	2,591	3,739
Amortized Cost	$ 11	$ 12	$ 20
Retail | Other retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	9	10	18
Retail | Other retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	0	0	0
Retail | Other retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized Cost	$ 2	$ 2	$ 2