ALLOWANCE FOR CREDIT LOSSES, NONACCRUAL LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Troubled Debt Restructuring Activity (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($) contract	Jun. 30, 2021 USD ($) contract	Jun. 30, 2022 USD ($) contract	Jun. 30, 2021 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1,178	2,017	3,376	3,659
Post-modification	$ 119	$ 252	$ 421	$ 295
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	19	11	46	20
Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	40	126	78	140
Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 60	$ 115	$ 297	$ 135
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	9	15	19	22
Post-modification	$ 27	$ 57	$ 58	$ 60
Commercial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	0	0	0
Commercial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	3	24	6
Commercial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 27	$ 54	$ 34	$ 54
Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	9	15	19	22
Post-modification	$ 27	$ 57	$ 58	$ 60
Commercial | Commercial and industrial | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	0	0	0
Commercial | Commercial and industrial | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	3	24	6
Commercial | Commercial and industrial | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 27	$ 54	$ 34	$ 54
Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	1,169	2,002	3,357	3,637
Post-modification	$ 92	$ 195	$ 363	$ 235
Retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	19	11	46	20
Retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	40	123	54	134
Retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 33	$ 61	$ 263	$ 81
Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	290	671	1,471	713
Post-modification	$ 76	$ 172	$ 326	$ 185
Retail | Residential mortgages | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	16	8	38	12
Retail | Residential mortgages | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	39	120	53	126
Retail | Residential mortgages | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 21	$ 44	$ 235	$ 47
Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	72	102	250	249
Post-modification	$ 6	$ 7	$ 17	$ 18
Retail | Home equity | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	1	2	3
Retail | Home equity | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	1	3	1	8
Retail | Home equity | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 5	$ 3	$ 14	$ 7
Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	147	379	312	1,048
Post-modification	$ 1	$ 6	$ 3	$ 14
Retail | Automobile | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	1	1	1
Retail | Automobile | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	0	0	0
Retail | Automobile | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 1	$ 5	$ 2	$ 13
Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	93	265	236	412
Post-modification	$ 5	$ 9	$ 11	$ 13
Retail | Education | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	0	0	0
Retail | Education | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	0	0	0
Retail | Education | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 5	$ 9	$ 11	$ 13
Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Contracts | contract	567	585	1,088	1,215
Post-modification	$ 4	$ 1	$ 6	$ 5
Retail | Other retail | Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	3	1	5	4
Retail | Other retail | Maturity Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	0	0	0	0
Retail | Other retail | Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Post-modification	$ 1	$ 0	$ 1	$ 1