LOANS AND LEASES - Age Analysis of Past Due Loans (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	$ 26,839	$ 24,968
90 Days Past Due and Still Accruing	8	6
Non-accrual loans	102	88
Non-accrual with No ACL	27	24
30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	68	59
Past due
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	178	153
Current
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	26,661	24,815
Total commercial loans and leases
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	17,581	16,465
90 Days Past Due and Still Accruing	0	0
Non-accrual loans	67	64
Non-accrual with No ACL	27	24
Total commercial loans and leases | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	10,746	9,899
90 Days Past Due and Still Accruing	0	0
Non-accrual loans	53	48
Non-accrual with No ACL	26	20
Total commercial loans and leases | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	6,220	5,977
90 Days Past Due and Still Accruing	0	0
Non-accrual loans	12	15
Non-accrual with No ACL	1	4
Total commercial loans and leases | Commercial leases
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	471	495
90 Days Past Due and Still Accruing	0	0
Non-accrual loans	1	1
Non-accrual with No ACL	0	0
Total commercial loans and leases | Other
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	144	94
90 Days Past Due and Still Accruing	0	0
Non-accrual loans	1	0
Non-accrual with No ACL	0	0
Total commercial loans and leases | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	24	16
Total commercial loans and leases | 30-89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	16	11
Total commercial loans and leases | 30-89 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	6	4
Total commercial loans and leases | 30-89 Days Past Due | Commercial leases
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	0	1
Total commercial loans and leases | 30-89 Days Past Due | Other
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	2	0
Total commercial loans and leases | Past due
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	91	80
Total commercial loans and leases | Past due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	69	59
Total commercial loans and leases | Past due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	18	19
Total commercial loans and leases | Past due | Commercial leases
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	1	2
Total commercial loans and leases | Past due | Other
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	3	0
Total commercial loans and leases | Current
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	17,490	16,385
Total commercial loans and leases | Current | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	10,677	9,840
Total commercial loans and leases | Current | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	6,202	5,958
Total commercial loans and leases | Current | Commercial leases
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	470	493
Total commercial loans and leases | Current | Other
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	141	94
Total consumer loans
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	9,258	8,503
90 Days Past Due and Still Accruing	8	6
Non-accrual loans	35	24
Non-accrual with No ACL	0	0
Total consumer loans | Direct installment
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	2,568	2,376
90 Days Past Due and Still Accruing	0	0
Non-accrual loans	7	7
Non-accrual with No ACL	0	0
Total consumer loans | Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	4,188	3,654
90 Days Past Due and Still Accruing	6	4
Non-accrual loans	20	10
Non-accrual with No ACL	0	0
Total consumer loans | Indirect installment
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	1,216	1,227
90 Days Past Due and Still Accruing	0	1
Non-accrual loans	2	2
Non-accrual with No ACL	0	0
Total consumer loans | Consumer lines of credit
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	1,286	1,246
90 Days Past Due and Still Accruing	2	1
Non-accrual loans	6	5
Non-accrual with No ACL	0	0
Total consumer loans | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	44	43
Total consumer loans | 30-89 Days Past Due | Direct installment
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	6	5
Total consumer loans | 30-89 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	21	20
Total consumer loans | 30-89 Days Past Due | Indirect installment
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	10	12
Total consumer loans | 30-89 Days Past Due | Consumer lines of credit
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	7	6
Total consumer loans | Past due
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	87	73
Total consumer loans | Past due | Direct installment
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	13	12
Total consumer loans | Past due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	47	34
Total consumer loans | Past due | Indirect installment
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	12	15
Total consumer loans | Past due | Consumer lines of credit
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	15	12
Total consumer loans | Current
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	9,171	8,430
Total consumer loans | Current | Direct installment
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	2,555	2,364
Total consumer loans | Current | Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	4,141	3,620
Total consumer loans | Current | Indirect installment
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	1,204	1,212
Total consumer loans | Current | Consumer lines of credit
Financing Receivable, Past Due [Line Items]
Total Loans and Leases	$ 1,271	$ 1,234