CREDIT QUALITY AND THE ALLOWANCE FOR CREDIT LOSSES - Modified (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2023
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 408,000,000	$ 944,000,000
Total as a % of Loan Class	0.27%	0.63%
Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		$ 593,000,000
30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		56,000,000
60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		5,000,000
90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		13,000,000
Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		277,000,000
Unfunded loan commitment
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		146,000,000
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 365,000,000	$ 836,000,000
Total as a % of Loan Class	0.47%	1.08%
Commercial | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		$ 524,000,000
Commercial | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		55,000,000
Commercial | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Commercial | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Commercial | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		257,000,000
Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 196,000,000	$ 344,000,000
Total as a % of Loan Class	0.42%	0.74%
Weighted-Average Interest Rate Reduction	2.32%	2.95%
Weighted-Average Term Extension (in Months)	17 months	14 months
Weighted-Average Payment Deferral	$ 427,833	$ 562,909
Commercial | Commercial and industrial | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		262,000,000
Commercial | Commercial and industrial | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Commercial | Commercial and industrial | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Commercial | Commercial and industrial | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Commercial | Commercial and industrial | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		82,000,000
Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 169,000,000	$ 492,000,000
Total as a % of Loan Class	0.57%	1.67%
Weighted-Average Interest Rate Reduction	1.25%	1.25%
Weighted-Average Term Extension (in Months)	7 months	8 months
Weighted-Average Payment Deferral	$ 77,482	$ 47,172
Commercial | Commercial real estate | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		262,000,000
Commercial | Commercial real estate | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		55,000,000
Commercial | Commercial real estate | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Commercial | Commercial real estate | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Commercial | Commercial real estate | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		175,000,000
Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 43,000,000	$ 108,000,000
Total as a % of Loan Class	0.06%	0.15%
Retail | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		$ 69,000,000
Retail | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		1,000,000
Retail | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		5,000,000
Retail | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		13,000,000
Retail | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		20,000,000
Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 33,000,000	$ 81,000,000
Total as a % of Loan Class	0.11%	0.26%
Weighted-Average Interest Rate Reduction	0.98%	1.55%
Weighted-Average Term Extension (in Months)	47 months	48 months
Weighted-Average Payment Deferral	$ 0	$ 0
Retail | Residential mortgages | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		52,000,000
Retail | Residential mortgages | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Residential mortgages | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		5,000,000
Retail | Residential mortgages | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		13,000,000
Retail | Residential mortgages | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		11,000,000
Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 3,000,000	$ 10,000,000
Total as a % of Loan Class	0.02%	0.07%
Weighted-Average Interest Rate Reduction	2.51%	2.24%
Weighted-Average Term Extension (in Months)	133 months	127 months
Weighted-Average Payment Deferral	$ 0	$ 2,343
Retail | Home equity | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		3,000,000
Retail | Home equity | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Home equity | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Home equity | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Home equity | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		7,000,000
Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 0	$ 0
Total as a % of Loan Class	0.00%	0.00%
Weighted-Average Interest Rate Reduction	0.00%	3.47%
Weighted-Average Term Extension (in Months)		20 months
Weighted-Average Payment Deferral	$ 0	$ 1,253
Retail | Automobile | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Automobile | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Automobile | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Automobile | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Automobile | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 4,000,000	$ 9,000,000
Total as a % of Loan Class	0.03%	0.07%
Weighted-Average Interest Rate Reduction	4.95%	4.97%
Weighted-Average Term Extension (in Months)	0 months	0 months
Weighted-Average Payment Deferral	$ 4,972	$ 4,171
Retail | Education | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		8,000,000
Retail | Education | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Education | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Education | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Education | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		1,000,000
Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 3,000,000	$ 8,000,000
Total as a % of Loan Class	0.06%	0.16%
Weighted-Average Interest Rate Reduction	18.86%	18.45%
Weighted-Average Term Extension (in Months)	0 months	0 months
Weighted-Average Payment Deferral	$ 0	$ 0
Retail | Other retail | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		6,000,000
Retail | Other retail | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		1,000,000
Retail | Other retail | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Other retail | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0
Retail | Other retail | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		1,000,000
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	9,000,000	22,000,000
Interest Rate Reduction | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Interest Rate Reduction | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Interest Rate Reduction | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Interest Rate Reduction | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	9,000,000	22,000,000
Interest Rate Reduction | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	2,000,000	6,000,000
Interest Rate Reduction | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1,000,000	1,000,000
Interest Rate Reduction | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Interest Rate Reduction | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	3,000,000	7,000,000
Interest Rate Reduction | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	3,000,000	8,000,000
Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	305,000,000	780,000,000
Term Extension | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	279,000,000	717,000,000
Term Extension | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	148,000,000	263,000,000
Term Extension | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	131,000,000	454,000,000
Term Extension | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	26,000,000	63,000,000
Term Extension | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	25,000,000	59,000,000
Term Extension | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1,000,000	4,000,000
Term Extension | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Term Extension | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Term Extension | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	48,000,000	80,000,000
Payment Delay | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	47,000,000	78,000,000
Payment Delay | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	47,000,000	78,000,000
Payment Delay | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Payment Delay | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1,000,000	2,000,000
Payment Delay | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Payment Delay | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Payment Delay | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Payment Delay | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1,000,000	2,000,000
Payment Delay | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Principal Forgiveness | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Principal Forgiveness | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Amount of Principal Forgiven	0	0
Principal Forgiveness | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Amount of Principal Forgiven	0	0
Principal Forgiveness | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Principal Forgiveness | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Amount of Principal Forgiven	0	0
Principal Forgiveness | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Amount of Principal Forgiven	0	0
Principal Forgiveness | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Amount of Principal Forgiven	0	0
Principal Forgiveness | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Amount of Principal Forgiven	0	0
Principal Forgiveness | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Amount of Principal Forgiven	1,000,000	4,000,000
Interest Rate Reduction and Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	44,000,000	59,000,000
Interest Rate Reduction and Term Extension | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	37,000,000	38,000,000
Interest Rate Reduction and Term Extension | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	1,000,000
Interest Rate Reduction and Term Extension | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	37,000,000	37,000,000
Interest Rate Reduction and Term Extension | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	7,000,000	21,000,000
Interest Rate Reduction and Term Extension | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	6,000,000	16,000,000
Interest Rate Reduction and Term Extension | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1,000,000	5,000,000
Interest Rate Reduction and Term Extension | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Interest Rate Reduction and Term Extension | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Interest Rate Reduction and Term Extension | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Term Extension and Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	2,000,000	3,000,000
Term Extension and Payment Delay | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	2,000,000	3,000,000
Term Extension and Payment Delay | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1,000,000	2,000,000
Term Extension and Payment Delay | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1,000,000	1,000,000
Term Extension and Payment Delay | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Term Extension and Payment Delay | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Term Extension and Payment Delay | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Term Extension and Payment Delay | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Term Extension and Payment Delay | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0
Term Extension and Payment Delay | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 0	$ 0