CREDIT QUALITY AND THE ALLOWANCE FOR CREDIT LOSSES - Modified (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended	15 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Sep. 30, 2024
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 498	$ 408	$ 1,331	$ 944		$ 1,486
Total as a % of Loan Class	0.35%	0.27%	0.94%	0.63%
Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				$ 593		931
30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				56		73
60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				5		69
90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				13		16
Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				277		397
Unfunded loan commitment
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			$ 75		$ 221
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 444	$ 365	$ 1,168	$ 836		1,271
Total as a % of Loan Class	0.62%	0.47%	1.63%	1.08%
Commercial | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				$ 524		801
Commercial | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				55		65
Commercial | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		64
Commercial | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Commercial | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				257		341
Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 104	$ 196	$ 289	$ 344		301
Total as a % of Loan Class	0.24%	0.42%	0.66%	0.74%
Weighted-Average Interest Rate Reduction	3.62%	2.32%	3.72%	2.95%
Weighted-Average Term Extension (in Months)	14 months	17 months	15 months	14 months
Weighted-Average Payment Deferral	$ 11	$ 0	$ 3	$ 1
Commercial | Commercial and industrial | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				262		211
Commercial | Commercial and industrial | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		35
Commercial | Commercial and industrial | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		1
Commercial | Commercial and industrial | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Commercial | Commercial and industrial | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				82		54
Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 340	$ 169	$ 879	$ 492		970
Total as a % of Loan Class	1.22%	0.57%	3.14%	1.67%
Weighted-Average Interest Rate Reduction	4.31%	1.25%	2.83%	1.25%
Weighted-Average Term Extension (in Months)	9 months	7 months	17 months	8 months
Weighted-Average Payment Deferral	$ 1	$ 0	$ 1	$ 0
Commercial | Commercial real estate | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				262		590
Commercial | Commercial real estate | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				55		30
Commercial | Commercial real estate | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		63
Commercial | Commercial real estate | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Commercial | Commercial real estate | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				175		287
Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 54	$ 43	$ 163	$ 108		215
Total as a % of Loan Class	0.08%	0.06%	0.23%	0.15%
Retail | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				$ 69		130
Retail | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				1		8
Retail | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				5		5
Retail | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				13		16
Retail | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				20		56
Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 22	$ 33	$ 86	$ 81		111
Total as a % of Loan Class	0.07%	0.11%	0.27%	0.26%
Weighted-Average Interest Rate Reduction	1.52%	0.98%	1.59%	1.55%
Weighted-Average Term Extension (in Months)	94 months	47 months	92 months	48 months
Weighted-Average Payment Deferral	$ 0	$ 0	$ 0	$ 0
Retail | Residential mortgages | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				52		73
Retail | Residential mortgages | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		6
Retail | Residential mortgages | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				5		4
Retail | Residential mortgages | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				13		16
Retail | Residential mortgages | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				11		12
Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 7	$ 3	$ 14	$ 10		19
Total as a % of Loan Class	0.04%	0.02%	0.09%	0.07%
Weighted-Average Interest Rate Reduction	4.32%	2.51%	4.03%	2.24%
Weighted-Average Term Extension (in Months)	51 months	133 months	75 months	127 months
Weighted-Average Payment Deferral	$ 0	$ 0	$ 0	$ 0
Retail | Home equity | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				3		11
Retail | Home equity | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Retail | Home equity | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Retail | Home equity | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Retail | Home equity | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				7		8
Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 0	$ 0	$ 0	$ 0		0
Total as a % of Loan Class	0.00%	0.00%	0.00%	0.00%
Weighted-Average Interest Rate Reduction	0.00%	0.00%	0.00%	3.47%
Weighted-Average Term Extension (in Months)				20 months
Weighted-Average Payment Deferral	$ 0	$ 0	$ 0	$ 0
Retail | Automobile | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Retail | Automobile | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Retail | Automobile | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Retail | Automobile | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Retail | Automobile | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 20	$ 4	$ 50	$ 9		70
Total as a % of Loan Class	0.18%	0.03%	0.45%	0.07%
Weighted-Average Interest Rate Reduction	4.44%	4.95%	4.42%	4.97%
Weighted-Average Term Extension (in Months)	24 months		24 months
Weighted-Average Payment Deferral	$ 0	$ 0	$ 0	$ 0
Retail | Education | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				8		34
Retail | Education | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		1
Retail | Education | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Retail | Education | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Retail | Education | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				1		35
Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 5	$ 3	$ 13	$ 8		15
Total as a % of Loan Class	0.10%	0.06%	0.27%	0.16%
Weighted-Average Interest Rate Reduction	20.79%	18.86%	20.23%	18.45%
Weighted-Average Payment Deferral	$ 0	$ 0	$ 0	$ 0
Retail | Other retail | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				6		12
Retail | Other retail | 30-59
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				1		1
Retail | Other retail | 60-89
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		1
Retail | Other retail | 90+
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0		0
Retail | Other retail | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				1		$ 1
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	11	9	29	22
Interest Rate Reduction | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Interest Rate Reduction | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Interest Rate Reduction | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Interest Rate Reduction | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	11	9	29	22
Interest Rate Reduction | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1	2	4	6
Interest Rate Reduction | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	2	1	3	1
Interest Rate Reduction | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Interest Rate Reduction | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	3	3	9	7
Interest Rate Reduction | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	5	3	13	8
Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	248	305	751	780
Term Extension | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	231	279	684	717
Term Extension | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	75	148	179	263
Term Extension | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	156	131	505	454
Term Extension | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	17	26	67	63
Term Extension | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	15	25	63	59
Term Extension | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1	1	2	4
Term Extension | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Term Extension | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1	0	2	0
Term Extension | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	65	48	234	80
Payment Delay | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	48	47	186	78
Payment Delay | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	25	47	86	78
Payment Delay | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	23	0	100	0
Payment Delay | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	17	1	48	2
Payment Delay | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1	0	9	0
Payment Delay | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Payment Delay | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Payment Delay | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	16	1	39	2
Payment Delay | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Principal Forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Principal Forgiveness | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Principal Forgiveness | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Amount of Principal Forgiven	0	0	0	0
Principal Forgiveness | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Amount of Principal Forgiven	0	0	0	0
Principal Forgiveness | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Principal Forgiveness | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Amount of Principal Forgiven	0	0	0	0
Principal Forgiveness | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Amount of Principal Forgiven	0	0	0	0
Principal Forgiveness | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Amount of Principal Forgiven	0	0	0	0
Principal Forgiveness | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Amount of Principal Forgiven	0	0	0	0
Principal Forgiveness | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Amount of Principal Forgiven	1	1	5	4
Interest Rate Reduction and Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	76	44	148	59
Interest Rate Reduction and Term Extension | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	68	37	131	38
Interest Rate Reduction and Term Extension | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1	0	1	1
Interest Rate Reduction and Term Extension | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	67	37	130	37
Interest Rate Reduction and Term Extension | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	8	7	17	21
Interest Rate Reduction and Term Extension | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	4	6	8	16
Interest Rate Reduction and Term Extension | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	4	1	9	5
Interest Rate Reduction and Term Extension | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Interest Rate Reduction and Term Extension | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Interest Rate Reduction and Term Extension | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Term Extension and Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	98	2	168	3
Term Extension and Payment Delay | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	97	2	167	3
Term Extension and Payment Delay | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	3	1	23	2
Term Extension and Payment Delay | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	94	1	144	1
Term Extension and Payment Delay | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1	0	1	0
Term Extension and Payment Delay | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1	0	1	0
Term Extension and Payment Delay | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Term Extension and Payment Delay | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Term Extension and Payment Delay | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	0	0
Term Extension and Payment Delay | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 0	$ 0	0	$ 0
Interest Rate Reduction, Term Extension and Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			1
Interest Rate Reduction, Term Extension and Payment Delay | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			0
Interest Rate Reduction, Term Extension and Payment Delay | Commercial | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			0
Interest Rate Reduction, Term Extension and Payment Delay | Commercial | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			0
Interest Rate Reduction, Term Extension and Payment Delay | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			1
Interest Rate Reduction, Term Extension and Payment Delay | Retail | Residential mortgages
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			1
Interest Rate Reduction, Term Extension and Payment Delay | Retail | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			0
Interest Rate Reduction, Term Extension and Payment Delay | Retail | Automobile
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			0
Interest Rate Reduction, Term Extension and Payment Delay | Retail | Education
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			0
Interest Rate Reduction, Term Extension and Payment Delay | Retail | Other retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			$ 0