FAIR VALUE MEASUREMENTS - Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt securities available for sale	$ 3,145	$ 3,275
Debt securities held to maturity	3,403	3,687
Net loans and leases, including loans held for sale	40	12
Derivative assets	145	96
Long-term borrowings	615
Derivative liabilities	469	411
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,637	1,674
Debt securities available for sale	306	257
Debt securities held to maturity	0	0
Net loans and leases, including loans held for sale	0	0
Loan servicing rights	0	0
Derivative assets	0	0
Accrued interest receivable	158	126
Deposits	28,822	31,158
Short-term borrowings	2,064	1,369
Long-term borrowings	0	0
Derivative liabilities	0	0
Accrued interest payable	60	31
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Debt securities available for sale	2,839	3,018
Debt securities held to maturity	3,403	3,687
Net loans and leases, including loans held for sale	92	91
Loan servicing rights	0	0
Derivative assets	145	96
Accrued interest receivable	0	0
Deposits	5,712	3,515
Short-term borrowings	0	0
Long-term borrowings	1,187	0
Derivative liabilities	466	399
Accrued interest payable	0	0
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Debt securities available for sale	0	0
Debt securities held to maturity	0	0
Net loans and leases, including loans held for sale	29,641	28,917
Loan servicing rights	77	71
Derivative assets	0	0
Accrued interest receivable	0	0
Deposits	0	0
Short-term borrowings	0	0
Long-term borrowings	736	1,061
Derivative liabilities	3	12
Accrued interest payable	0	0
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,637	1,674
Debt securities available for sale	3,145	3,275
Debt securities held to maturity	3,922	4,087
Net loans and leases, including loans held for sale	31,860	29,977
Loan servicing rights	60	55
Derivative assets	145	96
Accrued interest receivable	158	126
Deposits	34,615	34,770
Short-term borrowings	2,066	1,372
Long-term borrowings	1,968	1,093
Derivative liabilities	469	411
Accrued interest payable	60	31
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,637	1,674
Debt securities available for sale	3,145	3,275
Debt securities held to maturity	3,403	3,687
Net loans and leases, including loans held for sale	29,733	29,008
Loan servicing rights	77	71
Derivative assets	145	96
Accrued interest receivable	158	126
Deposits	34,534	34,673
Short-term borrowings	2,064	1,369
Long-term borrowings	1,923	1,061
Derivative liabilities	469	411
Accrued interest payable	$ 60	$ 31