LOANS AND ALLOWANCE FOR CREDIT LOSSES - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Allowance for credit loss - loans
Beginning balance			$ 79,114
Ending balance	$ 84,944	$ 76,200	84,944	$ 76,200	$ 79,114
Accrued interest receivable on loans	36,800		36,800
Amounts outstanding	462,200		462,200		610,000
Net loans	10,665,318		10,665,318		10,353,251
Foreclosures in progress	8,300		8,300		6,300
Sale of secondary market eligible residential consumer mortgage loans			66,700	39,900
Liabilities for off-balance-sheet
Beginning balance	859	869	1,132	913	913
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	0	213	(273)	169
Ending balance	859	1,082	859	1,082	1,132
Asset Pledged as Collateral \| Borrowings from FHLB and FRB
Allowance for credit loss - loans
Net loans	6,780,000		6,780,000		6,720,000
Business lending
Allowance for credit loss - loans
Charge-offs			(6,184)		(3,157)
Consumer mortgage
Allowance for credit loss - loans
Charge-offs			(56)		(384)
Consumer indirect
Allowance for credit loss - loans
Charge-offs			(10,142)		(11,607)
Consumer direct
Allowance for credit loss - loans
Charge-offs			(1,939)		(2,744)
Home equity
Allowance for credit loss - loans
Charge-offs			(116)		(164)
Commercial Portfolio Segment \| Business lending
Allowance for credit loss - loans
Beginning balance	41,154	31,128	37,201	26,854	26,854
Charge-offs	(564)	(1,114)	(6,184)	(1,640)
Recoveries	187	199	636	330
Provision	3,135	3,536	12,259	8,205
Ending balance	43,912	33,749	43,912	33,749	37,201
Residential Portfolio Segment \| Consumer mortgage
Allowance for credit loss - loans
Beginning balance	14,196	14,303	15,017	15,333	15,333
Charge-offs	(33)	(188)	(56)	(345)
Recoveries	8	5	19	39
Provision	(114)	674	(923)	(233)
Ending balance	14,057	14,794	14,057	14,794	15,017
Residential Portfolio Segment \| Home equity
Allowance for credit loss - loans
Beginning balance	1,483	1,519	1,548	1,628	1,628
Charge-offs	(93)	(58)	(116)	(115)
Recoveries	0	2	1	5
Provision	136	103	93	48
Ending balance	1,526	1,566	1,526	1,566	1,548
Consumer Portfolio Segment \| Consumer indirect
Allowance for credit loss - loans
Beginning balance	19,885	20,124	20,895	18,585	18,585
Charge-offs	(3,557)	(3,000)	(10,142)	(8,669)
Recoveries	2,052	1,816	6,160	4,956
Provision	1,940	2,146	3,407	6,214
Ending balance	20,320	21,086	20,320	21,086	20,895
Consumer Portfolio Segment \| Consumer direct
Allowance for credit loss - loans
Beginning balance	4,133	3,368	3,453	3,269	3,269
Charge-offs	(722)	(661)	(1,939)	(2,169)
Recoveries	251	228	807	710
Provision	467	1,037	1,808	2,162
Ending balance	4,129	3,972	4,129	3,972	3,453
Unallocated
Allowance for credit loss - loans
Beginning balance	1,000	1,000	1,000	1,000	1,000
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	0	0	0	0
Ending balance	1,000	1,000	1,000	1,000	1,000
Allowances for credit losses
Allowance for credit loss - loans
Beginning balance	81,851	71,442	79,114	66,669	66,669
Charge-offs	(4,969)	(5,021)	(18,437)	(12,938)
Recoveries	2,498	2,250	7,623	6,040
Provision	5,564	7,496	16,644	16,396
Ending balance	84,944	76,167	84,944	76,167	79,114
Total allowances for credit losses
Allowance for credit loss
Beginning balance	82,710	72,311	80,246	67,582	67,582
Charge-offs	(4,969)	(5,021)	(18,437)	(12,938)
Recoveries	2,498	2,250	7,623	6,040
Provision	5,564	7,709	16,371	16,565
Ending balance	$ 85,803	$ 77,249	$ 85,803	$ 77,249	$ 80,246

Amount of Federal Home Loan Bank (FHLB) advances foreclosures in progress.

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Amount of net allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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Amount that has been written off for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

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Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss, excluding acquisition-related provision for credit losses based on assessment of uncollectible from the counterparty to reduce the account to their net realizable value.

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Amount, after allowance for credit loss, of accrued interest on financing receivable.

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Amortized cost excluding accrued interest, after allowance for credit loss, of financing receivable. Excludes net investment in lease.

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Amount of expected credit loss for credit exposure on off-balance-sheet commitment, including but not limited to, loan commitment, standby letter of credit, financial guarantee not accounted for as insurance. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

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