FAIR VALUE - Summary of carrying amounts and estimated fair values for financial instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Assets:
Cash and cash equivalents	$ 276,222	$ 254,442	$ 271,484	$ 196,512
Held-to-maturity securities, net of ACL	70,826	66,617
Total	2,383,199	2,530,915
Accrued interest receivable	10,455	11,428
Liabilities:
FHLB borrowings – fixed rate	62,373	125,711
Subordinated notes	52,508	52,340
Accrued interest payable	3,339	3,793
Carrying Amount
Assets:
Cash and cash equivalents	276,222	254,442
Held-to-maturity securities, net of ACL	76,745	74,102
Total	2,355,757	2,493,258
Accrued interest receivable	10,455	11,428
Liabilities:
Term deposits	533,452	496,452
Non-term deposits	1,969,602	2,032,587
FHLB borrowings – fixed rate		41,175
FHLB borrowings – floating rate	50,000	50,000
Federal Reserve borrowings – fixed rate	12,373	34,536
Accrued interest payable	3,339	3,793
Subordinated Notes | Carrying Amount
Liabilities:
Subordinated notes	52,508	52,340
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimate of Fair Value Measurement
Assets:
Cash and cash equivalents	276,222	254,442
Held-to-maturity securities, net of ACL	247	242
Accrued interest receivable	10,455	11,428
Liabilities:
Term deposits	480,413	414,613
Non-term deposits	1,969,602	2,032,587
Federal Reserve borrowings – fixed rate	2,373	3,539
Accrued interest payable	3,339	3,793
Significant Other Observable Inputs (Level 2) | Estimate of Fair Value Measurement
Assets:
Held-to-maturity securities, net of ACL	62,829	58,231
Liabilities:
FHLB borrowings – fixed rate		41,372
FHLB borrowings – floating rate	49,993	49,986
Federal Reserve borrowings – fixed rate	9,979	30,936
Significant Unobservable Inputs (Level 3) | Estimate of Fair Value Measurement
Assets:
Held-to-maturity securities, net of ACL	7,750	8,144
Total	2,286,056	2,395,468
Liabilities:
Term deposits	54,016	82,564
Significant Unobservable Inputs (Level 3) | Subordinated Notes | Estimate of Fair Value Measurement
Liabilities:
Subordinated notes	$ 47,727	$ 48,228