FAIR VALUE - Summary of carrying amounts and estimated fair values for financial instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Assets:
Cash and cash equivalents	$ 257,889	$ 254,442	$ 295,067	$ 196,512
Held-to-maturity securities, net of ACL	64,908	66,617
Total	2,475,524	2,530,915
Accrued interest receivable	11,919	11,428
Liabilities:
FHLB borrowings – fixed rate	69,484	125,711
Subordinated notes	52,397	52,340
Accrued interest payable	2,415	3,793
Carrying Amount
Assets:
Cash and cash equivalents	257,889	254,442
Held-to-maturity securities, net of ACL	72,303	74,102
Total	2,438,972	2,493,258
Accrued interest receivable	11,919	11,428
Liabilities:
Term deposits	442,834	496,452
Non-term deposits	2,089,136	2,032,587
FHLB borrowings – fixed rate	56,314	41,175
FHLB borrowings – floating rate		50,000
Federal Reserve borrowings – fixed rate	13,170	34,536
Accrued interest payable	2,415	3,793
Subordinated Notes | Carrying Amount
Liabilities:
Subordinated notes	52,397	52,340
Quoted Prices in Active Markets for Identical Assets (Level 1)
Liabilities:
Term deposits due within one year	362,600	414,600
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimate of Fair Value Measurement
Assets:
Cash and cash equivalents	257,889	254,442
Held-to-maturity securities, net of ACL	245	242
Accrued interest receivable	11,919	11,428
Liabilities:
Term deposits	362,564	414,613
Non-term deposits	2,089,136	2,032,587
Federal Reserve borrowings – fixed rate	3,171	3,539
Accrued interest payable	2,415	3,793
Significant Other Observable Inputs (Level 2) | Estimate of Fair Value Measurement
Assets:
Held-to-maturity securities, net of ACL	56,332	58,231
Liabilities:
FHLB borrowings – fixed rate	56,340	41,372
FHLB borrowings – floating rate		49,986
Federal Reserve borrowings – fixed rate	9,940	30,936
Significant Unobservable Inputs (Level 3) | Estimate of Fair Value Measurement
Assets:
Held-to-maturity securities, net of ACL	8,331	8,144
Total	2,340,551	2,395,468
Liabilities:
Term deposits	80,249	82,564
Significant Unobservable Inputs (Level 3) | Subordinated Notes | Estimate of Fair Value Measurement
Liabilities:
Subordinated notes	$ 47,441	$ 48,228