FAIR VALUE - Summary of carrying amounts and estimated fair values for financial instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Assets:
Cash and cash equivalents	$ 271,484	$ 196,512	$ 165,248	$ 386,983
Held-to-maturity securities, net of ACL	66,487	74,718
Total	2,530,459	2,469,413
Accrued interest receivable	11,633	10,445
Liabilities:
FHLB borrowings – fixed rate	259,930	146,886
Subordinated notes	52,279	52,132
Accrued interest payable	3,203	1,125
Loans accounted for under the fair value option	15,464	23,321
Carrying Amount
Assets:
Cash and cash equivalents	271,484	196,512
Held-to-maturity securities, net of ACL	75,539	81,056
Total	2,491,820	2,428,909
Accrued interest receivable	11,633	10,445
Liabilities:
Term deposits	373,459	224,090
Non-term deposits	2,046,537	2,181,139
FHLB borrowings – fixed rate	175,067	141,498
FHLB borrowings – floating rate	50,000
Federal Reserve borrowings – fixed rate	34,863	5,388
Accrued interest payable	3,203	1,125
Subordinated Notes | Carrying Amount
Liabilities:
Subordinated notes	52,279	52,132
Quoted Prices in Active Markets for Identical Assets (Level 1)
Liabilities:
Term deposits due within one year	322,100	181,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimate of Fair Value Measurement
Assets:
Cash and cash equivalents	271,484	196,512
Held-to-maturity securities, net of ACL	239	234
Accrued interest receivable	11,633	10,445
Liabilities:
Term deposits	322,144	181,036
Non-term deposits	2,046,537	2,181,139
Federal Reserve borrowings – fixed rate	3,865	5,388
Accrued interest payable	3,203	1,125
Significant Other Observable Inputs (Level 2) | Estimate of Fair Value Measurement
Assets:
Held-to-maturity securities, net of ACL	58,120	67,433
Liabilities:
FHLB borrowings – fixed rate	175,269	141,867
FHLB borrowings – floating rate	49,978
Federal Reserve borrowings – fixed rate	30,862
Significant Unobservable Inputs (Level 3) | Estimate of Fair Value Measurement
Assets:
Held-to-maturity securities, net of ACL	8,128	7,051
Total	2,383,883	2,356,085
Liabilities:
Term deposits	50,460	43,586
Significant Unobservable Inputs (Level 3) | Subordinated Notes | Estimate of Fair Value Measurement
Liabilities:
Subordinated notes	$ 47,370	$ 60,384