FAIR VALUE - Carrying amounts and estimated fair values of other financial instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Financial assets:
Held-to-maturity securities	$ 1,131,615	$ 1,121,816
Carrying Value
Financial assets:
Net loans	9,813,409	9,637,929
Held-to-maturity securities	1,217,472	1,172,174
Financial liabilities:
Deposits	13,352,022	12,928,121
Overnight borrowings	0	53,000
Securities sold under agreement to repurchase, short-term	287,241	304,595
Other Federal Home Loan Bank borrowings	395,122	407,603
Fair Value
Financial assets:
Net loans	9,366,599	9,293,902
Held-to-maturity securities	1,131,615	1,121,816
Financial liabilities:
Deposits	13,326,950	12,907,605
Overnight borrowings	0	53,000
Securities sold under agreement to repurchase, short-term	287,241	304,595
Other Federal Home Loan Bank borrowings	$ 391,311	$ 410,385