FAIR VALUE - Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Carrying Value [Member]
Financial assets:
Net loans	$ 7,313,258	$ 7,355,083
Financial liabilities:
Deposits	12,204,237	11,224,974
Securities sold under agreement to repurchase, short-term	257,767	284,008
Other Federal Home Loan Bank borrowings	5,959	6,658
Subordinated notes payable	3,297	3,303
Subordinated debt held by unconsolidated subsidiary trusts	0	77,320
Fair Value [Member]
Financial assets:
Net loans	7,565,204	7,655,044
Financial liabilities:
Deposits	12,215,473	11,239,628
Securities sold under agreement to repurchase, short-term	257,767	284,008
Other Federal Home Loan Bank borrowings	6,031	6,758
Subordinated notes payable	3,297	3,303
Subordinated debt held by unconsolidated subsidiary trusts	$ 0	$ 77,320