FAIR VALUE - Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Reported Value Measurement [Member]
Financial assets:
Net loans	$ 7,233,083	$ 7,355,083
Financial liabilities:
Deposits	12,723,821	11,224,974
Securities sold under agreement to repurchase, short-term	316,763	284,008
Other Federal Home Loan Bank borrowings	2,912	6,658
Subordinated notes payable	3,283	3,303
Subordinated debt held by unconsolidated subsidiary trusts	0	77,320
Estimate of Fair Value Measurement [Member]
Financial assets:
Net loans	7,608,456	7,655,044
Financial liabilities:
Deposits	12,730,674	11,239,628
Securities sold under agreement to repurchase, short-term	316,763	284,008
Other Federal Home Loan Bank borrowings	2,956	6,758
Subordinated notes payable	3,283	3,303
Subordinated debt held by unconsolidated subsidiary trusts	$ 0	$ 77,320