FAIR VALUE - Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Carrying Value [Member]
Financial assets [Abstract]
Net loans	$ 7,393,681	$ 6,840,632
Financial liabilities [Abstract]
Deposits	11,120,590	8,994,967
Overnight Federal Home Loan Bank borrowings	0	8,300
Securities sold under agreement to repurchase, short-term	280,767	241,708
Other Federal Home Loan Bank borrowings	7,681	3,750
Subordinated notes payable	13,735	13,795
Subordinated debt held by unconsolidated subsidiary trusts	77,320	77,320
Fair Value [Member]
Financial assets [Abstract]
Net loans	7,710,461	7,028,663
Financial liabilities [Abstract]
Deposits	11,135,779	8,997,551
Overnight Federal Home Loan Bank borrowings	0	8,300
Securities sold under agreement to repurchase, short-term	280,767	241,708
Other Federal Home Loan Bank borrowings	7,810	3,755
Subordinated notes payable	13,735	13,795
Subordinated debt held by unconsolidated subsidiary trusts	$ 77,320	$ 77,320