FAIR VALUE MEASURES - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financial assets:
Cash and due from banks	$ 203,349	$ 246,234
Time deposits in other banks	249	249
Financial liabilities:
Other borrowings	425,155	1,398,709
Subordinated deferrable interest debentures	124,345	127,560
FDIC loss-share payable	0	19,642
Carrying Amount
Financial assets:
Cash and due from banks	203,349	246,234
Federal funds sold and interest-bearing accounts	1,913,957	375,615
Time deposits in other banks	249	249
Loans, net	14,183,077	12,736,499
Accrued interest receivable	76,254	52,362
Financial liabilities:
Deposits	16,957,823	14,027,073
Securities sold under agreements to repurchase	11,641	20,635
Other borrowings	425,155	1,398,709
Subordinated deferrable interest debentures	124,345	127,560
FDIC loss-share payable		19,642
Accrued interest payable	5,487	11,524
Fair Value
Financial assets:
Cash and due from banks	203,349	246,234
Federal funds sold and interest-bearing accounts	1,913,957	375,615
Time deposits in other banks	249	249
Loans, net	14,096,711	12,806,709
Accrued interest receivable	76,254	52,362
Financial liabilities:
Deposits	16,968,606	14,035,686
Securities sold under agreements to repurchase	11,641	20,635
Other borrowings	431,783	1,402,510
Subordinated deferrable interest debentures	116,280	126,815
FDIC loss-share payable		19,657
Accrued interest payable	5,487	11,524
Fair Value | Level 1
Financial assets:
Cash and due from banks	203,349	246,234
Federal funds sold and interest-bearing accounts	1,913,957	375,615
Time deposits in other banks	0	0
Loans, net	0	0
Accrued interest receivable	0	0
Financial liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	11,641	20,635
Other borrowings	0	0
Subordinated deferrable interest debentures	0	0
FDIC loss-share payable		0
Accrued interest payable	0	0
Fair Value | Level 2
Financial assets:
Cash and due from banks	0	0
Federal funds sold and interest-bearing accounts	0	0
Time deposits in other banks	249	249
Loans, net	0	0
Accrued interest receivable	3,567	5,179
Financial liabilities:
Deposits	16,968,606	14,035,686
Securities sold under agreements to repurchase	0	0
Other borrowings	431,783	1,402,510
Subordinated deferrable interest debentures	116,280	126,815
FDIC loss-share payable		0
Accrued interest payable	5,487	11,524
Fair Value | Level 3
Financial assets:
Cash and due from banks	0	0
Federal funds sold and interest-bearing accounts	0	0
Time deposits in other banks	0	0
Loans, net	14,096,711	12,806,709
Accrued interest receivable	72,687	47,183
Financial liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	0	0
Other borrowings	0	0
Subordinated deferrable interest debentures	0	0
FDIC loss-share payable		19,657
Accrued interest payable	$ 0	$ 0