FAIR VALUE MEASURES - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financial assets:
Cash and due from banks	$ 257,026	$ 246,234
Federal funds sold and interest-bearing accounts	494,765	375,615
Time deposits in other banks	249	249
Loans, net	14,563,586	12,736,499
Accrued interest receivable	79,403	52,362
Financial liabilities:
Deposits	16,063,806	14,027,073
Securities sold under agreements to repurchase	9,103	20,635
Other borrowings	875,255	1,398,709
Subordinated deferrable interest debentures	123,860	127,560
FDIC loss-share payable	19,476	19,642
Accrued interest payable	6,443	11,524
Fair Value
Financial assets:
Cash and due from banks	257,026	246,234
Federal funds sold and interest-bearing accounts	494,765	375,615
Time deposits in other banks	249	249
Loans, net	14,657,861	12,806,709
Accrued interest receivable	79,403	52,362
Financial liabilities:
Deposits	16,076,073	14,035,686
Securities sold under agreements to repurchase	9,103	20,635
Other borrowings	877,424	1,402,510
Subordinated deferrable interest debentures	116,193	126,815
FDIC loss-share payable	19,639	19,657
Accrued interest payable	6,443	11,524
Fair Value | Level 1
Financial assets:
Cash and due from banks	257,026	246,234
Federal funds sold and interest-bearing accounts	494,765	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	9,103	20,635
Other borrowings	0	0
Subordinated deferrable interest debentures	0	0
FDIC loss-share payable	0	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	4,485	5,179
Financial liabilities:
Deposits	16,076,073	14,035,686
Securities sold under agreements to repurchase	0	0
Other borrowings	877,424	1,402,510
Subordinated deferrable interest debentures	116,193	126,815
FDIC loss-share payable	0	0
Accrued interest payable	6,443	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	14,657,861	12,806,709
Accrued interest receivable	74,918	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	19,639	19,657
Accrued interest payable	$ 0	$ 0