FAIR VALUE MEASURES - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financial assets:
Cash and due from banks	$ 246,234	$ 172,036
Time deposits in other banks	249	10,812
Financial liabilities:
Other borrowings	1,398,709	151,774
Subordinated deferrable interest debentures	127,560	89,187
FDIC loss-share payable	19,642	19,487
Carrying Amount
Financial assets:
Cash and due from banks	246,234	172,036
Federal funds sold and interest-bearing accounts	375,615	507,491
Time deposits in other banks	249	10,812
Loans, net	12,736,499	8,454,442
Accrued interest receivable	52,362	36,970
Financial liabilities:
Deposits	14,027,073	9,649,313
Securities sold under agreements to repurchase	20,635	20,384
Other borrowings	1,398,709	151,774
Subordinated deferrable interest debentures	127,560	89,187
FDIC loss-share payable	19,642	19,487
Accrued interest payable	11,524	5,669
Fair Value
Financial assets:
Cash and due from banks	246,234	172,036
Federal funds sold and interest-bearing accounts	375,615	507,491
Time deposits in other banks	249	10,812
Loans, net	12,806,709	8,365,293
Accrued interest receivable	52,362	36,970
Financial liabilities:
Deposits	14,035,686	9,645,617
Securities sold under agreements to repurchase	20,635	20,384
Other borrowings	1,402,510	152,873
Subordinated deferrable interest debentures	126,815	90,180
FDIC loss-share payable	19,657	19,576
Accrued interest payable	11,524	5,669
Fair Value | Level 1
Financial assets:
Cash and due from banks	246,234	172,036
Federal funds sold and interest-bearing accounts	375,615	507,491
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	20,635	20,384
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	10,812
Loans, net	0	0
Accrued interest receivable	5,179	5,456
Financial liabilities:
Deposits	14,035,686	9,645,617
Securities sold under agreements to repurchase	0	0
Other borrowings	1,402,510	152,873
Subordinated deferrable interest debentures	126,815	90,180
FDIC loss-share payable	0	0
Accrued interest payable	11,524	5,669
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	12,806,709	8,365,293
Accrued interest receivable	47,183	31,514
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	19,576
Accrued interest payable	$ 0	$ 0