FAIR VALUE MEASURES - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Cash and due from banks	$ 139,313	$ 127,164
Federal funds sold and interest-bearing accounts	191,345	71,221
Loans, net	5,992,880	5,212,153
Accrued interest receivable	26,005	22,278
Financial liabilities:
Deposits	6,625,845	5,575,163
Securities sold under agreements to repurchase	30,638	53,505
Other borrowings	250,554	492,321
Subordinated deferrable interest debentures	85,550	84,228
FDIC loss-share payable	8,803	6,313
Accrued interest payable	3,258	1,501
Fair Value
Financial assets:
Cash and due from banks	139,313	127,164
Federal funds sold and interest-bearing accounts	191,345	71,221
Loans, net	5,960,963	5,236,034
Accrued interest receivable	26,005	22,278
Financial liabilities:
Deposits	6,627,773	5,575,288
Securities sold under agreements to repurchase	30,638	53,505
Other borrowings	251,759	492,321
Subordinated deferrable interest debentures	74,243	67,321
FDIC loss-share payable	9,548	8,243
Accrued interest payable	3,258	1,501
Fair Value | Level 1
Financial assets:
Cash and due from banks	139,313	127,164
Federal funds sold and interest-bearing accounts	191,345	71,221
Loans, net	0	0
Accrued interest receivable	26,005	22,278
Financial liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	30,638	53,505
Other borrowings	0	0
Subordinated deferrable interest debentures	0	0
FDIC loss-share payable	0	0
Accrued interest payable	3,258	1,501
Fair Value | Level 2
Financial assets:
Cash and due from banks	0	0
Federal funds sold and interest-bearing accounts	0	0
Loans, net	0	0
Accrued interest receivable	0	0
Financial liabilities:
Deposits	6,627,773	5,575,288
Securities sold under agreements to repurchase	0	0
Other borrowings	271,759	492,321
Subordinated deferrable interest debentures	74,243	67,321
FDIC loss-share payable	0	0
Accrued interest payable	0	0
Fair Value | Level 3
Financial assets:
Cash and due from banks	0	0
Federal funds sold and interest-bearing accounts	0	0
Loans, net	5,960,963	5,236,034
Accrued interest receivable	0	0
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	9,548	8,243
Accrued interest payable	$ 0	$ 0