FAIR VALUE MEASURES - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financial assets:
Cash and due from banks	$ 151,539	$ 139,313
Federal funds sold and interest-bearing deposits in banks	273,170	191,345
Time deposits in other banks	11,558	0
Loans, net	8,424,156	5,992,880
Accrued interest receivable	33,423	26,005
Financial liabilities:
Deposits	8,761,593	6,625,845
Securities sold under agreements to repurchase	11,002	30,638
Other borrowings	862,136	250,554
Subordinated deferrable interest debentures	88,646	85,550
FDIC loss-share payable	18,716	8,803
Liability for USPF acquisition contingent consideration	3,699
Accrued interest payable	4,703	3,258
Fair Value
Financial assets:
Cash and due from banks	151,539	139,313
Federal funds sold and interest-bearing deposits in banks	273,170	191,345
Time deposits in other banks	11,558
Loans, net	8,415,555	5,960,963
Accrued interest receivable	33,423	26,005
Financial liabilities:
Deposits	8,766,345	6,627,773
Securities sold under agreements to repurchase	11,002	30,638
Other borrowings	863,276	251,759
Subordinated deferrable interest debentures	85,249	74,243
FDIC loss-share payable	18,997	9,548
Liability for USPF acquisition contingent consideration	3,699
Accrued interest payable	4,703	3,258
Fair Value | Level 1
Financial assets:
Cash and due from banks	151,539	139,313
Federal funds sold and interest-bearing deposits in banks	273,170	191,345
Time deposits in other banks	0
Loans, net	0	0
Accrued interest receivable	33,423	26,005
Financial liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	11,002	30,638
Other borrowings	0	0
Subordinated deferrable interest debentures	0	0
FDIC loss-share payable	0	0
Liability for USPF acquisition contingent consideration	0
Accrued interest payable	4,703	3,258
Fair Value | Level 2
Financial assets:
Cash and due from banks	0	0
Federal funds sold and interest-bearing deposits in banks	0	0
Time deposits in other banks	11,558
Loans, net	0	0
Accrued interest receivable	0	0
Financial liabilities:
Deposits	8,766,345	6,627,773
Securities sold under agreements to repurchase	0	0
Other borrowings	863,276	251,759
Subordinated deferrable interest debentures	85,249	74,243
FDIC loss-share payable	0	0
Liability for USPF acquisition contingent consideration	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 deposits in banks	0	0
Time deposits in other banks	0
Loans, net	8,415,555	5,960,963
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	18,997	9,548
Liability for USPF acquisition contingent consideration	3,699
Accrued interest payable	$ 0	$ 0