FAIR VALUE MEASURES - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial assets:
Cash and due from banks	$ 158,453	$ 139,313
Federal funds sold and interest-bearing deposits in banks	470,804	191,345
Time deposits in other banks	11,558	0
Loans, net	8,470,220	5,992,880
Accrued interest receivable	36,846	26,005
Financial liabilities:
Deposits	9,181,363	6,625,845
Securities sold under agreements to repurchase	14,071	30,638
Other borrowings	656,831	250,554
Subordinated deferrable interest debentures	88,986	85,550
FDIC loss-share payable	18,740	8,803
Liability for USPF acquisition contingent consideration	2,514
Accrued interest payable	4,462	3,258
Fair Value
Financial assets:
Cash and due from banks	158,453	139,313
Federal funds sold and interest-bearing deposits in banks	470,804	191,345
Time deposits in other banks	11,558
Loans, net	8,421,188	5,960,963
Accrued interest receivable	36,846	26,005
Financial liabilities:
Deposits	9,175,256	6,627,773
Securities sold under agreements to repurchase	14,071	30,638
Other borrowings	658,197	251,759
Subordinated deferrable interest debentures	85,400	74,243
FDIC loss-share payable	18,901	9,548
Liability for USPF acquisition contingent consideration	2,514
Accrued interest payable	4,462	3,258
Fair Value | Level 1
Financial assets:
Cash and due from banks	158,453	139,313
Federal funds sold and interest-bearing deposits in banks	470,804	191,345
Time deposits in other banks	0
Loans, net	0	0
Accrued interest receivable	0	26,005
Financial liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	14,071	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	0	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	5,498	0
Financial liabilities:
Deposits	9,175,256	6,627,773
Securities sold under agreements to repurchase	0	0
Other borrowings	658,197	251,759
Subordinated deferrable interest debentures	85,400	74,243
FDIC loss-share payable	0	0
Liability for USPF acquisition contingent consideration	0
Accrued interest payable	4,462	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	8,421,188	5,960,963
Accrued interest receivable	31,348	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	18,901	9,548
Liability for USPF acquisition contingent consideration	2,514
Accrued interest payable	$ 0	$ 0