DISCLOSURES ABOUT FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Financial Assets:
Available-for-sale securities	$ 635,333	$ 705,539
Held-to-maturity securities	63,993	13,146
Loans held for investment	3,222,108	3,190,485	$ 3,123,355
Real estate acquired by foreclosure	802	802
Level 1
Financial Assets:
Cash and short term investments	231,251	140,681
Available-for-sale securities	0	0
Held-to-maturity securities	0	0
Other securities	42,962	26,617
Loans held for sale	0	0
Loans held for investment	0	0
Real estate acquired by foreclosure	0	0
Total	274,213	167,298
Financial Liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	0	0
Other borrowed funds	0	0
Subordinated debentures and subordinated notes	0	0
Total	0	0
Level 2
Financial Assets:
Cash and short term investments	0	0
Available-for-sale securities	635,333	705,539
Held-to-maturity securities	63,993	13,146
Other securities	0	0
Loans held for sale	4,992	7,156
Loans held for investment	0	0
Real estate acquired by foreclosure	0	0
Total	704,318	725,841
Financial Liabilities:
Deposits	3,429,588	3,392,144
Securities sold under agreements to repurchase	4,141	5,173
Other borrowed funds	412,026	324,873
Subordinated debentures and subordinated notes	46,170	45,356
Total	3,891,925	3,767,546
Level 3
Financial Assets:
Cash and short term investments	0	0
Available-for-sale securities	0	0
Held-to-maturity securities	0	0
Other securities	0	0
Loans held for sale	0	0
Loans held for investment	3,201,649	3,206,145
Real estate acquired by foreclosure	802	802
Total	3,202,451	3,206,947
Financial Liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	0	0
Other borrowed funds	0	0
Subordinated debentures and subordinated notes	0	0
Total	0	0
Carrying Value
Financial Assets:
Cash and short term investments	231,251	140,681
Available-for-sale securities	635,333	705,539
Held-to-maturity securities	64,530	13,275
Other securities	42,962	26,617
Loans held for sale	4,992	7,156
Loans held for investment	3,187,022	3,190,485
Real estate acquired by foreclosure	802	802
Total	4,166,892	4,084,555
Financial Liabilities:
Deposits	3,426,050	3,397,143
Securities sold under agreements to repurchase	4,141	5,173
Other borrowed funds	412,000	325,000
Subordinated debentures and subordinated notes	48,019	47,737
Total	3,890,210	3,775,053
Fair Value
Financial Assets:
Cash and short term investments	231,251	140,681
Available-for-sale securities	635,333	705,539
Held-to-maturity securities	63,993	13,146
Other securities	42,962	26,617
Loans held for sale	4,992	7,156
Loans held for investment	3,201,649	3,206,145
Real estate acquired by foreclosure	802	802
Total	4,180,982	4,100,086
Financial Liabilities:
Deposits	3,429,588	3,392,144
Securities sold under agreements to repurchase	4,141	5,173
Other borrowed funds	412,026	324,873
Subordinated debentures and subordinated notes	46,170	45,356
Total	$ 3,891,925	$ 3,767,546