DISCLOSURES ABOUT FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Financial Assets:
Available-for-sale securities	$ 664,045	$ 705,539
Held-to-maturity securities	65,037	13,146
Loans held for investment	3,136,336	3,190,485	$ 3,012,275
Real estate acquired by foreclosure	802	802
Level 1
Financial Assets:
Cash and short term investments	142,144	140,681
Available-for-sale securities	0	0
Held-to-maturity securities	0	0
Other securities	26,631	26,617
Loans held for sale	0	0
Loans held for investment	0	0
Real estate acquired by foreclosure	0	0
Total	168,775	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	664,045	705,539
Held-to-maturity securities	65,037	13,146
Other securities	0	0
Loans held for sale	7,461	7,156
Loans held for investment	0	0
Real estate acquired by foreclosure	0	0
Total	736,543	725,841
Financial Liabilities:
Deposits	3,454,086	3,392,144
Securities sold under agreements to repurchase	4,948	5,173
Other borrowed funds	230,160	324,873
Subordinated debentures and subordinated notes	46,189	45,356
Total	3,735,383	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	3,116,497	3,206,145
Real estate acquired by foreclosure	802	802
Total	3,117,299	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	142,144	140,681
Available-for-sale securities	664,045	705,539
Held-to-maturity securities	65,101	13,275
Other securities	26,631	26,617
Loans held for sale	7,461	7,156
Loans held for investment	3,098,103	3,190,485
Real estate acquired by foreclosure	802	802
Total	4,004,287	4,084,555
Financial Liabilities:
Deposits	3,453,727	3,397,143
Securities sold under agreements to repurchase	4,948	5,173
Other borrowed funds	230,000	325,000
Subordinated debentures and subordinated notes	47,878	47,737
Total	3,736,553	3,775,053
Fair Value
Financial Assets:
Cash and short term investments	142,144	140,681
Available-for-sale securities	664,045	705,539
Held-to-maturity securities	65,037	13,146
Other securities	26,631	26,617
Loans held for sale	7,461	7,156
Loans held for investment	3,116,497	3,206,145
Real estate acquired by foreclosure	802	802
Total	4,022,617	4,100,086
Financial Liabilities:
Deposits	3,454,086	3,392,144
Securities sold under agreements to repurchase	4,948	5,173
Other borrowed funds	230,160	324,873
Subordinated debentures and subordinated notes	46,189	45,356
Total	$ 3,735,383	$ 3,767,546