BORROWING ARRANGEMENTS - Additional Information (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) contract	Dec. 31, 2018 USD ($) credit_line agreement Oustanding_Letters_of_Credit Rate	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Debt face amount		$ 270,000,000
Number of credit lines maintained by the Company | credit_line		3
Federal funds purchased		$ 28,000,000	$ 0
Number of outstanding letters of credit | Oustanding_Letters_of_Credit		1
Letters of credit outstanding		$ 195,000
FHLB Advances, unused funds		1,170,000,000
Letters of credit, FHLB, as collateral		0
Securities sold under agreements to repurchase		$ 8,800,000	9,500,000
Maturity of repurchase agreements		2 years
Frost Bank
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity		$ 40,000,000
Line of credit facility, capacity available for issuance of letters of credit		5,000,000
TIB - The Independent Bankers Bank
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity		15,000,000
Comerica Bank
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity		$ 7,500,000
Minimum
Line of Credit Facility [Line Items]
Federal home loan bank, advances, general debt obligations, disclosures, interest rate at period end		1.37%
Maximum
Line of Credit Facility [Line Items]
Federal home loan bank, advances, general debt obligations, disclosures, interest rate at period end		4.799%
Maturity range of FHLB advances		9 years 6 months 3 days
Variable Rate Advance Agreements
Line of Credit Facility [Line Items]
Debt face amount		$ 310,000,000	$ 280,000,000
Three-Month London Interbank Offered Rate (LIBOR) | Variable Rate Advance Agreements
Line of Credit Facility [Line Items]
Description of variable rate basis		three-month LIBOR
Three-Month London Interbank Offered Rate (LIBOR) | Variable Rate Advance Agreements | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate | Rate		(0.003%)
Three-Month London Interbank Offered Rate (LIBOR) | Variable Rate Advance Agreements | Maximum
Line of Credit Facility [Line Items]
Number of variable rate advance agreements | agreement		2
Basis spread on variable rate | Rate		0.021%
One-Month London Interbank Offered Rate (LIBOR) | Variable Rate Advance Agreements
Line of Credit Facility [Line Items]
Description of variable rate basis		one-month LIBOR
One-Month London Interbank Offered Rate (LIBOR) | Variable Rate Advance Agreements | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate | Rate		0.072%
One-Month London Interbank Offered Rate (LIBOR) | Variable Rate Advance Agreements | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate | Rate		0.164%
Interest Rate Swaps
Line of Credit Facility [Line Items]
Derivative, instruments terminated | contract	2
Derivative, terminated, notional amount	$ 40,000,000
Interest Rate Swaps | Minimum
Line of Credit Facility [Line Items]
Derivative, fixed interest rate		0.932%
Derivative, term of contract		4 years
Interest Rate Swaps | Maximum
Line of Credit Facility [Line Items]
Derivative, fixed interest rate		2.833%
Derivative, term of contract		10 years