BORROWING ARRANGEMENTS - Narrative (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) credit_line agreement oustanding_letters_of_credit Rate	Dec. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Debt face amount	$ 270,000,000.0
Number of credit lines maintained by the Company | credit_line	3
Federal funds purchased	$ 0	$ 28,000,000.0
FHLB borrowings, unused funds	1,370,000,000
Letters of credit, FHLB, as collateral	0
Securities sold under agreements to repurchase	$ 28,400,000	8,800,000
Maturity of repurchase agreements, less than	1 year
Frost Bank
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity	$ 40,000,000.0
Line of credit facility, capacity available for issuance of letters of credit	$ 5,000,000.0
Number of outstanding letters of credit | oustanding_letters_of_credit	0
TIB - The Independent Bankers Bank
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity	$ 15,000,000.0
Comerica Bank
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity	$ 7,500,000
Minimum
Line of Credit Facility [Line Items]
FHLB borrowings, interest rate at period end	1.35%
Maturity range of FHLB borrowings	2 years
Maximum
Line of Credit Facility [Line Items]
FHLB borrowings, interest rate at period end	4.799%
Maturity range of FHLB borrowings	8 years 6 months
Variable Rate Agreements [Member]
Line of Credit Facility [Line Items]
Debt face amount		$ 310,000,000.0
Three-Month London Interbank Offered Rate (LIBOR) | Variable Rate Agreements [Member]
Line of Credit Facility [Line Items]
Description of variable rate basis	three-month LIBOR
Three-Month London Interbank Offered Rate (LIBOR) | Variable Rate Agreements [Member] | Maximum
Line of Credit Facility [Line Items]
Number of variable rate agreements | agreement	3
One-Month London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Securities sold under agreements to repurchase	$ 20,000,000.0
One-Month London Interbank Offered Rate (LIBOR) | Securities sold under agreements to repurchase
Line of Credit Facility [Line Items]
Description of variable rate basis	one-month LIBOR
One-Month London Interbank Offered Rate (LIBOR) | Variable Rate Agreements [Member]
Line of Credit Facility [Line Items]
Description of variable rate basis	one-month LIBOR
Cash Flow Hedging | Variable Rate Agreements [Member]
Line of Credit Facility [Line Items]
Debt face amount	$ 270,000,000.0
Interest rate swap contracts, average interest rate during the period | Rate	1.58%
Interest rate swap contracts, average weighted maturity	3 years 9 months 18 days