BORROWING ARRANGEMENTS - Narrative (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) credit_line agreement Rate	Dec. 31, 2019 USD ($)
Line of Credit Facility [Line Items]
Debit face amount	$ 670,000,000.0
Number of credit lines maintained by the Company | credit_line	3
Federal funds purchased	$ 0	$ 0
FHLB borrowings, unused funds	1,150,000,000
Letters of credit, FHLB, as collateral	0
Securities sold under agreements to repurchase	$ 23,200,000	28,400,000
Maturity of repurchase agreements, less than	1 year
Federal Reserve Discount Window [Member]
Line of Credit Facility [Line Items]
Other borrowings	$ 0
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	1
Letters of Credit Outstanding, Amount	$ 1,000,000.0
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
Federal Reserve Discount Window [Member]
Line of Credit Facility [Line Items]
Additional Funding Available	$ 722,300,000
Minimum
Line of Credit Facility [Line Items]
FHLB borrowings, interest rate at period end	0.10%
Maturity range of FHLB borrowings	4 days
Maximum
Line of Credit Facility [Line Items]
FHLB borrowings, interest rate at period end	4.799%
Maturity range of FHLB borrowings	7 years 6 months
Variable Rate Agreements [Member]
Line of Credit Facility [Line Items]
Debit face amount	$ 670,000,000.0	$ 310,000,000.0
Three-Month London Interbank Offered Rate (LIBOR) | Variable Rate Agreements and Fixed Short-term Pay Agreements [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable and Stated Rate Basis	three-month LIBOR
Three-Month London Interbank Offered Rate (LIBOR) | Variable Rate Agreements and Fixed Short-term Pay Agreements [Member] | Maximum
Line of Credit Facility [Line Items]
Debt Instrument, Number Of Variable Rate Agreements and Fixed Rate Short-term Pay Agreements | agreement	6
One-Month London Interbank Offered Rate (LIBOR) | Variable Rate Agreements and Fixed Short-term Pay Agreements [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable and Stated Rate Basis	one-month LIBOR
Cash Flow Hedging | Variable Rate Agreements and Fixed Short-term Pay Agreements [Member]
Line of Credit Facility [Line Items]
Interest rate swap contracts, average interest rate during the period | Rate	1.12%
Interest rate swap contracts, average weighted maturity	3 years 9 months 18 days