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Borrowing Arrangements Narrative (Details)
9 Months Ended
Sep. 30, 2021
USD ($)
oustanding_letters_of_credit
credit_line
agreement
Rate
Dec. 31, 2020
USD ($)
Line of Credit Facility [Line Items]    
Number of credit lines maintained by the Company | credit_line 3  
Carrying value of investment securities pledged as collateral $ 1,480,000,000 $ 1,560,000,000
Federal funds purchased 0 0
Letters of Credit, Federal Home Loan Bank, as collateral $ 0  
Maturity of repurchase agreements (less than) 1 year  
Securities sold under repurchase agreements $ 20,700,000 23,200,000
Federal Home Loan Bank borrowings, additional funding available 1,220,000,000  
Borrowings with variable interest rate, face amount 605,000,000  
Frost Bank    
Line of Credit Facility [Line Items]    
Line of credit, maximum borrowing capacity 40,000,000  
Line of credit, capacity available for issuance of letters of credit $ 5,000,000  
Number of outstanding letters of credit | oustanding_letters_of_credit 1  
Letters of Credit Outstanding, Amount $ 91,000  
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  
Federal Reserve Discount Window    
Line of Credit Facility [Line Items]    
Additional funding available $ 476,100,000  
Minimum    
Line of Credit Facility [Line Items]    
Federal Home Loan Bank borrowings interest rates 0.13%  
Federal Home Loan Bank borrowings maturities 4 days  
Maximum    
Line of Credit Facility [Line Items]    
Federal Home Loan Bank borrowings interest rates 4.799%  
Federal Home Loan Bank borrowings maturities 6 years 9 months 18 days  
Variable Rate Agreements and Fixed Short-term Pay Agreements    
Line of Credit Facility [Line Items]    
Borrowings with variable interest rate, face amount $ 605,000,000 670,000,000
Three-Month London Interbank Offered Rate (LIBOR) | Variable Rate Agreements and Fixed Short-term Pay Agreements    
Line of Credit Facility [Line Items]    
Description of variable and fixed rate basis three-month LIBOR  
Three-Month London Interbank Offered Rate (LIBOR) | Variable Rate Agreements and Fixed Short-term Pay Agreements | Maximum    
Line of Credit Facility [Line Items]    
Debt Instrument, Number Of Variable Rate Advance Agreements | agreement 6  
One-Month London Interbank Offered Rate (LIBOR) | Variable Rate Agreements and Fixed Short-term Pay Agreements    
Line of Credit Facility [Line Items]    
Description of variable and fixed rate basis one-month LIBOR  
Cash flow hedging | Variable Rate Agreements and Fixed Short-term Pay Agreements    
Line of Credit Facility [Line Items]    
Debt Instrument, Interest Rate During Period | Rate 1.18%  
Derivative, Average Remaining Maturity 3 years 4 months 24 days  
Federal Reserve Discount Window    
Line of Credit Facility [Line Items]    
Other borrowings $ 0 $ 0