Borrowing Arrangements Narrative (Details)	9 Months Ended
Sep. 30, 2019 USD ($) Oustanding_Letters_of_Credit credit_line agreement Rate	Dec. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Federal Home Loan Bank borrowings with variable interest rate, 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
Federal Home Loan Bank borrowings, additional funding available	1,270,000,000
Letters of Credit, Federal Home Loan Bank, as collateral	0
Securities sold under repurchase agreements	$ 9,600,000	8,800,000
Maturity of repurchase agreements (less than)	10 months
Frost Bank
Line of Credit Facility [Line Items]
Line of credit, maximum borrowing capacity	$ 40,000,000.0
Line of credit, 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]
Federal Home Loan Bank borrowings interest rates	1.37%
Federal Home Loan Bank borrowings maturities	1 day
Maximum
Line of Credit Facility [Line Items]
Federal Home Loan Bank borrowings interest rates	4.799%
Federal Home Loan Bank borrowings maturities	8 years 9 months 18 days
Variable Rate Advance Agreements
Line of Credit Facility [Line Items]
Federal Home Loan Bank borrowings with variable interest rate, face amount	$ 310,000,000.0	$ 310,000,000
Three-Month London Interbank Offered Rate (LIBOR) \| Variable Rate Advance Agreements
Line of Credit Facility [Line Items]
Federal Home Loan Bank borrowings, description of variable rate basis	three-month LIBOR
Three-Month London Interbank Offered Rate (LIBOR) \| Variable Rate Advance Agreements \| Maximum
Line of Credit Facility [Line Items]
Debt Instrument, Number Of Variable Rate Advance Agreements \| agreement	3
Cash flow hedging \| Variable Rate Advance Agreements
Line of Credit Facility [Line Items]
Federal Home Loan Bank borrowings with variable interest rate, face amount	$ 270,000,000.0
Debt Instrument, Interest Rate During Period \| Rate	1.58%
Derivative, Average Remaining Maturity	4 years 1 month 6 days

Debt Instrument, Number Of Variable Rate Advance Agreements

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Debt Instrument, Weighted Average Maturity

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Represents the range of years until maturity from lowest to highest that are applicable to Federal Home Loan Bank outstanding as of the balance sheet date.

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- Definition

The amount of letters of credit outstanding at year end from the Federal Home Loan Bank as collateral for its public fund deposits.

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Line of Credit Facility, Number of Credit Lines Maintained by the Company

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Number of Outstanding Letters of Credit

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Repurchase Agreement Counterparty, Maturity of Agreements

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The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

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Face (par) amount of debt instrument at time of issuance.

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The average effective interest rate during the reporting period.

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Amount of short term borrowing where a bank borrows, at the federal funds rate, from another bank.

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Amount of advances or credit lines available from the Federal Home Loan Bank (FHLB).

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States the interest rate applicable to Federal Home Loan Bank advances outstanding as of the Balance Sheet date.

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The maximum amount of borrowing capacity under a line of credit that is available as of the balance sheet date for a specific purpose other than for financing goods acquired for inventory or imminent delivery to a customer.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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