Document And Entity Information
us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions
Share-Based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount
Statistical Measurement [Axis]
us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions
Share-Based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount
Note To Financial Statement Details Textual
Significant Accounting Policies
Note 2 - Business Combinations
Income tax expense
Income tax expense
ecd_NonRule10b51ArrTrmntdFlag
Non-Rule 10b5-1 Arrangement Terminated [Flag]
Note 3 - Earnings Per Common Share
ecd_NonRule10b51ArrAdoptedFlag
Non-Rule 10b5-1 Arrangement Adopted [Flag]
Note 4 - Investment Securities
Trading Arrangements, by Individual [Table]
Note 5 - Loans and Allowance for Credit Losses
Material Terms of Trading Arrangement [Text Block]
Note 6 - Goodwill and Other Intangible Assets
Deposit Account [Member]
Note 7 - Stockholders' Equity
Note 8 - Stock-based Compensation
Financial Instrument [Axis]
Investment in Debt and Equity Securities and Other Trading Assets [Text Block]
Note 9 - Derivative Financial Instruments
us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Gross Unrealized Losses
Note 10 - Fair Values of Financial Instruments
Gross Unrealized Gains
us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Minimum [Member]
Note 11 - Income Taxes
us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Gross Unrealized Losses
Note 12 - Commitments and Contingencies
Gross Unrealized Gains
us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Statistical Measurement [Domain]
Note 13 - Leases
Maximum [Member]
Note 2 - Business Combinations - Schedule of Business Combination Information (Details)
Note 2 - Business Combinations - Schedule of PCD Loans Acquired (Details)
Note 3 - Earnings Per Common Share - Earnings Per Share (Details)
us-gaap_BusinessAcquisitionSharePrice
Price per share, based on Investar common stock price as of December 31, 2025 (in dollars per share)
Note 3 - Earnings Per Common Share - Antidilutive Securities (Details)
Note 4 - Investment Securities - Amortized Cost and Fair Value of Investment Securities Classified as AFS (Details)
Stock dividends payable
Carrying value of dividends payable.
Note 4 - Investment Securities - Sales of Investments Classified as AFS (Details)
Note 4 - Investment Securities - Amortized Cost and Fair Value of Investment Securities Classified as HTM (Details)
Schedule of Investments [Table]
Note 4 - Investment Securities - Summary of Continuous Unrealized Loss Position for Securities Classified as AFS (Details)
Note 4 - Investment Securities - Summary of Continuous Unrealized Loss Position for Securities Classified as HTM (Details)
Note 4 - Investment Securities - Contractual Maturity (Details)
Note 5 - Loans and Allowance for Credit Losses - Summary of Loans by Categories (Details)
Note 5 - Loans and Allowance for Credit Losses - Aging Analysis of Loans (Details)
Note 5 - Loans and Allowance for Credit Losses - Schedule of Nonaccrual Loans (Details)
Note 5 - Loans and Allowance for Credit Losses - Loan Portfolio by Category and Credit Quality Indicator (Details)
Note 5 - Loans and Allowance for Credit Losses - Loans to Related Parties (Details)
Note 5 - Loans and Allowance for Credit Losses - Allowance for Loan Losses (Details)
Note 6 - Goodwill and Other Intangible Assets - Schedule of Goodwill (Details)
Note 6 - Goodwill and Other Intangible Assets - Core Deposit Intangible Assets (Details)
Note 6 - Goodwill and Other Intangible Assets - Amortization Schedule for Core Deposit Intangible Assets (Details)
Commercial Mortgage-Backed Securities [Member]
Note 7 - Stockholders' Equity - Accumulated Other Comprehensive Income (Loss) (Details)
Note 8 - Stock-based Compensation - Schedule of Stock Option Activity (Details)
Note 8 - Stock-based Compensation - Schedule of Restricted Stock Activity (Details)
Counterparty Name [Axis]
Note 9 - Derivative Financial Instruments - Schedule of Derivatives Instruments (Details)
Effective tax rate
us-gaap_EffectiveIncomeTaxRateContinuingOperations
Note 9 - Derivative Financial Instruments - Offsetting of Derivative Instruments (Details)
Note 10 - Fair Values of Financial Instruments - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details)
us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent
Note 10 - Fair Values of Financial Instruments - Reconciliation for Assets Measured at Fair Value on Recurring Basis Using significant Unobservable Inputs (Details)
Debt Securities, Available-for-Sale [Table]
Note 10 - Fair Values of Financial Instruments - Quantitative Information About Significant Unobservable Inputs Used in Fair Value Measurement (Details)
Note 10 - Fair Values of Financial Instruments - Estimated Fair Value of Financial Instruments (Details)
Note 11 - Income Taxes - Summary of Provision for Federal Income Taxes (Details)
Exercised, weighted average price (in dollars per share)
Note 12 - Commitments and Contingencies - Commitments to Extend Credit (Details)
Note 12 - Commitments and Contingencies - Summary of ACL on Unfunded Loan Commitments (Details)
Note 13 - Leases - Quantitative Information Regarding Operating Leases (Details)
Note 13 - Leases - Future Minimum Lease Payment (Details)
Notes To Financial Statements
Salaries and employee benefits
Notes To Financial Statements [Abstract]
Income Tax Jurisdiction [Domain]
us-gaap_DividendsCommonStockCash
Common stock dividends declared
Financial Instruments [Domain]
us-gaap_LesseeOperatingLeaseRemainingLeaseTerm
Lessee, Operating Lease, Remaining Lease Term (Year)
Common Stock Dividends per share declared (in dollars per share)
Weighted Average [Member]
us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet
Accrued interest receivable
Counterparty Name [Domain]
Preferred Stock, Dividend Rate
us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions
12 Months or More, Fair Value
us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
us-gaap_IncreaseDecreaseInOtherAccruedLiabilities
Accrued taxes and other liabilities
Less than 12 Months, Fair Value
us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Award Type [Axis]
Adjustments to reconcile net income to net cash provided by operating activities:
Preferred stock, no par value per share; 5,000,000 shares authorized; 6.5% Series A Non-Cumulative Perpetual Convertible Preferred Stock; 32,500 shares ($1,000 liquidation preference) issued and outstanding at June 30, 2026 and December 31, 2025
Plan Name [Axis]
us-gaap_Liabilities
Total liabilities
Plan Name [Domain]
us-gaap_AllocatedShareBasedCompensationExpense
Share-Based Payment Arrangement, Expense
us-gaap_LiabilitiesAndStockholdersEquity
Total liabilities and stockholders’ equity
Pledged Status [Domain]
Title and Position [Axis]
Pledged Status [Axis]
Location, Statement of Financial Position, Balance [Axis]
us-gaap_ShareBasedCompensation
Stock-based compensation
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Additional Shares Authorized (in shares)
Lessee, Lease, Description [Table]
Location, Statement of Income, Balance [Axis]
Share-Based Payment Arrangement [Text Block]
Net change in:
Preferred stock, shares issued (in shares)
us-gaap_FiniteLivedIntangibleAssetUsefulLife
Finite-Lived Intangible Asset, Useful Life (Year)
Preferred stock, shares outstanding (in shares)
Preferred stock, shares authorized (in shares)
Other interest income
istr_DebtSecuritiesAvailableforsaleUnrealizedLossPositionPercentOfSecuritiesCostBasis
Debt Securities, Available-for-Sale, Unrealized Loss Position, Percent of Securities Cost Basis
The percent of securities cost basis for the unrealized loss position of debt securities available for sale.
Portion at Fair Value Measurement [Member]
istr_DebtSecuritiesAvailableforsaleUnrealizedLossPositionPercentOfAmortizedCostOfInvestmentSecuritiesPortfolio
Debt Securities, Available-for-Sale, Unrealized Loss Position, Percent of Amortized Cost of Investment Securities Portfolio
The percent of amortized cost of investment securities portfolio for unrealized loss position of debt securities available for sale.
Reported Value Measurement [Member]
Fair Value, by Balance Sheet Grouping [Table]
us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value
us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect
Net change in cash and cash equivalents
Offsetting Interest Rate Swap [Member]
Related to offsetting interest rate swaps.
us-gaap_CashAndCashEquivalentsFairValueDisclosure
Cash and cash equivalents
Estimate of Fair Value Measurement [Member]
New Accounting Pronouncements, Policy [Policy Text Block]
us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations
Cash and cash equivalents, beginning of period
Cash and cash equivalents, end of period
Share-Based Payment Arrangement, Tranche One [Member]
Statement [Table]
Fair Value, off-Balance-Sheet Risk [Table]
us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Unrealized Losses
Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table Text Block]
Loans, net of allowance
us-gaap_LoansReceivableFairValueDisclosure
us-gaap_InvestmentsFairValueDisclosure
Investments, Fair Value Disclosure
Fair Value
us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition
Provision for other real estate owned
Represents the provision for other real estate owned.
Goodwill and other intangible assets, net
Intangible Asset, Including Goodwill, after Accumulated Amortization
Interest rate swaps - gross liabilities
us-gaap_DerivativeLiabilities
us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions
Financing Receivable, Allowance for Credit Loss [Table Text Block]
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Balance, shares (in shares)
Balance, shares (in shares)
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
Balance, beginning of period, weighted average grant date fair value (in dollars per share)
Balance, end of period, weighted average grant date fair value (in dollars per share)
Financing Receivable, Past Due [Table Text Block]
Granted, shares (in shares)
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Granted, weighted average grant date fair value (in dollars per share)
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
Forfeited, shares (in shares)
Financing Receivable Credit Quality Indicators [Table Text Block]
Product and Service [Domain]
Product and Service [Axis]
Earned and issued, weighted average grant date fair value (in dollars per share)
Change in the fair value of equity securities
Change in the fair value of equity securities
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
Earned and issued, shares (in shares)
us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue
Total lease payments
Residential Mortgage-Backed Securities [Member]
Weighted-average discount rate
Accrued taxes and other liabilities
us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent
Commercial Real Estate [Member]
2027
us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths
Interest and fees on loans
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Authorized (in shares)
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Available for Grant (in shares)
istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss1
Less than 12 months, unrealized losses
Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for less than 12 months.
Less than 12 months, fair value
istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPositionLessThan12MonthsFairValue1
Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for less than 12 months.
Remainder of 2026
us-gaap_LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear
Commitments to Extend Credit [Member]
istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss1
12 months or more, unrealized losses
Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for 12 months or longer.
Exercisable, end of period, shares (in shares)
Lessee, Operating Lease, Liability, to be Paid, Maturity [Table Text Block]
12 months or more, fair value
istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPosition12MonthsOrLongerFairValue1
Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for 12 months or longer.
Exercisable, end of period, weighted average price (in dollars per share)
Assets [Abstract]
Measurement Frequency [Axis]
2029
us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Outstanding, beginning of period, shares (in shares)
Outstanding, end of period, shares (in shares)
Commitments and Contingencies Disclosure [Text Block]
2028
us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
Outstanding, beginning of period, weighted average price (in dollars per share)
Outstanding, end of period, weighted average price (in dollars per share)
Statement [Line Items]
Fair Value Hierarchy and NAV [Axis]
2030
us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour
Common stock, shares issued (in shares)
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]
Common stock, shares outstanding (in shares)
Award Type [Domain]
Common stock, shares authorized (in shares)
istr_DebtSecuritiesHeldtomaturityUnrealizedLossPositionAccumulatedLoss1
Total, unrealized losses
Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.
Total, fair value
istr_DebtSecuritiesHeldtomaturityUnrealizedLossPositionFairValue1
Fair value of investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.
us-gaap_OperatingLeaseLeaseIncome
Operating Lease, Lease Income
istr_DebtSecuritiesHeldtomaturityFairValueDisclosure
Investment securities - HTM
Fair value of investment in debt security measured at amortized cost (held-to-maturity).
New loans/changes in relationship
Greater Than Ninety Days and Accruing [Member]
Relating to greater than 90 days and accruing.
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]
Schedule of Share-Based Compensation Arrangements by Share-Based Payment Award [Table]
Common stock, $1.00 par value per share; 40,000,000 shares authorized; 13,777,385 and 9,798,948 shares issued and outstanding at June 30, 2026 and December 31, 2025, respectively
us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo
2028
us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree
2029
us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths
2027
Repurchase agreements
Advanced proceeds from preferred stock offering
us-gaap_PolicyTextBlockAbstract
Accounting Policies
us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear
Remainder of 2026
Financing Receivable, Credit Ratio [Table]
us-gaap_DisclosureTextBlockAbstract
Notes to Financial Statements
us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour
2030
us-gaap_TableTextBlock
Notes Tables
us-gaap_ComprehensiveIncomeNetOfTax
Total comprehensive income
Proceeds from long-term FHLB advances
Related Party [Member]
Significant Accounting Policies [Text Block]
Proceeds from maturities, prepayments and calls of investment securities AFS
Proceeds from maturities, prepayments and calls of investment securities HTM
us-gaap_LesseeOperatingLeaseLiabilityUndiscountedExcessAmount
Less: imputed interest
Total lease obligations
Operating Lease, Liability
Schedule of Related Party Transactions [Table Text Block]
istr_GainLossOnSalesOfForeclosedAssets
Loss (gain) on sale of other real estate owned, net
The net gain (loss) resulting from sales and other disposals of assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceeding or defeasance; excludes write-downs.
Construction Loans [Member]
istr_FederalHomeLoanBankStockDividends
FHLB stock dividend
Dividends received on federal home loan bank stock.
Deposits
us-gaap_DepositsFairValueDisclosure
Distributions from other investments
Amount of distribution received from investments.
Net (accretion) amortization of purchase accounting adjustments
The expense charged against earnings for the periodic recognition of purchase accounting adjustments.
us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Balance
Balance
Capitalization, Equity [Table]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table]
Subordinated debt
us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations
Income Tax Jurisdiction, Domestic Federal [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table]
Standby Letters of Credit [Member]
istr_DerivativeCollateralPost
Derivative, Collateral Post
Amount of collateral post for derivative activities as of balance sheet date.
Intangible Asset, Finite-Lived, Class [Axis]
Statement of Stockholders' Equity [Abstract]
Proceeds from sales of other real estate owned
Statement of Comprehensive Income [Abstract]
Loss Contingency Nature [Axis]
Equity securities at fair value
us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent
Equity Securities, FV-NI
Intangible Asset, Finite-Lived, Class [Domain]
Non-employee Directors and Certain Officers [Member]
Represents non-employee directors and certain officers.
istr_OtherRealEstateOwnedWriteDowns
Other Real Estate Owned, Write Downs
Represents the amount of write downs on other real estate owned during the period.
Statement of Cash Flows [Abstract]
Financing Receivable Portfolio Segment [Axis]
Debt Securities, Held-to-Maturity, Unrealized Loss Position, Fair Value [Table Text Block]
Tabular disclosure of fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (held-to-maturity), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.
W F B [Member]
Represents WFB.
Customers [Member]
Represents customers.
Off-Balance-Sheet, Allowance for Credit Loss [Table Text Block]
Tabular disclosure of of expected credit loss for credit exposure on off-balance-sheet commitment, including but not limited to, loan commitment, standby letter of credit, financial guarantee not accounted for as insurance. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.
Other Financial Institutions [Member]
Represents other financial institutions.
Other comprehensive income (loss), net
Total other comprehensive (loss) income
STOCKHOLDERS’ EQUITY
Core Deposits [Member]
Equity Components [Axis]
Unfunded Loan Commitment [Member]
us-gaap_PaymentsOfStockIssuanceCosts
Payments of stock issuance costs
Asset Not Pledged as Collateral and Asset Pledged as Collateral without Right [Member]
Financing Receivable Portfolio Segment [Domain]
Proceeds from stock options exercised
Equity Component [Domain]
Fair Value, Inputs, Level 2 [Member]
Fair Value, Inputs, Level 3 [Member]
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member]
us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedLiability
Total, offset
Equity [Text Block]
AOCI Attributable to Parent [Member]
Fair Value, Inputs, Level 1 [Member]
ALABAMA
Additional Paid-in Capital [Member]
Corporate Debt Securities [Member]
Retained Earnings [Member]
us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedGross
Total
Repurchase agreements
us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseGross
Loss Contingency, Nature [Domain]
Financial Instrument Performance Status [Domain]
Bank premises and equipment, net of accumulated depreciation of $25,268 and $23,836, respectively
Subordinated debt, net of unamortized issuance costs
Financial Asset, Aging [Axis]
Financial Instrument Performance Status [Axis]
Repurchase agreements, offset
LOUISIANA
Financing Receivable, Nonaccrual [Table Text Block]
Loans
Loans
us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedGross
Total
Deferred tax asset
Financial Asset, Aging [Domain]
istr_LoanParticipationsAndWholeLoansSoldToAndServicedForOthersNotOnBalanceSheet
Loan Participations and Whole Loans Sold to and Serviced for Others Not on Balance Sheet
Principal balance on loan participations and whole loans sold and serviced for others that is excluded from the balance sheet.
istr_LoanParticipationsAndWholeLoansSoldToAndServicedForOthersUnpaidPrincipalBalance
Loan Participations and Whole Loans Sold to and Serviced For Others Unpaid Principal Balance
Unpaid principal balance on loan participations and whole loans sold and serviced for others, including the portion that was sold and not reflected on the balance sheet.
Reversal of credit losses
Provision for (reversal of) credit losses
Provision for Loan, Lease, and Other Losses
us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAsset
Total, offset
Amortized Cost
Held to maturity securities at amortized cost (fair value of $49,450 and $50,540, respectively)
Financial Asset, 30 to 59 Days Past Due [Member]
Amortized Cost
Available for sale, Amortized cost
Total debt securities, AFS, amortized cost
TEXAS
Real Estate Portfolio Segment [Member]
Portfolio segment of the company's total financing receivables related to real estate.
1-4 Family [Member]
Represents the 1-4 family real estate loans.
Intangible Asset and Goodwill [Text Block]
Multifamily Loans [Member]
Represents multifamily loans.
Farmland [Member]
Represents farmland loans.
Commercial Real Estate Loan [Member]
Represents the commercial real estate loan.
us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest
Less: allowance for credit losses
Balance, beginning of period
Balance, end of period
Financial Asset, 60 to 89 Days Past Due [Member]
Share-Based Payment Arrangement, Restricted Stock Unit, Activity [Table Text Block]
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Share-Based Payment Arrangement, Option, Activity [Table Text Block]
Year five
us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYear
Prior
us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYear
Year three
us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYear
Year four
us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYear
Fair Value Disclosures [Text Block]
Current year
us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYear
Property Insurance Settlement [Member]
Relating to property insurance settlement.
Fair Value, by Balance Sheet Grouping [Table Text Block]
Year two
us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear
Share-Based Payment Arrangement, Option [Member]
Offsetting Assets and Liabilities [Table Text Block]
Tabular disclosure of derivative and other financial assets and liabilities that are subject to offsetting, including master netting arrangements.
Revolving
us-gaap_FinancingReceivableExcludingAccruedInterestRevolving
FHLB short-term advances and repurchase agreements
FHLB short-term advances and repurchase agreements
us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments
Net accretion of securities
Preferred Stock, Liquidation Preference Per Share (in dollars per share)
istr_FinancingReceivableImpairedDuringPeriodAllowanceForCreditLossesIndividuallyEvaluatedForImpairment
Financing Receivable, Impaired During Period, Allowance For Credit Losses, Individually Evaluated For Impairment
Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment during the period.
us-gaap_PaymentsToAcquireEquitySecuritiesFvNi
Purchases of equity securities at fair value
Proceeds from redemption or sale of equity securities at fair value
Lease, Cost [Table Text Block]
Marketing
Cash flows from financing activities:
Restricted Stock and Restricted Stock Units [Member]
Represents restricted stock and restricted stock units.
Derivative Instruments and Hedging Activities Disclosures [Table]
Junior subordinated debt
us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
Other Commitments [Table Text Block]
Other Commitments [Table]
Loss on sale or disposition of bank premises and equipment, net
Loss on sale or disposition of bank premises and equipment, net
Bank premises and equipment, accumulated depreciation
Accumulated other comprehensive loss
us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
Accumulated amortization
us-gaap_DerivativeCollateralObligationToReturnSecurities
Interest rate swaps, not offset by financial instruments
us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired
Cash acquired from acquisition of Wichita Falls Bancshares, Inc., net of cash paid
us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimSecurities
Repurchase agreements, not offset by financial instruments
us-gaap_FinancingReceivableDeferredIncome
Financing Receivable, Deferred Commitment Fee
us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimCash
Repurchase agreements, not offset, cash collateral
us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest
Income before income tax expense
us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedCollateralObligationToReturnCash
Total, not offset, cash collateral
Data processing
us-gaap_DerivativeCollateralRightToReclaimSecurities
Interest rate swaps, not offset by financial instruments
istr_IntangibleAssetIncludingGoodwillAccumulatedImpairmentLoss
Intangible Asset, Including Goodwill, Accumulated Impairment Loss
Amount of accumulated impairment loss on intangible asset, including goodwill.
us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedCollateralObligationToReturnSecurities
Total, not offset by financial instruments
Surplus
us-gaap_PaymentsToAcquireOtherProductiveAssets
Purchases of bank premises and equipment
us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimSecurities
Total, not offset by financial instruments
Segment Reporting, Policy [Policy Text Block]
us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimCash
Total, not offset, cash collateral
Use of Estimates, Policy [Policy Text Block]
us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Period (Year)
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1
Share-Based Compensation Arrangement by Share-Based Payment Award, Award Requisite Service Period (Year)
Lessee, Operating Leases [Text Block]
Stock-based compensation
us-gaap_PaymentsToAcquireOtherInvestments
Purchases of other investments
Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table]
us-gaap_StockRepurchasedAndRetiredDuringPeriodValue
Shares repurchased
Other Real Estate Owned, Fair Value Disclosure
istr_OtherRealEstateOwnedFairValueDisclosure
Other Real Estate Owned, Fair Value Disclosure
The fair value amount of other real estate owned.
us-gaap_NoninterestExpense
Total noninterest expense
Impaired loans, measurement input
istr_ImpairedFinancingReceivableMeasurementInput
Value of input used to measure impaired financing receivable.
NONINTEREST EXPENSE
Common stock issued in acquisition of Wichita Falls Bancshares, Inc., net of issuance costs
Options exercised
Measurement Input, Collateral Discounts and Estimated Costs to Sell [Member]
Represents the measurement input as collateral discounts and estimated costs to sell.
Financial Asset, Not Past Due [Member]
Measurement Input, Collateral Discounts and Discount Rates [Member]
Represents the measurement input as collateral discounts and discount rates.
Goodwill [Table]
us-gaap_OffBalanceSheetCreditLossLiability
Balance, beginning of period
Balance, end of period
Restricted Stock Units (RSUs) [Member]
Fair Value Measurement Inputs and Valuation Techniques [Table Text Block]
us-gaap_ReclassificationFromAociCurrentPeriodNetOfTaxAttributableToParent
Reclassification of realized gain, net
us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
Exercised, shares (in shares)
Fair Value Measurement Inputs and Valuation Techniques [Table]
Unrealized loss, net
Related and Nonrelated Parties [Axis]
Class of Stock [Domain]
istr_CommitmentToFundInvestmentInSBICQualifiedFunds
Commitment to Fund Investment in SBIC Qualified Funds
Unfunded commitments to investment in Small Business Investment Company qualified funds.
Commitments to extend credit
us-gaap_UnusedCommitmentsToExtendCredit
Dilutive effect of Series A Preferred Stock (in shares)
us-gaap_StockOptionPlanExpense
Stock or Unit Option Plan Expense
Cash flows from investing activities:
us-gaap_NetCashProvidedByUsedInOperatingActivities
Net cash provided by operating activities
Thereafter
istr_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFour
Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid after fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).
us-gaap_NetCashProvidedByUsedInInvestingActivities
Net cash provided by investing activities
istr_PolicyTable
Intangible Asset, Finite-Lived, and Capitalized Cost, Software to be Sold, Leased, or Marketed, Estimated Amortization Expense [Table Text Block]
us-gaap_NetCashProvidedByUsedInFinancingActivities
Net cash (used in) provided by financing activities
Advances from Federal Home Loan Bank
us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt
Purchases of securities AFS
Proceeds from sales
Proceeds from sales of investment securities AFS
Proceeds from Sale of Debt Securities, Available-for-Sale
Total assets
us-gaap_AssetsFairValueDisclosure
Fair Value Measured at Net Asset Value Per Share [Member]
Loans, Notes, Trade and Other Receivables Disclosure [Text Block]
us-gaap_GoodwillImpairmentLoss
Goodwill, Impairment Loss
Net decrease in customer deposits
US Government Corporations and Agencies Securities [Member]
Accounting Policies [Abstract]
US States and Political Subdivisions Debt Securities [Member]
us-gaap_FinancingReceivableExcludingAccruedInterestAfterAllowanceForCreditLoss
Loans, net
Measurement Input, Appraised Value [Member]
Measurement Input Type [Axis]
Income Statement [Abstract]
(Reversal of) provision for credit losses on unfunded loan commitments
Fair Value
Available for sale securities at fair value (amortized cost of $459,375 and $416,002, respectively)
Total debt securities, AFS, fair value
Investment securities - AFS
us-gaap_NumberOfReportableSegments
Number of Reportable Segments
us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestAfterAllowanceForCreditLoss
Total debt securities, HTM, amortized cost
Deferred taxes
Net increase in repurchase agreements
Schedule of Available-for-Sale Securities Reconciliation [Table Text Block]
Net income
Net income (loss)
Net income
us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Net income available to common shareholders
Other real estate owned, net
Amortization of subordinated debt issuance costs
us-gaap_AmortizationOfIntangibleAssets
Intangible Asset, Finite-Lived, Amortization Expense
us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts
Common stock issued in acquisition of Wichita Falls Bancshares, Inc., net of issuance costs
Valuation Technique, Discounted Cash Flow [Member]
Debt Securities, Held-to-Maturity, Past Due [Table]
Measurement Input Type [Domain]
us-gaap_InterestIncomeExpenseNet
Net interest income
us-gaap_LoansAndLeasesReceivableRelatedPartiesProceeds
Repayments/changes in relationship
Income Tax Disclosure [Text Block]
Other Real Estate Owned, Measurement Input
us-gaap_OtherRealEstateOwnedMeasurementInput
Accounting Standards Update and Change in Accounting Principle [Table]
PCD loans, fair value
Occupancy
Other operating expenses
Related and Nonrelated Parties [Domain]
PCD loans, principal balance acquired
us-gaap_FinancingReceivablePurchasedWithCreditDeteriorationDiscountPremium
PCD loans, non-credit premium/(discount)
us-gaap_LifeInsuranceCorporateOrBankOwnedChangeInValue
Net change in value of BOLI
Related Party Transaction [Table]
Income from bank owned life insurance
istr_BusinessCombinationRecognizedLiabilityAssumedNotesPayable
Notes payable
Amount of notes payable assumed in business combination and recognized at acquisition date.
istr_BusinessCombinationRecognizedAssetAcquiredNonmarketableEquitySecurities
Nonmarketable equity securities
Amount of nonmarketable equity securities acquired in business combination and recognized at acquisition date.
us-gaap_NoninterestIncome
Total noninterest income
istr_BusinessCombinationRecognizedLiabilityAssumedAdvanceFromFederalHomeLoanBank
Repurchase agreements
Amount of federal home loan bank assumed in business combination and recognized at acquisition date.
NONINTEREST INCOME
us-gaap_PaymentsForProceedsFromLoansReceivable
Net decrease in loans
istr_BusinessCombinationRecognizedAssetAcquiredBankOwnedLifeInsurance
BOLI
Amount of bank owned life insurance acquired in business combination and recognized at acquisition date.
PSL loans, fair value
Amount at purchase price of financing receivable purchased seasoned loans.
Total, principal balance acquired
Amount of gross contractual receivable purchased, acquired in business combination.
istr_FinancingReceivablePurchasedSeasonedLoansDiscountPremium
PSL loans, non-credit premium/(discount)
Amount of discount (premium) attributable to other factors on financing receivable purchased seasoned loans.
Measurement Basis [Axis]
us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1
Intangible Asset, Finite-Lived, Remaining Amortization Period (Year)
ACL on PSL loans at acquisition
PSL loans, ACL
Amount of allowance for credit loss at acquisition date of financing receivable purchased seasoned loans.
Schedule of Receivables Acquired [Table Text Block]
Tabular disclosure of receivables acquired during the period.
PSL loans, principal balance acquired
Amount at par value of financing receivable purchased seasoned loans.
Gain (loss) on sale of other real estate owned, net
istr_BusinessCombinationRecognizedLiabilityAssumedOtherBorrowings
Other borrowings
Amount of other borrowings assumed in business combination and recognized at acquisition date.
Other operating income
us-gaap_NoninterestIncomeOtherOperatingIncome
Fair Value Hierarchy and NAV [Domain]
Total, fair value
Fair value of receivable purchased, acquired in business combination.
Gain on sale of loans
Gain on sale of loans
Measurement Frequency [Domain]
Transfer from loans to other real estate owned
istr_BusinessCombinationAcquiredReceivablePcdAndNonpcdDiscount
Total, non-credit premium/(discount)
Discount of receivable purchased, acquired in business combination.
istr_BusinessCombinationAcquiredReceivablePcdAndPslAllowanceForCreditLossAtAcquisitionDate
Total, ACL
Amount of allowance for credit loss at acquisition date of financing receivable.
Vesting [Axis]
istr_BusinessCombinationConsiderationTransferredEquityInterestFairValueOfSharesIssued
Fair value of Investar common stock issued
The fair value of the shares issued for the business combination.
Statement of Financial Position [Abstract]
Vesting [Domain]
istr_RepaymentsOfLongtermDebtExcludingThanFhlbAdvances
Repayment of long-term debt
The amount of repayments for the long-term debt excluding FHLB advances.
ACL on unfunded loan commitments at acquisition
Amount of allowance for credit loss on off-balance-sheet commitment, including but not limited to, loan commitment, standby letter of credit, financial guarantee not accounted for as insurance. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.
istr_IntangibleAssetFinitelivedEstimatedAmortizationExpenseAfterYearFour
Thereafter
Amount of amortization expense for finite-lived intangible asset expected to be recognized after fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). Excludes goodwill and capitalized cost for software to be sold, leased, or marketed.
us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings
istr_IncreaseDecreaseInShorttermFhlbAdvances
Net increase in short-term FHLB advances
Increase (Decrease) in short-term FHLB advances.
Provision for (reversal of) credit losses on loans
Financing Receivable, Excluding Accrued Interest, Credit Loss Expense (Reversal)
us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
Unrealized gain included in other comprehensive loss
us-gaap_DividendsPreferredStock
Preferred stock dividends declared
Recoveries
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Recovery
istr_BusinessCombinationRecognizedLiabilityAssumedDeposits
Deposits
The amount of deposits assumed in the business combination.
Other assets
us-gaap_OtherAssets
Current-period gross charge-offs, Total
us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff
Charge-offs
istr_BusinessCombinationRecognizedAssetAcquiredReceivableDebt
Net loans
Amount of debt assumed in the business combination.
FHLB long-term advances
istr_FederalHomeLoanBankLongTermBorrowingsFairValueDisclosure
Fair value portion of long-term borrowing from Federal Home Loan Bank (FHLBank).
Fair Value, Nonrecurring [Member]
Professional fees
us-gaap_PaymentsForRepurchaseOfCommonStock
Payments to repurchase common stock
us-gaap_IncreaseDecreaseInOtherOperatingAssets
Other assets
Document Type
Due after five years through ten years, HTM, amortized cost
Due after ten years, HTM, amortized cost
Document Information [Line Items]
Document Quarterly Report
Due within one year, HTM, amortized cost
Due after one year through five years, HTM, amortized cost
ACL on PCD loans at acquisition
PCD loans, ACL
Weighted-average remaining lease term (in years) (Year)
Document Information [Table]
Antidilutive Securities (in shares)
Entity Interactive Data Current
us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
Business Combination, Consideration Transferred, Equity Interest, Share Issued, Number of Shares (in shares)
us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod
Share-Based Compensation Arrangement by Share-Based Payment Award, Expiration Period (Year)
Bank owned life insurance
Document Transition Report
us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Weighted average diluted shares outstanding (in shares)
EARNINGS PER COMMON SHARE
Dilutive effect of stock compensation (in shares)
Weighted average basic shares outstanding (in shares)
us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
us-gaap_NotesReceivableNet
Financing Receivable, after Allowance for Credit Loss
Derivative Instrument [Axis]
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block]
Basic earnings per common share (in dollars per share)
Basic earnings per common share (in dollars per share)
Antidilutive Security, Excluded EPS Calculation [Table]
Earnings Per Share, Basic, by Common Class, Including Two-Class Method [Table]
us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage
Nonaccrual with no allowance
Earnings Per Share, Diluted, by Common Class, Including Two-Class Method [Table]
Nonaccrual loans
Effective Income Tax Rate Reconciliation [Table]
Title and Position [Domain]
Interest-bearing
Debt Securities, Available-for-Sale [Table Text Block]
us-gaap_Deposits
Deposits
Total deposits
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Table]
Deposits:
Income Tax Jurisdiction [Axis]
Acquisition of WFB
Goodwill, Acquired During Period
Noninterest-bearing
us-gaap_FiniteLivedIntangibleAssetsNet
Total
Document Fiscal Period Focus
Gross carrying amount
Document Fiscal Year Focus
Depreciation and amortization
Amendment Flag
Common Stock [Member]
Equity Securities, Held In Correspondent Banks [Member]
Represents held in correspondent banks for equity securities.
Financing Receivable, Past Due [Table]
us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount
Financing Receivable, Unamortized Loan Cost (Fee) and Purchase Premium (Discount)
us-gaap_Goodwill
Goodwill
Beginning balance
Ending balance
Gross gains
us-gaap_DebtSecuritiesAvailableForSaleRealizedGain
Gain on call or sale of investment securities, net
Gain on call or sale of investment securities, net
Document Period End Date
INTEREST INCOME
us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Grants in Period, Gross (in shares)
Business Combination, Consideration Transferred, Equity Interest [Table]
Legal Entity [Axis]
us-gaap_IndefiniteLivedTrademarks
Indefinite-Lived Trademarks
us-gaap_InterestAndDividendIncomeOperating
Total interest income
us-gaap_FinancialLiabilitiesFairValueDisclosure
Financial Liabilities Fair Value Disclosure
Consumer Portfolio Segment [Member]
Total operating lease cost
Financing Receivable, Allowance for Credit Loss [Table]
Commercial Portfolio Segment [Member]
Business Combination [Domain]
Loans individually evaluated for impairment
Balance of loans individually evaluated for impairment
Entity File Number
Balance of loans collectively evaluated for impairment
Entity Tax Identification Number
SUPPLEMENTAL DISCLOSURES OF NONCASH INVESTING AND FINANCING ACTIVITIES
Interest rate swaps
Derivative Assets
Business Combination [Axis]
Ending allowance balance for loans collectively evaluated for impairment
Interest rate swaps
Derivative Liabilities
us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Less than 12 Months, Unrealized Losses
us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
12 Months or More, Unrealized Losses
Entity [Domain]
Entity Central Index Key
us-gaap_FinancingReceivableAllowanceForCreditLosses
Financing Receivable, Allowance for Credit Loss
us-gaap_DebtSecurities
Debt Securities
Marketable Securities and Mutual Funds [Member]
Asset obtained to generate income or appreciate in value.
Entity Registrant Name
Doubtful [Member]
Entity Current Reporting Status
Business Combination [Table]
us-gaap_DerivativeGainLossOnDerivativeNet
Derivative, Gain (Loss) on Derivative, Net
us-gaap_DebtSecuritiesAvailableForSaleRealizedLoss
Gross losses
Special Mention [Member]
Substandard [Member]
Entity Incorporation, State or Country Code
Pass [Member]
Current Fiscal Year End Date
us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents
Cash and cash equivalents
us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities
Investment securities
Entity Filer Category
Preferred Stock Dividends per share declared (in dollars per share)
us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
Total assets acquired
Entity Shell Company
us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities
Total liabilities assumed
Entity Emerging Growth Company
Nonperforming Financial Instruments [Member]
Notional
Derivative, Notional Amount
Entity Small Business
Investments [Domain]
Valuation Approach and Technique [Axis]
us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet
Fair value of net assets acquired
Equity securities at fair value
us-gaap_EquityFairValueDisclosure
us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets
Other assets
Interest rate swaps - gross assets
us-gaap_DerivativeAssets
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]
us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment
Bank premises and equipment
us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles
Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Finite-Lived
Preferred stock dividends declared
Nonaccrual with allowance
Amortized cost of financing receivable on nonaccrual status with allowance for credit loss. Excludes net investment in lease.
INTEREST EXPENSE
Interest on borrowings
Interest-bearing balances due from other banks
Derivative Instruments and Hedging Activities Disclosure [Text Block]
Valuation Approach and Technique [Domain]
us-gaap_BusinessCombinationConsiderationTransferred1
Business Combination, Consideration Transferred
Interest on deposits
Entity Address, State or Province
Entity Address, City or Town
Retained earnings
us-gaap_InterestExpenseOperating
Total interest expense
Entity Address, Address Line One
City Area Code
Due after one year through five years, AFS, amortized cost
us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther
Other liabilities
Due after five years through ten years, AFS, amortized cost
Local Phone Number
Due within one year, AFS, amortized cost
Entity Address, Postal Zip Code
Derivative [Table]
Preferred Stock [Member]
Title of 12(b) Security
us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentFinancialAssets
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Assets
Security Exchange Name
Due after five years through ten years, AFS, fair value
Trading Symbol
Due after ten years, AFS, fair value
Due within one year, AFS, fair value
Due after one year through five years, AFS, fair value
Investment securities:
Investment securities, measurement input
us-gaap_DebtSecuritiesAvailableForSaleMeasurementInput
Due after ten years, AFS, amortized cost
Class of Stock [Axis]
Class of Financing Receivable [Axis]
Class of Financing Receivable [Domain]
Entity Common Stock, Shares Outstanding
Interest rate swaps, offset
us-gaap_DerivativeAssetFairValueGrossLiability
Interest rate swaps, offset
us-gaap_DerivativeLiabilityFairValueGrossAsset
Earnings Per Share [Text Block]
Preferred stock, no par value (in dollars per share)
Interest rate swaps, net
us-gaap_DerivativeAssetFairValueAfterOffsetAndDeductionSubjectToMasterNettingArrangement
Unrealized gain (loss) available for sale, tax
Unrealized (loss) gain, available for sale, net of tax (benefit) expense of ($188), $339, ($565) and $1,821, respectively
Internal Credit Assessment [Axis]
us-gaap_DerivativeAssetSecurityPurchasedUnderAgreementToResellSecurityBorrowedAfterOffsetSubjectToMasterNettingArrangement
Total, net amounts in Balance Sheet
us-gaap_DerivativeCollateralRightToReclaimCash
Interest rate swaps, not offset, cash collateral
Debt Securities, Held-to-Maturity [Table]
Acquisition expense
Business Combination, Acquisition-Related Cost, Expense
Debt Securities, Held-to-Maturity [Table Text Block]
Repurchase agreements, net amounts in Balance Sheet
Fair Value
Held to maturity securities, fair value
Total debt securities, HTM, fair value
us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
Maturities, prepayments, and calls
us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss
Net interest income after provision for (reversal of) credit losses
us-gaap_DerivativeCollateralObligationToReturnCash
Interest rate swaps, not offset, cash collateral
Interest rate swaps, net
us-gaap_DerivativeLiabilityFairValueAfterOffsetAndDeductionSubjectToMasterNettingArrangement
us-gaap_DerivativeAssetSecurityPurchasedUnderAgreementToResellSecurityBorrowedAfterOffsetAndDeductionSubjectToMasterNettingArrangement
Total, net
us-gaap_DerivativeLiabilitySecuritySoldUnderAgreementToRepurchaseSecurityLoanedAfterOffsetSubjectToMasterNettingArrangement
Total, net amounts in Balance Sheet
Investments Classified by Contractual Maturity Date [Table Text Block]
Internal Credit Assessment [Domain]
Repurchase agreements, net
Due after five years through ten years, HTM, fair value
Common stock, par value (in dollars per share)
Due after ten years, HTM, fair value
Taxable
Due within one year, HTM, fair value
Due after one year through five years, HTM, fair value
Ending allowance balance for loans individually evaluated for impairment
Basis of Accounting, Policy [Policy Text Block]
Tax-exempt
us-gaap_DerivativeLiabilitySecuritySoldUnderAgreementToRepurchaseSecurityLoanedAfterOffsetAndDeductionSubjectToMasterNettingArrangement
Total, net
Derivative Contract [Domain]
us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax
Reclassification of realized gain, tax
Cash and due from banks
us-gaap_Assets
Assets
Total assets
us-gaap_CashCashEquivalentsAndFederalFundsSold
Cash and cash equivalents
us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
Reclassification of realized gain, available for sale, net of tax expense of $3, $0, $3 and $0, respectively
Financing Receivable, Nonaccrual [Table]
Geographical [Domain]
Geographical [Axis]
Interest on investment securities:
us-gaap_NotesReceivableGross
Financing Receivable, before Allowance for Credit Loss
Balance, beginning of period
Balance, end of period
Diluted earnings per common share (in dollars per share)
Diluted earnings per common share (in dollars per share)
us-gaap_TradingSecurities
Debt Securities, Trading, and Equity Securities, FV-NI
us-gaap_PaymentsToAcquireBusinessesGross
Payments to Acquire Businesses, Gross
Schedule of Derivative Instruments [Table Text Block]
us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation
Surrendered shares
Interest rate swaps - gross liabilities
us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
Investment Type [Axis]
Accounts and Financing Receivables [Table]
Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block]
Interest rate swaps - gross assets
us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
Consolidation, Policy [Policy Text Block]
Accrued interest receivable
Interest Receivable
us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLossStatementOfFinancialPositionExtensibleList
Financing Receivable, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration]
Interest Rate Swap [Member]
Intangible Asset, Finite-Lived [Table Text Block]
Federal Income Tax Note [Table]
Intangible Asset, Finite-Lived [Table]
us-gaap_DebtSecuritiesAvailableForSaleAccruedInterestAfterAllowanceForCreditLossStatementOfFinancialPositionExtensibleList
Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration]
istr_FinancingReceivableImpairedDuringPeriodGross
Financing Receivable, Impaired During Period Gross
Represents the amount of financing receivables impaired during the period.
Business Combination [Table Text Block]
Current-period gross charge-offs, prior
us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff
Current-period gross charge-offs, revolving
us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingWriteoff
Current-period gross charge-offs, year four
us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff
Current-period gross charge-offs, year five
us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff
Current-period gross charge-offs, year two
us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff
Current-period gross charge-offs, year three
us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff
Current-period gross charge-offs, current year
us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYearWriteoff
Business Combination [Text Block]
us-gaap_StockholdersEquity
Total stockholders’ equity
Balance
Balance
Forfeited, weighted average grant date fair value (in dollars per share)
us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLoss
Financing Receivable, Accrued Interest, after Allowance for Credit Loss
Goodwill [Table Text Block]
A 2017 Long Term Incentive Compensation Plan [Member]
Represents a 2017 Long-Term Incentive Compensation Plan.
us-gaap_PaymentsOfDividendsCommonStock
Cash dividends paid on common stock
Key Personnel [Member]
Represents key personnel.
Stock, Class of Stock [Table]
us-gaap_OperatingLeaseRightOfUseAsset
Operating Lease, Right-of-Use Asset
us-gaap_RepaymentsOfFederalHomeLoanBankBorrowings
Repayment of long-term FHLB advances
Employees [Member]
Represents information related to employees.
us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock
Cash dividends paid on preferred stock
us-gaap_NumberOfStores
Number of Stores
us-gaap_DebtSecuritiesAvailableForSaleAccruedInterestAfterAllowanceForCreditLoss
Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss
Credit and Debit Card [Member]
ecd_Rule10b51ArrTrmntdFlag
Rule 10b5-1 Arrangement Terminated [Flag]
Noninterest income
ecd_Rule10b51ArrAdoptedFlag
Rule 10b5-1 Arrangement Adopted [Flag]
Commitments and contingencies (Note 12)