Document And Entity Information Statistical Measurement [Axis] us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500NetAssetAvailableForBenefitAllocationToWithdrawingParticipantNotYetPaid Amounts allocated to withdrawing participants Note To Financial Statement Details Textual Document Fiscal Period Focus EBP, Basis of Accounting [Policy Text Block] EBP, Note Receivable from Participant [Policy Text Block] Significant Accounting Policies Document Fiscal Year Focus Note 5 - Fair Value Measurements istr_EBPAnnualDeferralIncreasePercent EBP, Annual Deferral Increase Percent The annual deferral increase percent. Note 10 - Reconciliation of Financial Statements to Form 5500 Net assets available for benefits: istr_EBPAutomaticEnrollmentMaxPercentagePerEmployee EBP, Automatic Enrollment, Max Percentage Per Employee The maximum percentage of automatic enrollment per employee for employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Schedule H, Line 4i - Schedule of Assets Amendment Flag Note 5 - Fair Value Measurements - Schedule of Plan's Assets (Details) us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionNetAssetAvailableForBenefit Net assets available for benefits per the Form 5500 Note 10 - Reconciliation of Financial Statements to Form 5500 - Reconciliation of Net Assets Available for Benefits (Details) us-gaap-ebp_EmployeeBenefitPlanAssetHeldForInvestmentPartyInInterest EBP, Asset Held for Investment, Party-in-Interest [true false] Schedule H, Line 4i - Schedule of Assets (Details) Schedule H, Line 4i - Schedule of Assets 2 (Details) Minimum [Member] Notes To Financial Statements Notes To Financial Statements [Abstract] dei_DocumentPeriodEndDate Document Period End Date Statistical Measurement [Domain] us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseForContribution Total contributions Maximum [Member] Other us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitTransferToFromExternalPlan EBP, Change in Net Asset Available for Benefit, Transfer to (from) Plan Legal Entity [Axis] EBP, Schedule of Asset Held for Investment [Table] us-gaap-ebp_EmployeeBenefitPlanParticipantLoanMaturityDate EBP, Asset Held for Investment, Participant Loan, Maturity Date Notes receivable from participants dei_EntityTaxIdentificationNumber Entity Tax Identification Number EBP, Investment, Fair Value and NAV [Text Block] Entity [Domain] Entity Central Index Key Net appreciation in fair value of investments dei_EntityRegistrantName Entity Registrant Name Entity Current Reporting Status EBP, Description of Plan [Text Block] us-gaap-ebp_EmployeeBenefitPlanAssetHeldForInvestment EBP, Asset Held for Investment EBP, Related Party and Party-in-Interest Transactions [Table] Current Fiscal Year End Date us-gaap-ebp_EmployeeBenefitPlanTaxDeterminationLetterDate EBP, Tax Determination Letter, Date us-gaap-ebp_EmployeeBenefitPlanAssetHeldForInvestmentParticipantLoanAfterDeemedDistribution EBP, Asset Held for Investment, Participant Loan, after Deemed Distribution Schedule of Assets [Text Block] Disclosure presenting the schedule of assets. Entity Shell Company Entity Emerging Growth Company Entity Small Business Investments [Domain] EBP, Employer, Common Stock [Member] Benefits paid to participants Accounting Policies [Abstract] Rollovers Employer EBP, Risk and Uncertainty [Table] us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseDecreaseForIncomeLossOnInvestment Net change in investments Valuation Approach and Technique [Domain] us-gaap-ebp_EmployeeBenefitPlanTaxQualificationStatusExtensibleEnumeration EBP, Tax Qualification Status [Extensible Enumeration] Valuation, Market Approach [Member] Statement of Stockholders' Equity [Abstract] istr_EBPMinimumAgeForVoluntaryCatchupContributions EBP, Minimum Age for Voluntary Catch-up Contributions The minimum age of participant to make voluntary catch-up contributions to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Non Pooled Accounts [Member] Relating to non-pooled accounts. EBP, Tax Status [Text Block] us-gaap-ebp_EmployeeBenefitPlanForfeitedNonvestedAccountDecreaseForAdministrativeExpense EBP, Forfeited Nonvested Account, Decrease from Administrative Expense istr_EmployeeBenefitPlanTerminationPlanVestingOnTermination EBP, Termination, Plan Vesting on Termination Percentage of award to vest on termination in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). us-gaap-ebp_EmployeeBenefitPlanInvestmentFairAndContractValue Total investments us-gaap-ebp_EmployeeBenefitPlanForfeitedNonvestedAccount EBP, Forfeited Nonvested Account us-gaap-ebp_EmployeeBenefitPlanInvestmentValuationTechniqueExtensibleEnumeration EBP, Investment, Valuation Technique [Extensible Enumeration] us-gaap-ebp_EmployeeBenefitPlanAssetHeldForInvestmentInvestmentExcludingPlanInterestInMasterTrustCurrentValue EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Current Value EBP, Reconciliation of Financial Statement to Form 5500 [Table Text Block] Equity Components [Axis] us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionNetIncomeLoss Net increase in net assets available for benefits per the Form 5500 Class of Stock [Axis] Equity Component [Domain] Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 3 [Member] us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseForDividendIncomeOnInvestment EBP, Change in Net Asset Available for Benefit, Increase from Dividend Income on Investment us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAllowanceForCreditLoss EBP, Note Receivable from Participant, Allowance for Credit Loss Fair Value, Inputs, Level 1 [Member] istr_EmployeeBenefitPlanCreditingInterestRate Employee Benefit Plan, Crediting Interest Rate Represents the crediting interest rate. Net increase in net assets available for benefits per the financial statements Net increase in net assets istr_EBPAwardVestingPercentageAfterSixYearsOfServices EBP, Award Vesting Percentage After Six Years of Services Represents the percentage vested after six years of service. EBP, Subsequent Event [Table] istr_EBPMaximumOfVestedValueOfParticipantsAccountsRequiredToMakeLumpSumCashPayments EBP, Maximum of Vested Value of Participant's Accounts Required to Make Lump Sum Cash Payments Maximum of Vested Value of Participant's Accounts Required to Make Lump Sum Cash Payments. Deductions from net assets attributed to: EBP, Reconciliation of Financial Statement to Form 5500 [Text Block] us-gaap-ebp_EmployeeBenefitPlanParticipantLoanInterestRate EBP, Asset Held for Investment, Participant Loan, Interest Rate Subsequent Event [Member] us-gaap-ebp_EmployeeBenefitPlanForfeitedNonvestedAccountDecreaseForEmployerContribution EBP, Forfeited Nonvested Account, Decrease from Employer Contribution EBP, Participant-Directed [Member] Contributions Subsequent Event Type [Domain] Subsequent Event Type [Axis] Investments Interest and dividends Investments, at fair value Investments, at contract value Investments, at fair value Common stock of Investar Holding Corporation Investments, at contract value us-gaap-ebp_EmployeeBenefitPlanInvestmentExcludingPlanInterestInMasterTrustContractValue Administrative expenses EBP, Change in Net Asset Available for Benefit, Decrease from Administrative Expense Investment, Identifier [Axis] istr_PolicyTable EBP, Accounting Policy [Table] Receivables EBP, Description of Plan [Table] Statement [Table] EBP, Reconciliation of Financial Statement to Form 5500 [Table] EBP, Tax Status [Table] Fair Value Hierarchy and NAV [Domain] us-gaap-ebp_EmployeeBenefitPlanPlanNumber EBP, Plan Number EBP, Risk and Uncertainty [Text Block] Statement of Financial Position [Abstract] us-gaap-ebp_EmployeeBenefitPlanFullyBenefitResponsiveInvestmentContractLimitationOnTransactingAtContractValueNotProbable EBP, Fully Benefit-Responsive Investment Contract, Limitation on Transacting at Contract Value, Not Probable [true false] us-gaap-ebp_EmployeeBenefitPlanFullyBenefitResponsiveInvestmentContractDescription EBP, Fully Benefit-Responsive Investment Contract, Description EBP, Traditional Guaranteed Investment Contract [Member] EBP, Fully Benefit-Responsive Investment Contract [Text Block] us-gaap-ebp_EmployeeBenefitPlanInvestmentTypeExtensibleEnumeration EBP, Investment, Current Value, Type [Extensible Enumeration] EBP, Statement of Net Asset Available for Benefit [Table] EBP, Participant-Directed and Nonparticipant-Directed Status [Domain] us-gaap-ebp_EmployeeBenefitPlanParticipantContributionAutomaticDeferralRate EBP, Participant Contribution, Automatic, Deferral Rate Investment Type [Axis] Mutual Fund [Member] us-gaap-ebp_EmployeeBenefitPlanEmployerContributionMatchingPercentage EBP, Employer Contribution, Matching Percentage us-gaap-ebp_EmployeeBenefitPlanInvestmentNumberOfShares EBP, Investment, Number of Shares (in shares) EBP, Schedule of Asset Held for Investment [Text Block] EBP, Use of Estimate [Policy Text Block] EBP, Subsequent Event [Text Block] us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAccountMaximumBorrowingAmount EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAccountMinimumBorrowingAmount EBP, Note Receivable from Participant, Account, Minimum Borrowing, Amount EBP, Related Party and Party-in-Interest [Domain] EBP, Employer, Common Stock Fund [Member] EBP 27-1560715 001 [Member] Represnts the company employee benefit plan. dei_DocumentType Document Type EBP, Summary of Accounting Policy [Text Block] us-gaap-ebp_EmployeeBenefitPlanInvestmentExcludingPlanInterestInMasterTrustCost EBP, Investment, Excluding Plan Interest in Master Trust, Cost Document Information [Line Items] EBP, Investment [Policy Text Block] Document Information [Table] EBP, Trustee [Member] Amounts allocated to withdrawing participants prior year end Entity Interactive Data Current Statement [Line Items] Fair Value Hierarchy and NAV [Axis] istr_EBPNoteReceivableFromParticipantTerm EBP, Note Receivable From Participant, Term (Year) The term length for the notes receivable from participant. EBP, Related Party and Party-in-Interest [Axis] istr_EBPAgeRequirement EBP Age Requirement The age requirement for the employee benefit plan. EBP, Payment to Participant [Policy Text Block] us-gaap-ebp_EmployeeBenefitPlanParticipantContributionPretaxMaximumAnnualCompensationAmount EBP, Participant Contribution, Pretax, Maximum Annual Compensation, Amount istr_EBPVestingPercentagePerYear EBP, Vesting Percentage Per Year The vesting percent per year for the employee benefit plan. us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAccountMaximumBorrowingToTotalAccountPercentage EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage us-gaap-ebp_EmployeeBenefitPlanEmployerContributionParticipantCompensationMatchedPercentage EBP, Employer Contribution, Participant Compensation Matched, Percentage istr_EBPAwardVestingPeriod EBP, Award Vesting Period (Year) The vesting period for the employee benefit plan. us-gaap-ebp_EmployeeBenefitPlanTaxDeterminationLetterObtained true us-gaap-ebp_EmployeeBenefitPlanAssetHeldForInvestmentInvestmentExcludingPlanInterestInMasterTrustParticipationStatusExtensibleEnumeration EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Participation Status [Extensible Enumeration] us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecrease Total deductions EBP, Administrative Expenses [Text Block] Disclosure of information about the administrative expenses. EBP, Related Party and Party-in-Interest Transactions [Text Block] EBP, Plan Termination [Text Block] us-gaap-ebp_EmployeeBenefitPlanNetAssetAvailableForBenefit Net assets available for benefits Net assets available for benefits per the financial statements Beginning of year End of year EBP, Fully Benefit-Responsive Investment Contract [Table] EBP, Pooled Separate Account [Member] EBP, Investment, Fair Value and NAV [Table Text Block] us-gaap_PolicyTextBlockAbstract Accounting Policies Participants EBP, Investment, Fair Value and NAV [Table] us-gaap_DisclosureTextBlockAbstract Notes to Financial Statements Retirement Plan Tax Status [Domain] us-gaap_TableTextBlock Notes Tables Qualified Plan [Member]