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Disclosure - Award Timing Disclosure Sheet http://xbrl.sec.gov/ecd/role/AwardTimingDisclosure Award Timing Disclosure Notes 10 false false R11.htm 995445 - Disclosure - Insider Trading Arrangements Sheet http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements Insider Trading Arrangements Notes 11 false false R12.htm 995447 - Disclosure - Insider Trading Policies and Procedures Sheet http://xbrl.sec.gov/ecd/role/InsiderTradingPoliciesProc Insider Trading Policies and Procedures Notes 12 false false R13.htm 995550 - Disclosure - Cybersecurity Risk Management and Strategy Disclosure Sheet http://xbrl.sec.gov/cyd/role/CybersecurityRiskManagementAndStrategyDisclosure Cybersecurity Risk Management and Strategy Disclosure Notes 13 false false R14.htm 995552 - Disclosure - Note 1 - Summary of Significant Accounting Policies Sheet http://investors.investarbank.com/20251231/role/statement-note-1-summary-of-significant-accounting-policies Note 1 - Summary of Significant Accounting Policies Notes 14 false false R15.htm 995553 - Disclosure - Note 2 - Investment Securities Sheet http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities Note 2 - Investment Securities Notes 15 false false R16.htm 995554 - Disclosure - Note 3 - Loans and Allowance for Credit Losses Sheet http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses Note 3 - Loans and Allowance for Credit Losses Notes 16 false false R17.htm 995555 - Disclosure - Note 4 - Other Real Estate Owned Sheet http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned Note 4 - Other Real Estate Owned Notes 17 false false R18.htm 995556 - Disclosure - Note 5 - Bank Premises and Equipment Sheet http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment- Note 5 - Bank Premises and Equipment Notes 18 false false R19.htm 995557 - Disclosure - Note 6 - Leases Sheet http://investors.investarbank.com/20251231/role/statement-note-6-leases Note 6 - 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Other Borrowed Funds Sheet http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds- Note 11 - Other Borrowed Funds Notes 24 false false R25.htm 995563 - Disclosure - Note 12 - Derivative Financial Instruments Sheet http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments Note 12 - Derivative Financial Instruments Notes 25 false false R26.htm 995564 - Disclosure - Note 13 - Stockholders' Equity Sheet http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity Note 13 - Stockholders' Equity Notes 26 false false R27.htm 995565 - Disclosure - Note 14 - Stock-Based Compensation Sheet http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation- Note 14 - Stock-Based Compensation Notes 27 false false R28.htm 995566 - Disclosure - Note 15 - Employee Benefits Sheet http://investors.investarbank.com/20251231/role/statement-note-15-employee-benefits Note 15 - Employee Benefits Notes 28 false false R29.htm 995567 - Disclosure - Note 16 - Income Taxes Sheet http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes Note 16 - Income Taxes Notes 29 false false R30.htm 995568 - Disclosure - Note 17 - Fair Values of Financial Instruments Sheet http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments Note 17 - Fair Values of Financial Instruments Notes 30 false false R31.htm 995569 - Disclosure - Note 18 - Regulatory Matters Sheet http://investors.investarbank.com/20251231/role/statement-note-18-regulatory-matters Note 18 - Regulatory Matters Notes 31 false false R32.htm 995570 - Disclosure - Note 19 - Commitments and Contingencies Sheet http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies Note 19 - Commitments and Contingencies Notes 32 false false R33.htm 995571 - Disclosure - Note 20 - Transactions with Related Parties Sheet http://investors.investarbank.com/20251231/role/statement-note-20-transactions-with-related-parties Note 20 - Transactions with Related Parties Notes 33 false false R34.htm 995572 - Disclosure - Note 21 - Parent Company Only Financial Statements Sheet http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements Note 21 - Parent Company Only Financial Statements Notes 34 false false R35.htm 995573 - Disclosure - Note 22 - Earnings Per Common Share Sheet http://investors.investarbank.com/20251231/role/statement-note-22-earnings-per-common-share Note 22 - Earnings Per Common Share Notes 35 false false R36.htm 995574 - Disclosure - Note 23 - Subsequent Events Sheet http://investors.investarbank.com/20251231/role/statement-note-23-subsequent-events- Note 23 - Subsequent Events Notes 36 false false R37.htm 995578 - Disclosure - Significant Accounting Policies (Policies) Sheet http://investors.investarbank.com/20251231/role/statement-significant-accounting-policies-policies Significant Accounting Policies (Policies) Policies http://investors.investarbank.com/20251231/role/statement-note-1-summary-of-significant-accounting-policies 37 false false R38.htm 995579 - Disclosure - Note 2 - Investment Securities (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-tables Note 2 - Investment Securities (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities 38 false false R39.htm 995580 - Disclosure - Note 3 - Loans and Allowance for Credit Losses (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-tables Note 3 - Loans and Allowance for Credit Losses (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses 39 false false R40.htm 995581 - Disclosure - Note 4 - Other Real Estate Owned (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned-tables Note 4 - Other Real Estate Owned (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned 40 false false R41.htm 995582 - Disclosure - Note 5 - Bank Premises and Equipment (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment-tables Note 5 - Bank Premises and Equipment (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment- 41 false false R42.htm 995583 - Disclosure - Note 6 - Leases (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-6-leases-tables Note 6 - Leases (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-6-leases 42 false false R43.htm 995584 - Disclosure - Note 7 - Goodwill and Other Intangible Assets (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-tables Note 7 - Goodwill and Other Intangible Assets (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets- 43 false false R44.htm 995585 - Disclosure - Note 8 - Deposits (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-8-deposits-tables Note 8 - Deposits (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-8-deposits- 44 false false R45.htm 995586 - Disclosure - Note 11 - Other Borrowed Funds (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds-tables Note 11 - Other Borrowed Funds (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds- 45 false false R46.htm 995587 - Disclosure - Note 12 - Derivative Financial Instruments (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-tables Note 12 - Derivative Financial Instruments (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments 46 false false R47.htm 995588 - Disclosure - Note 13 - Stockholders' Equity (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity-tables Note 13 - Stockholders' Equity (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity 47 false false R48.htm 995589 - Disclosure - Note 14 - Stock-Based Compensation (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-tables Note 14 - Stock-Based Compensation (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation- 48 false false R49.htm 995590 - Disclosure - Note 16 - Income Taxes (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-tables Note 16 - Income Taxes (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes 49 false false R50.htm 995591 - Disclosure - Note 17 - Fair Values of Financial Instruments (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-tables Note 17 - Fair Values of Financial Instruments (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments 50 false false R51.htm 995592 - Disclosure - Note 18 - Regulatory Matters (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-18-regulatory-matters-tables Note 18 - Regulatory Matters (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-18-regulatory-matters 51 false false R52.htm 995593 - Disclosure - Note 19 - Commitments and Contingencies (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies-tables Note 19 - Commitments and Contingencies (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies 52 false false R53.htm 995594 - Disclosure - Note 21 - Parent Company Only Financial Statements (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements-tables Note 21 - Parent Company Only Financial Statements (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements 53 false false R54.htm 995595 - Disclosure - Note 22 - Earnings Per Common Share (Tables) Sheet http://investors.investarbank.com/20251231/role/statement-note-22-earnings-per-common-share-tables Note 22 - Earnings Per Common Share (Tables) Tables http://investors.investarbank.com/20251231/role/statement-note-22-earnings-per-common-share 54 false false R55.htm 995596 - Disclosure - Note 1 - Summary of Significant Accounting Policies (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-1-summary-of-significant-accounting-policies-details-textual Note 1 - Summary of Significant Accounting Policies (Details Textual) Details 55 false false R56.htm 995597 - Disclosure - Note 2 - Investment Securities (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-details-textual Note 2 - Investment Securities (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-tables 56 false false R57.htm 995598 - Disclosure - Note 2 - Investment Securities - Amortized Cost and Fair Value of Investment Securities Classified as AFS (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-amortized-cost-and-fair-value-of-investment-securities-classified-as-afs-details Note 2 - Investment Securities - Amortized Cost and Fair Value of Investment Securities Classified as AFS (Details) Details 57 false false R58.htm 995599 - Disclosure - Note 2 - Investment Securities - Sales of Investments Classified as AFS (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-sales-of-investments-classified-as-afs-details Note 2 - Investment Securities - Sales of Investments Classified as AFS (Details) Details 58 false false R59.htm 995600 - Disclosure - Note 2 - Investment Securities - Amortized Cost and Fair Value of Investment Securities Classified as HTM (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-amortized-cost-and-fair-value-of-investment-securities-classified-as-htm-details Note 2 - Investment Securities - Amortized Cost and Fair Value of Investment Securities Classified as HTM (Details) Details 59 false false R60.htm 995601 - Disclosure - Note 2 - Investment Securities - Summary of Continuous Unrealized Loss Position for Securities Classified as AFS (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-summary-of-continuous-unrealized-loss-position-for-securities-classified-as-afs-details Note 2 - Investment Securities - Summary of Continuous Unrealized Loss Position for Securities Classified as AFS (Details) Details 60 false false R61.htm 995602 - Disclosure - Note 2 - Investment Securities - Summary of Continuous Unrealized Loss Position for Securities Classified as HTM (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-summary-of-continuous-unrealized-loss-position-for-securities-classified-as-htm-details Note 2 - Investment Securities - Summary of Continuous Unrealized Loss Position for Securities Classified as HTM (Details) Details 61 false false R62.htm 995603 - Disclosure - Note 2 - Investment Securities - Contractual Maturity (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-contractual-maturity-details Note 2 - Investment Securities - Contractual Maturity (Details) Details 62 false false R63.htm 995604 - Disclosure - Note 3 - Loans and Allowance for Credit Losses (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-details-textual Note 3 - Loans and Allowance for Credit Losses (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-tables 63 false false R64.htm 995605 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Summary of Loans by Categories (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-summary-of-loans-by-categories-details Note 3 - Loans and Allowance for Credit Losses - Summary of Loans by Categories (Details) Details 64 false false R65.htm 995606 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Aging Analysis of Loans (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-aging-analysis-of-loans-details Note 3 - Loans and Allowance for Credit Losses - Aging Analysis of Loans (Details) Details 65 false false R66.htm 995607 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Schedule of Nonaccrual Loans (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-schedule-of-nonaccrual-loans-details Note 3 - Loans and Allowance for Credit Losses - Schedule of Nonaccrual Loans (Details) Details 66 false false R67.htm 995608 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Loan Portfolio by Category and Credit Quality Indicator (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-loan-portfolio-by-category-and-credit-quality-indicator-details Note 3 - Loans and Allowance for Credit Losses - Loan Portfolio by Category and Credit Quality Indicator (Details) Details 67 false false R68.htm 995609 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Loans to Related Parties (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-loans-to-related-parties-details Note 3 - Loans and Allowance for Credit Losses - Loans to Related Parties (Details) Details 68 false false R69.htm 995610 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Allowance for Loan Losses (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-allowance-for-loan-losses-details Note 3 - Loans and Allowance for Credit Losses - Allowance for Loan Losses (Details) Details 69 false false R70.htm 995611 - Disclosure - Note 4 - Other Real Estate Owned (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned-details-textual Note 4 - Other Real Estate Owned (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned-tables 70 false false R71.htm 995612 - Disclosure - Note 4 - Other Real Estate Owned - Real Estate Owned (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned-real-estate-owned-details Note 4 - Other Real Estate Owned - Real Estate Owned (Details) Details 71 false false R72.htm 995613 - Disclosure - Note 5 - Bank Premises and Equipment (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment-details-textual Note 5 - Bank Premises and Equipment (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment-tables 72 false false R73.htm 995614 - Disclosure - Note 5 - Bank Premises and Equipment - Bank Premises and Equipment (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment-bank-premises-and-equipment-details Note 5 - Bank Premises and Equipment - Bank Premises and Equipment (Details) Details 73 false false R74.htm 995615 - Disclosure - Note 6 - Leases (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-6-leases-details-textual Note 6 - Leases (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-6-leases-tables 74 false false R75.htm 995616 - Disclosure - Note 6 - Leases - Quantitative Information Regarding Operating Leases (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-6-leases-quantitative-information-regarding-operating-leases-details Note 6 - Leases - Quantitative Information Regarding Operating Leases (Details) Details 75 false false R76.htm 995617 - Disclosure - Note 6 - Leases - Future Minimum Lease Payment (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-6-leases-future-minimum-lease-payment-details Note 6 - Leases - Future Minimum Lease Payment (Details) Details 76 false false R77.htm 995618 - Disclosure - Note 7 - Goodwill and Other Intangible Assets (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-details-textual Note 7 - Goodwill and Other Intangible Assets (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-tables 77 false false R78.htm 995619 - Disclosure - Note 7 - Goodwill and Other Intangible Assets - Core Deposit Intangible Assets (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-core-deposit-intangible-assets-details Note 7 - Goodwill and Other Intangible Assets - Core Deposit Intangible Assets (Details) Details 78 false false R79.htm 995620 - Disclosure - Note 7 - Goodwill and Other Intangible Assets - Amortization Schedule for Core Deposit Intangible Assets (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-amortization-schedule-for-core-deposit-intangible-assets-details Note 7 - Goodwill and Other Intangible Assets - Amortization Schedule for Core Deposit Intangible Assets (Details) Details 79 false false R80.htm 995621 - Disclosure - Note 8 - Deposits (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-8-deposits-details-textual Note 8 - Deposits (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-8-deposits-tables 80 false false R81.htm 995622 - Disclosure - Note 8 - Deposits - Deposits (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-8-deposits-deposits-details Note 8 - Deposits - Deposits (Details) Details 81 false false R82.htm 995623 - Disclosure - Note 8 - Deposits - Scheduled Maturates of Time Deposits (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-8-deposits-scheduled-maturates-of-time-deposits-details Note 8 - Deposits - Scheduled Maturates of Time Deposits (Details) Details 82 false false R83.htm 995624 - Disclosure - Note 9 - Securities Sold Under Agreements to Repurchase (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-9-securities-sold-under-agreements-to-repurchase-details-textual Note 9 - Securities Sold Under Agreements to Repurchase (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-9-securities-sold-under-agreements-to-repurchase- 83 false false R84.htm 995625 - Disclosure - Note 10 - Subordinated Debt Securities (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-10-subordinated-debt-securities-details-textual Note 10 - Subordinated Debt Securities (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-10-subordinated-debt-securities- 84 false false R85.htm 995626 - Disclosure - Note 11 - Other Borrowed Funds (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds-details-textual Note 11 - Other Borrowed Funds (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds-tables 85 false false R86.htm 995627 - Disclosure - Note 11 - Other Borrowed Funds - Federal Home Loan Bank Advances (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds-federal-home-loan-bank-advances-details Note 11 - Other Borrowed Funds - Federal Home Loan Bank Advances (Details) Details 86 false false R87.htm 995628 - Disclosure - Note 11 - Other Borrowing Funds - Junior Subordinated Debt (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowing-funds-junior-subordinated-debt-details Note 11 - Other Borrowing Funds - Junior Subordinated Debt (Details) Details 87 false false R88.htm 995629 - Disclosure - Note 12 - Derivative Financial Instruments (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-details-textual Note 12 - Derivative Financial Instruments (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-tables 88 false false R89.htm 995630 - Disclosure - Note 12 - Derivative Financial Instruments - Schedule of Derivatives Instruments (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-schedule-of-derivatives-instruments-details Note 12 - Derivative Financial Instruments - Schedule of Derivatives Instruments (Details) Details 89 false false R90.htm 995631 - Disclosure - Note 12 - Derivative Financial Instruments - Offsetting of Derivative Instruments (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-offsetting-of-derivative-instruments-details Note 12 - Derivative Financial Instruments - Offsetting of Derivative Instruments (Details) Details 90 false false R91.htm 995632 - Disclosure - Note 13 - Stockholders' Equity (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity-details-textual Note 13 - Stockholders' Equity (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity-tables 91 false false R92.htm 995633 - Disclosure - Note 13 - Stockholders' Equity - Accumulated Other Comprehensive Income (Loss) (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity-accumulated-other-comprehensive-income-loss-details Note 13 - Stockholders' Equity - Accumulated Other Comprehensive Income (Loss) (Details) Details 92 false false R93.htm 995634 - Disclosure - Note 14 - Stock-Based Compensation (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-details-textual Note 14 - Stock-Based Compensation (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-tables 93 false false R94.htm 995635 - Disclosure - Note 14 - Stock-Based Compensation - Schedule of Stock Option Activity (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-schedule-of-stock-option-activity-details Note 14 - Stock-Based Compensation - Schedule of Stock Option Activity (Details) Details 94 false false R95.htm 995636 - Disclosure - Note 14 - Stock-Based Compensation - Schedule of Stock Option Valuation Assumptions (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-schedule-of-stock-option-valuation-assumptions-details Note 14 - Stock-Based Compensation - Schedule of Stock Option Valuation Assumptions (Details) Details 95 false false R96.htm 995637 - Disclosure - Note 14 - Stock-Based Compensation - Schedule of Restricted Stock Activity (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-schedule-of-restricted-stock-activity-details Note 14 - Stock-Based Compensation - Schedule of Restricted Stock Activity (Details) Details 96 false false R97.htm 995638 - Disclosure - Note 15 - Employee Benefits (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-15-employee-benefits-details-textual Note 15 - Employee Benefits (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-15-employee-benefits 97 false false R98.htm 995639 - Disclosure - Note 16 - Income Taxes (Details Textual) Sheet http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-details-textual Note 16 - Income Taxes (Details Textual) Details http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-tables 98 false false R99.htm 995640 - Disclosure - Note 16 - Income Taxes - Income Tax Expense (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-income-tax-expense-details Note 16 - Income Taxes - Income Tax Expense (Details) Details 99 false false R100.htm 995641 - Disclosure - Note 16 - Income Taxes - Summary of Provision for Federal Income Taxes (Details) Sheet http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-summary-of-provision-for-federal-income-taxes-details Note 16 - Income Taxes - 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