Basis of Presentation (Policies)	3 Months Ended
Mar. 31, 2026
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited consolidated financial statements have been prepared in accordance with generally accepted accounting principles for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by generally accepted accounting principles. The consolidated financial statements and the footnotes of First Guaranty Bancshares, Inc. ("First Guaranty") thereto should be read in conjunction with the audited consolidated financial statements and note disclosures for First Guaranty previously filed with the Securities and Exchange Commission in First Guaranty's Annual Report on Form 10-K for the year ended December 31, 2025. The consolidated financial statements include the accounts of First Guaranty Bancshares, Inc. and its wholly owned subsidiary First Guaranty Bank (the "Bank"). All significant intercompany balances and transactions have been eliminated in consolidation. In the opinion of management, the accompanying unaudited consolidated financial statements contain all adjustments necessary for a fair presentation of the consolidated financial statements. Those adjustments are of a normal recurring nature. The results of operations at March 31, 2026 and for the three-month periods ended March 31, 2026 and 2025 are not necessarily indicative of the results expected for the full year or any other interim period. The preparation of financial statements in conformity with generally accepted accounting principles requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from those estimates. Material estimates that are particularly susceptible to significant change in the near-term relate to the determination of the allowance for credit losses, the valuation of real estate acquired in connection with foreclosures or in satisfaction of loans, the valuation of the deferred tax asset, and the valuation of investment securities.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Accounting Standards Adopted in 2026 None. Accounting Pronouncements Not Yet Adopted ASU 2024-03, "Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures" (ASU 2024-03") requires the disaggregation of certain expenses in the notes to the financial statements, to provide enhanced transparency into the expense captions presented on the face of the income statement. The amendments in ASU 2024-03 are effective for annual reporting periods beginning after December 15, 2026, and interim reporting periods beginning after December 15, 2027. Early adoption is permitted. The amendments in this ASU may be applied either prospectively or retrospectively. First Guaranty is currently evaluating the impact that this update will have on its disclosures in the consolidated financial statements. ASU 2025-08, "Financial Instruments—Credit Losses (Topic 326): Purchased Loans" (ASU 2025-08") amendments expand the application of the gross‑up approach to certain acquired loans, referred to as “purchased seasoned loans,” which will be recorded at purchase price plus an allowance for expected credit losses at acquisition rather than through a provision for credit losses. The guidance is effective for annual reporting periods beginning after December 15, 2026, including interim periods within those fiscal years, and is to be applied prospectively. Early adoption is permitted. First Guaranty is currently evaluating the impact that adoption of this guidance will have on its consolidated financial statements.
Fair Value Measurements	Fair Value Measurements The fair value of a financial instrument is the current amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. A fair value measurement assumes that the transaction to sell the asset or transfer the liability occurs in the principal market for the asset or liability or, in the absence of a principal market, the most advantageous market for the asset or liability. Valuation techniques use certain inputs to arrive at fair value. Inputs to valuation techniques are the assumptions that market participants would use in pricing the asset or liability. They may be observable or unobservable. First Guaranty uses a fair value hierarchy for valuation inputs that gives the highest priority to quoted prices in active markets for identical assets or liabilities and the lowest priority to unobservable inputs. The fair value hierarchy is as follows: Level 1 Inputs – Unadjusted quoted market prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2 Inputs – Inputs other than quoted prices included in Level 1 that are observable for the asset or liability, either directly or indirectly. These might include quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, inputs other than quoted prices that are observable for the asset or liability (such as interest rates, volatilities, prepayment speeds or credit risks) or inputs that are derived principally from or corroborated by market data by correlation or other means. Level 3 Inputs – Unobservable inputs for determining the fair values of assets or liabilities that reflect an entity's own assumptions about the assumptions that market participants would use in pricing the assets or liabilities. A description of the valuation methodologies used for instruments measured at fair value follows, as well as the classification of such instruments within the valuation hierarchy. Securities available for sale. Securities are classified within Level 1 where quoted market prices are available in an active market. Inputs include securities that have quoted prices in active markets for identical assets. If quoted market prices are unavailable, fair value is estimated using quoted prices of securities with similar characteristics, at which point the securities would be classified within Level 2 of the hierarchy. Securities classified within Level 3 in First Guaranty's portfolio as of March 31, 2026 include corporate debt and municipal securities. Loan individually evaluated for impairment. Fair value is measured by either the fair value of the collateral if the loan is collateral dependent (Level 2 or Level 3), or the present value of expected future cash flows, discounted at the loan's effective interest rate (Level 3). Fair value of the collateral is determined by appraisals or by independent valuation. Other real estate owned. Properties are recorded at the balance of the loan or at estimated fair value less estimated selling costs, whichever is less, at the date acquired. Fair values of other real estate owned ("OREO") are determined by sales agreement or appraisal, and costs to sell are based on estimation per the terms and conditions of the sales agreement or amounts commonly used in real estate transactions. Inputs include appraisal values or recent sales activity for similar assets in the property's market; thus, OREO measured at fair value would be classified within either Level 2 or Level 3 of the hierarchy. First Guaranty transferred $4.4 million of existing bank owned properties previously used as either operating branches or future branch development to other real estate owned. Those properties were available for sale as of March 31, 2026. The properties are included in Level 3 as of March 31, 2026. Certain non-financial assets and non-financial liabilities are measured at fair value on a non-recurring basis including assets and liabilities related to reporting units measured at fair value in the testing of goodwill impairment, as well as intangible assets and other non-financial long-lived assets measured at fair value for impairment assessment. ASC 825-10 provides First Guaranty with an option to report selected financial assets and liabilities at fair value. The fair value option established by this statement permits First Guaranty to choose to measure eligible items at fair value at specified election dates and report unrealized gains and losses on items for which the fair value option has been elected in earnings at each reporting date subsequent to implementation. First Guaranty has chosen not to elect the fair value option for any items that are not already required to be measured at fair value in accordance with accounting principles generally accepted in the United States.

Basis of Presentation (Policies)

3 Months Ended

Mar. 31, 2026

Organization, Consolidation and Presentation of Financial Statements [Abstract]

The accompanying unaudited consolidated financial statements have been prepared in accordance with generally accepted accounting principles for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by generally accepted accounting principles. The consolidated financial statements and the footnotes of First Guaranty Bancshares, Inc. ("First Guaranty") thereto should be read in conjunction with the audited consolidated financial statements and note disclosures for First Guaranty previously filed with the Securities and Exchange Commission in First Guaranty's Annual Report on Form 10-K for the year ended December 31, 2025.

The consolidated financial statements include the accounts of First Guaranty Bancshares, Inc. and its wholly owned subsidiary First Guaranty Bank (the "Bank"). All significant intercompany balances and transactions have been eliminated in consolidation.

In the opinion of management, the accompanying unaudited consolidated financial statements contain all adjustments necessary for a fair presentation of the consolidated financial statements. Those adjustments are of a normal recurring nature. The results of operations at March 31, 2026 and for the three-month periods ended March 31, 2026 and 2025 are not necessarily indicative of the results expected for the full year or any other interim period. The preparation of financial statements in conformity with generally accepted accounting principles requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from those estimates. Material estimates that are particularly susceptible to significant change in the near-term relate to the determination of the allowance for credit losses, the valuation of real estate acquired in connection with foreclosures or in satisfaction of loans, the valuation of the deferred tax asset, and the valuation of investment securities.

Recent Accounting Pronouncements

Accounting Standards Adopted in 2026

None.

Accounting Pronouncements Not Yet Adopted

ASU 2024-03, "Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures" (ASU 2024-03") requires the disaggregation of certain expenses in the notes to the financial statements, to provide enhanced transparency into the expense captions presented on the face of the income statement. The amendments in ASU 2024-03 are effective for annual reporting periods beginning after December 15, 2026, and interim reporting periods beginning after December 15, 2027. Early adoption is permitted. The amendments in this ASU may be applied either prospectively or retrospectively. First Guaranty is currently evaluating the impact that this update will have on its disclosures in the consolidated financial statements.

ASU 2025-08, "Financial Instruments—Credit Losses (Topic 326): Purchased Loans" (ASU 2025-08") amendments expand the application of the gross‑up approach to certain acquired loans, referred to as “purchased seasoned loans,” which will be recorded at purchase price plus an allowance for expected credit losses at acquisition rather than through a provision for credit losses. The guidance is effective for annual reporting periods beginning after December 15, 2026, including interim periods within those fiscal years, and is to be applied prospectively. Early adoption is permitted. First Guaranty is currently evaluating the impact that adoption of this guidance will have on its consolidated financial statements.

Fair Value Measurements

The fair value of a financial instrument is the current amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. A fair value measurement assumes that the transaction to sell the asset or transfer the liability occurs in the principal market for the asset or liability or, in the absence of a principal market, the most advantageous market for the asset or liability. Valuation techniques use certain inputs to arrive at fair value. Inputs to valuation techniques are the assumptions that market participants would use in pricing the asset or liability. They may be observable or unobservable. First Guaranty uses a fair value hierarchy for valuation inputs that gives the highest priority to quoted prices in active markets for identical assets or liabilities and the lowest priority to unobservable inputs. The fair value hierarchy is as follows:

Level 1 Inputs – Unadjusted quoted market prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date.

Level 2 Inputs – Inputs other than quoted prices included in Level 1 that are observable for the asset or liability, either directly or indirectly. These might include quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, inputs other than quoted prices that are observable for the asset or liability (such as interest rates, volatilities, prepayment speeds or credit risks) or inputs that are derived principally from or corroborated by market data by correlation or other means.

Level 3 Inputs – Unobservable inputs for determining the fair values of assets or liabilities that reflect an entity's own assumptions about the assumptions that market participants would use in pricing the assets or liabilities.

A description of the valuation methodologies used for instruments measured at fair value follows, as well as the classification of such instruments within the valuation hierarchy.

Securities available for sale. Securities are classified within Level 1 where quoted market prices are available in an active market. Inputs include securities that have quoted prices in active markets for identical assets. If quoted market prices are unavailable, fair value is estimated using quoted prices of securities with similar characteristics, at which point the securities would be classified within Level 2 of the hierarchy. Securities classified within Level 3 in First Guaranty's portfolio as of March 31, 2026 include corporate debt and municipal securities.

Loan individually evaluated for impairment. Fair value is measured by either the fair value of the collateral if the loan is collateral dependent (Level 2 or Level 3), or the present value of expected future cash flows, discounted at the loan's effective interest rate (Level 3). Fair value of the collateral is determined by appraisals or by independent valuation.

Other real estate owned. Properties are recorded at the balance of the loan or at estimated fair value less estimated selling costs, whichever is less, at the date acquired. Fair values of other real estate owned ("OREO") are determined by sales agreement or appraisal, and costs to sell are based on estimation per the terms and conditions of the sales agreement or amounts commonly used in real estate transactions. Inputs include appraisal values or recent sales activity for similar assets in the property's market; thus, OREO measured at fair value would be classified within either Level 2 or Level 3 of the hierarchy. First Guaranty transferred $4.4 million of existing bank owned properties previously used as either operating branches or future branch development to other real estate owned. Those properties were available for sale as of March 31, 2026. The properties are included in Level 3 as of March 31, 2026.

Certain non-financial assets and non-financial liabilities are measured at fair value on a non-recurring basis including assets and liabilities related to reporting units measured at fair value in the testing of goodwill impairment, as well as intangible assets and other non-financial long-lived assets measured at fair value for impairment assessment.

ASC 825-10 provides First Guaranty with an option to report selected financial assets and liabilities at fair value. The fair value option established by this statement permits First Guaranty to choose to measure eligible items at fair value at specified election dates and report unrealized gains and losses on items for which the fair value option has been elected in earnings at each reporting date subsequent to implementation.

First Guaranty has chosen not to elect the fair value option for any items that are not already required to be measured at fair value in accordance with accounting principles generally accepted in the United States.