Significant Accounting Policies (Policies)	3 Months Ended
Significant Accounting Policies (Policies)	Mar. 31, 2024
Accounting Policies [Abstract]
Basis of Accounting, Policy [Policy Text Block]	Basis of Presentation The accompanying unaudited consolidated financial statements of First National Corporation (the Company) and its subsidiary, First Bank (the Bank), have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information and in accordance with guidance provided by the Securities and Exchange Commission (SEC). Accordingly, they do not include all of the information and footnotes required by GAAP for annual year-end financial statements. All significant intercompany balances and transactions have been eliminated. In the opinion of management, the accompanying unaudited consolidated financial statements contain all adjustments and reclassifications of a normal and recurring nature considered necessary to present fairly the financial positions at March 31, 2024 and December 31, 2023, the statements of income and comprehensive income (loss) for the three months ended March 31, 2024 and 2023, the cash flows for the three months ended March 31, 2024 and 2023, and the changes in shareholders’ equity for the three months ended March 31, 2024 and 2023. The statements should be read in conjunction with the consolidated financial statements and related notes included in the Annual Report on Form 10-K for the year ended December 31, 2023. Operating results for the three months ended March 31, 2024 are not necessarily indicative of the results that may be expected for the year ending December 31, 2024. Certain items in the prior period financial statements have been reclassified to conform to the current presentation. These reclassifications had no effect on prior year net income or shareholders' equity.
Use of Estimates, Policy [Policy Text Block]	Significant Accounting Policies and Estimates Application of the principles of GAAP and practices within the banking industry requires management to make estimates, assumptions, and judgements that affect the amounts reported in the financial statements and accompanying notes. These estimates, assumptions, and judgements are based on information available as of the date of the financial statements; accordingly, as this information changes, the financial statements may reflect different estimates, assumptions and judgements. Certain policies inherently rely more extensively on the use of estimates, assumptions, and judgements and as such may have a greater possibility of producing results that could be materially different than originally reported. Material estimates that are particularly susceptible to significant changes in the near term include estimates related to the determination of the allowance for credit losses. The Company’s significant accounting policies followed in the preparation of the unaudited consolidated financial statements are disclosed in Note 1 of the audited financial statements and notes for the year ended December 31, 2023 and are contained in the Company’s 2023 Annual Report on Form 10-K.
Business Combinations Policy [Policy Text Block]	Business Combination On March 25, 2024, the Company entered into an agreement to acquire Touchstone Bankshares, Inc. (Touchstone) for an aggregate purchase price of $47.0 million in stock. The financial position and results of operations of Touchstone are not reflected in the Company’s financial statements as of March 31, 2024. At the time of closing of the acquisition, Touchstone is expected to have twelve retail bank offices serving south-central Virginia and northern North Carolina, and an administrative office in Prince George, Virginia. The closing of the acquisition is subject to customary closing conditions, including shareholder and regulatory approvals. As of March 31, 2024, Touchstone reported total assets of $673.2 million, gross loans of $506.0 million, and total deposits of $557.6 million. In connection with the transaction, the Company expects to issue approximately 2.7 million shares of its common stock to the shareholders of Touchstone. Upon completion of the transaction, Touchstone Bank, which is Touchstone’s wholly owned banking subsidiary, is expected to be merged with and into First Bank. The acquisition will be accounted for as a business combination under ASC 805, Business Combinations. Under acquisition accounting, assets acquired and liabilities assumed are recorded at their acquisition date fair values, and any excess of the purchase price over the aggregate fair value of the net assets acquired is recognized as goodwill.
New Accounting Pronouncements, Policy [Policy Text Block]	Adoption of New Accounting Pronouncements In July 2023, the Financial Accounting Standards Board (FASB) issued ASU 2023-03, “Presentation of Financial Statements (Topic 205), Income Statement—Reporting Comprehensive Income (Topic 220), Distinguishing Liabilities from Equity (Topic 480), Equity (Topic 505), and Compensation—Stock Compensation (Topic 718)”. This ASU amends the FASB Accounting Standards Codification for SEC paragraphs pursuant to SEC Staff Accounting Bulletin No. 120, SEC Staff Announcement at the March 24, 2022 EITF Meeting, and Staff Accounting Bulletin Topic 6.B, Accounting Series Release 280—General Revision of Regulation S-X: Income or Loss Applicable to Common Stock. ASU 2023-03 is effective upon addition to the FASB Codification. The Company does not expect the adoption of ASU 2023-03 to have a material impact on its consolidated financial statements. In March 2023, the FASB issued ASU 2023-02, “Investments—Equity Method and Joint Ventures (Topic 323): Accounting for Investments in Tax Credit Structures Using the Proportional Amortization Method”. These amendments allow reporting entities to elect to account for qualifying tax equity investments using the proportional amortization method, regardless of the program giving rise to the related income tax credits. The ASU is effective for public business entities for fiscal years beginning after December 15, 2023, including interim periods within those fiscal years. Early adoption is permitted for all entities in any interim period. The Company does not expect the adoption of ASU 2023-02 to have a material impact on its consolidated financial statements. In March 2023, the FASB issued ASU 2023-01, “Leases (Topic 842): Common Control Arrangements”. These amendments require entities to amortize leasehold improvements associated with common control leases over the useful life to the common control group. The ASU is effective for fiscal years beginning after December 15, 2023, including interim periods within those fiscal years. Early adoption is permitted. If an entity adopts the amendments in an interim period, it must adopt them as of the beginning of the fiscal year that includes that interim period. Transition can be done either retrospectively or prospectively. The Company does not expect the adoption of ASU 2023-01 to have a material impact on its consolidated financial statements. In June 2022, the FASB issued ASU 2022-03, “Fair Value Measurement (Topic 820): Fair Value Measurement of Equity Securities Subject to Contractual Sale Restrictions.” ASU 2022-03 clarifies that a contractual restriction on the sale of an equity security is not considered part of the unit of account of the equity security and, therefore, is not considered in measuring fair value. The ASU is effective for fiscal years, including interim periods within those fiscal years, beginning after December 15, 2023. Early adoption is permitted. The Company does not expect the adoption of ASU 2022-03 to have a material impact on its consolidated financial statements. Recent Accounting Pronouncements In March 2024, the Financial Accounting Standards Board (FASB) issued ASU 2024-01, “Compensation – Stock Compensation (Topic 718): Scope Application of Profits Interest and Similar Awards”. This ASU provides an illustrative example intended to demonstrate how entities that account for profits interest and similar awards would determine whether a profits interest award should be accounted for in accordance with Topic 718. This ASU is effective for annual periods beginning after December 15, 2024, and interim periods within those annual periods. Early adoption is permitted. If an entity adopts the amendments in an interim period, it must adopt them as of the beginning of the annual period that includes that interim period. Transition can be done either retrospectively or prospectively. The Company does not expect the adoption of ASU 2024-01 to have a material impact on its (consolidated) financial statements. In March 2024, the Financial Accounting Standards Board (FASB) issued ASU 2024-02, “Codification Improvements – Amendments to Remove References to the Concepts Statements”. This ASU contains amendments to the Codification that remove references to various Concepts Statements. In most instances, the references are extraneous and not required to understand or apply the guidance. In other instances, the references were used in prior Statements to provide guidance in certain topical areas. This ASU is effective for fiscal years beginning after December 15, 2024. Early adoption is permitted. The amendments should be applied prospectively to all new transactions recognized on or after the date that the entity first applies the amendments or retrospectively to the beginning of the earliest comparative period presented in which the amendments were first applied. If an entity adopts the amendments retrospectively, it should adjust the opening balance of retained earnings as of the beginning of the earliest comparative period presented. The Company does not expect the adoption of ASU 2024-02 to have a material impact on its (consolidated) financial statements.