Significant Accounting Policies (Policies)	6 Months Ended
Significant Accounting Policies (Policies)	Jun. 30, 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 June 30, 2024 and December 31, 2023, the statements of income and comprehensive income for the six months ended June 30, 2024 and 2023, the cash flows for the six months ended June 30, 2024 and 2023, and the changes in shareholders’ equity for the three and six months ended June 30, 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 six months ended June 30, 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 June 30, 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 Company has received regulatory approvals from the Federal Reserve Bank of Richmond, acting under authority delegated by the Board of Governors of the Federal Reserve System, and the Bureau of Financial Institutions division of the State Corporation Commission of the Commonwealth of Virginia. The closing of the acquisition is subject to customary closing conditions, including shareholder approval. As of June 30, 2024, Touchstone reported total assets of $662.7 million, gross loans of $500.6 million, and total deposits of $546.7 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 aggregate fair value of the net assets acquired over the purchase price is recognized as bargain purchase gain.
New Accounting Pronouncements, Policy [Policy Text Block]	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 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. In December 2023, the FASB issued ASU 2023-09, “Income Taxes (Topic 740): Improvements to Income Tax Disclosures.” The amendments in this ASU require an entity to disclose specific categories in the rate reconciliation and provide additional information for reconciling items that meet a quantitative threshold, which is greater than five percent of the amount computed by multiplying pretax income by the entity’s applicable statutory rate, on an annual basis. Additionally, the amendments in this ASU require an entity to disclose the amount of income taxes paid (net of refunds received) disaggregated by federal, state, and foreign taxes and the amount of income taxes paid (net of refunds received) disaggregated by individual jurisdictions that are equal to or greater than five percent of total income taxes paid (net of refunds received). Lastly, the amendments in this ASU require an entity to disclose income (or loss) from continuing operations before income tax expense (or benefit) disaggregated between domestic and foreign and income tax expense (or benefit) from continuing operations disa