General	3 Months Ended
Mar. 31, 2019
Accounting Policies [Abstract]
General	General 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. Accordingly, they do not include all of the information and footnotes required by GAAP. 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, 2019 and December 31, 2018, the statements of income and comprehensive income for the three months ended March 31, 2019 and 2018 and the cash flows and changes in shareholders’ equity for the three months ended March 31, 2019 and 2018. 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, 2018. Operating results for the three months ended March 31, 2019 are not necessarily indicative of the results that may be expected for the year ending December 31, 2019. Adoption of New Accounting Standards On January 1, 2019, the Company adopted ASU No. 2016-02, “Leases (Topic 842).” Among other things, in the amendments in ASU 2016-02, lessees are required to recognize the following for all leases (with the exception of short-term leases) at the commencement date: (1) A lease liability, which is a lessee‘s obligation to make lease payments arising from a lease, measured on a discounted basis; and (2) A right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. Under the new guidance, lessor accounting is largely unchanged. Certain targeted improvements were made to align, where necessary, lessor accounting with the lessee accounting model and Topic 606, Revenue from Contracts with Customers. Lessees (for capital and operating leases) and lessors (for sales-type, direct financing, and operating leases) must apply a modified retrospective transition approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements. The modified retrospective approach does not require any transition accounting for leases that expired before the earliest comparative period presented. Lessees and lessors may not apply a full retrospective transition approach. The FASB made subsequent amendments to Topic 842 through ASU 2018-10 (“Codification Improvements to Topic 842, Leases.”) and ASU 2018-11 (“Leases (Topic 842): Targeted Improvements.”) Among these amendments is the provision in ASU 2018-11 that provides entities with an additional (and optional) transition method to adopt the new leases standard. Under this new transition method, an entity initially applies the new leases standard at the adoption date and recognizes a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. Consequently, an entity’s reporting for the comparative periods presented in the financial statements in which it adopts the new leases standard will continue to be in accordance with current GAAP (Topic 840, Leases). The adoption of this standard did not have a material effect on the Company's consolidated financial statements. For further information about the Company's leases, see Note 16. On January 1, 2019, the Company adopted ASU No. 2017-08, "Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20), Premium Amortization on Purchased Callable Debt Securities.” The amendments in ASU 2017-08 shorten the amortization period for certain callable debt securities purchased at a premium. Under the new guidance, premiums on these qualifying callable debt securities are amortized to the earliest call date. Discounts on purchased debt securities continue to be accreted to maturity. The adoption of this standard did not have a material effect on the Company's consolidated financial statements and no cumulative effect adjustment was recorded. On January 1, 2019, the Company adopted ASU No. 2017-12, "Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities." The amendments in ASU 2017-12 modify the designation and measurement guidance for hedge accounting as well as provide for increased transparency regarding the presentation of economic results on both the financial statements and related footnotes. Certain aspects of hedge effectiveness assessments were simplified upon implementation of this updated. The new guidance also provides for a reclassification of certain debt securities from held to maturity to available for sale if the security is eligible to be hedged using the last-of-layer method. Any unrealized gain or loss existing at the time of transfer is recorded in accumulated comprehensive income or loss. As a permitted activity, the reclassification of securities will not taint future held to maturity classification so long as the securities transferred are eligible to be hedged under the last-of-layer method. Accordingly, on January 1, 2019, the Company reclassified eligible held to maturity securities with amortized costs totaling $23.4 million as available for sale. The unrealized loss associated with the reclassified securities totaled $431 thousand and was included in the Company's accumulated other comprehensive loss on the date of reclassification. Recent Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, “Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” The amendments in this ASU, among other things, require the measurement of all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions, and reasonable and supportable forecasts. Financial institutions and other organizations will now use forward-looking information to better inform their credit loss estimates. Many of the loss estimation techniques applied today will still be permitted, although the inputs to those techniques will change to reflect the full amount of expected credit losses. In addition, the ASU amends the accounting for credit losses on available-for-sale debt securities and purchased financial assets with credit deterioration. The amendments in this ASU are effective for SEC filers for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The Company is currently assessing the impact that ASU 2016-13 will have on its consolidated financial statements. The Company has formed a committee to address the compliance requirements of this ASU, which has analyzed gathered data, definined loan pools and segments, and selected methods for applying the concepts included in this ASU. The Company is in the process of testing selected models, building policy and processing documentation, modeling the impact of the ASU on the capital and strategic plans, performing model validation, and finalizing policies and procedures. This guidance may result in material changes in the Company's accounting for credit losses of financial instruments.

General

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. Accordingly, they do not include all of the information and footnotes required by GAAP. 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, 2019 and December 31, 2018, the statements of income and comprehensive income for the three months ended March 31, 2019 and 2018 and the cash flows and changes in shareholders’ equity for the three months ended March 31, 2019 and 2018. 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, 2018. Operating results for the three months ended March 31, 2019 are not necessarily indicative of the results that may be expected for the year ending December 31, 2019.

Adoption of New Accounting Standards

On January 1, 2019, the Company adopted ASU No. 2016-02, “Leases (Topic 842).” Among other things, in the amendments in ASU 2016-02, lessees are required to recognize the following for all leases (with the exception of short-term leases) at the commencement date: (1) A lease liability, which is a lessee‘s obligation to make lease payments arising from a lease, measured on a discounted basis; and (2) A right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. Under the new guidance, lessor accounting is largely unchanged. Certain targeted improvements were made to align, where necessary, lessor accounting with the lessee accounting model and Topic 606, Revenue from Contracts with Customers. Lessees (for capital and operating leases) and lessors (for sales-type, direct financing, and operating leases) must apply a modified retrospective transition approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements. The modified retrospective approach does not require any transition accounting for leases that expired before the earliest comparative period presented. Lessees and lessors may not apply a full retrospective transition approach. The FASB made subsequent amendments to Topic 842 through ASU 2018-10 (“Codification Improvements to Topic 842, Leases.”) and ASU 2018-11 (“Leases (Topic 842): Targeted Improvements.”) Among these amendments is the provision in ASU 2018-11 that provides entities with an additional (and optional) transition method to adopt the new leases standard. Under this new transition method, an entity initially applies the new leases standard at the adoption date and recognizes a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. Consequently, an entity’s reporting for the comparative periods presented in the financial statements in which it adopts the new leases standard will continue to be in accordance with current GAAP (Topic 840, Leases). The adoption of this standard did not have a material effect on the Company's consolidated financial statements. For further information about the Company's leases, see Note 16.

On January 1, 2019, the Company adopted ASU No. 2017-08, "Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20), Premium Amortization on Purchased Callable Debt Securities.” The amendments in ASU 2017-08 shorten the amortization period for certain callable debt securities purchased at a premium. Under the new guidance, premiums on these qualifying callable debt securities are amortized to the earliest call date. Discounts on purchased debt securities continue to be accreted to maturity. The adoption of this standard did not have a material effect on the Company's consolidated financial statements and no cumulative effect adjustment was recorded.

On January 1, 2019, the Company adopted ASU No. 2017-12, "Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities." The amendments in ASU 2017-12 modify the designation and measurement guidance for hedge accounting as well as provide for increased transparency regarding the presentation of economic results on both the financial statements and related footnotes. Certain aspects of hedge effectiveness assessments were simplified upon implementation of this updated. The new guidance also provides for a reclassification of certain debt securities from held to maturity to available for sale if the security is eligible to be hedged using the last-of-layer method. Any unrealized gain or loss existing at the time of transfer is recorded in accumulated comprehensive income or loss. As a permitted activity, the reclassification of securities will not taint future held to maturity classification so long as the securities transferred are eligible to be hedged under the last-of-layer method. Accordingly, on January 1, 2019, the Company reclassified eligible held to maturity securities with amortized costs totaling $23.4 million as available for sale. The unrealized loss associated with the reclassified securities totaled $431 thousand and was included in the Company's accumulated other comprehensive loss on the date of reclassification.

Recent Accounting Pronouncements

In June 2016, the FASB issued ASU No. 2016-13, “Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” The amendments in this ASU, among other things, require the measurement of all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions, and reasonable and supportable forecasts. Financial institutions and other organizations will now use forward-looking information to better inform their credit loss estimates. Many of the loss estimation techniques applied today will still be permitted, although the inputs to those techniques will change to reflect the full amount of expected credit losses. In addition, the ASU amends the accounting for credit losses on available-for-sale debt securities and purchased financial assets with credit deterioration. The amendments in this ASU are effective for SEC filers for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The Company is currently assessing the impact that ASU 2016-13 will have on its consolidated financial statements. The Company has formed a committee to address the compliance requirements of this ASU, which has analyzed gathered data, definined loan pools and segments, and selected methods for applying the concepts included in this ASU. The Company is in the process of testing selected models, building policy and processing documentation, modeling the impact of the ASU on the capital and strategic plans, performing model validation, and finalizing policies and procedures. This guidance may result in material changes in the Company's accounting for credit losses of financial instruments.