| Document And Entity Information | Feb. 18, 2021 |
| Document Information [Line Items] | |
| Entity, Registrant Name | FIRST NATIONAL CORPORATION |
| Document, Type | 8-K |
| Document, Period End Date | Feb. 18, 2021 |
| Entity, Incorporation, State or Country Code | VA |
| Entity, File Number | 1-38874 |
| Entity, Tax Identification Number | 54-1232965 |
| Entity, Address, Address Line One | 112 West King Street |
| Entity, Address, City or Town | Strasburg |
| Entity, Address, State or Province | VA |
| Entity, Address, Postal Zip Code | 22657 |
| City Area Code | 540 |
| Local Phone Number | 465-9121 |
| Written Communications | true |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | FXNC |
| Security Exchange Name | NASDAQ |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0000719402 |
