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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2016
Mar. 29, 2017
Jun. 30, 2016
Document And Entity Information [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2016    
Document Fiscal Year Focus 2016    
Document Fiscal Period Focus FY    
Trading Symbol FXNC    
Entity Registrant Name FIRST NATIONAL CORP /VA/    
Entity Central Index Key 0000719402    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Smaller Reporting Company    
Entity Common Stock, Shares Outstanding   4,940,766  
Entity Public Float     $ 39,525,026