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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2017
Feb. 24, 2018
Jun. 30, 2017
Document And Entity Information [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2017    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Trading Symbol FMAO    
Entity Registrant Name FARMERS & MERCHANTS BANCORP INC    
Entity Central Index Key 0000792966    
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 Accelerated Filer    
Entity Common Stock, Shares Outstanding   9,265,980  
Entity Public Float     $ 286,494,250