Document and Entity Information (USD $)	6 Months Ended
	Jun. 30, 2011	Jul. 27, 2011	Jun. 30, 2010
Document and Entity Information [Abstract]
Entity Registrant Name	FARMERS & MERCHANTS BANCORP INC
Entity Central Index Key	0000792966
Document Type	10-Q
Document Period End Date	Jun. 30, 2011
Amendment Flag	false
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q2
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Public Float			$ 86,373,431.65
Entity Common Stock, Shares Outstanding		4,682,697