Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	May 04, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	FARMERS & MERCHANTS BANCORP
Entity Central Index Key	0001085913
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 Common Stock, Shares Outstanding		821,073
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018