Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	May 07, 2021
Cover [Abstract]
Entity Registrant Name	FARMERS & MERCHANTS BANCORP
Entity Central Index Key	0001085913
Current Fiscal Year End Date	--12-31
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Document Transition Report	false
Entity File Number	000-26099
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	94-3327828
Entity Address, Address Line One	111 W. Pine Street
Entity Address, City or Town	Lodi
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	95240
City Area Code	209
Local Phone Number	367-2300
Title of 12(b) Security	Common Stock
Trading Symbol	FMCB
Security Exchange Name	NONE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		789,646