Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2021	Feb. 28, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Document Transition Report	false
Entity File Number	000-26099
Entity Registrant Name	FARMERS & MERCHANTS BANCORP
Entity Central Index Key	0001085913
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(g) Security	Common Stock, $0.01 Par Value Per Share
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
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 Public Float			$ 605,182,616
Entity Common Stock, Shares Outstanding		785,146
ICFR Auditor Attestation Flag	true
Auditor Name	Moss Adams LLP
Auditor Location	San Francisco, California
Auditor Firm ID	659