Document and Entity Information - USD ($)	Dec. 31, 2020	Feb. 28, 2021	Jun. 30, 2020
Cover [Abstract]
Entity Registrant Name	FARMERS & MERCHANTS BANCORP
Entity Central Index Key	0001085913
Current Fiscal Year End Date	--12-31
Document Type	10-K/A
Amendment Flag	true
Amendment Description	Farmers & Merchants Bancorp (the “Company”) is filing this Amendment No. 1 on Form 10-K/A (this “Amendment”) to amend our Annual Report on Form 10-K for the year ended December 31, 2020 (“Form 10-K”), originally filed with the Securities and Exchange Commission (the “SEC”) on March 15, 2021. The sole purpose of this Amendment is to include the information required by Items 10 through 14 of Part III of Form 10-K, which was previously omitted from our Form 10-K in reliance on General Instruction G(3) to Form 10-K, which permits the information in the above referenced items to be incorporated in the Form 10-K by reference from our definitive proxy statement if such statement is filed no later than 120 days after our fiscal year-end. We are filing this Amendment to include Part III information in our Form 10-K because a definitive proxy statement containing such information will not be filed by April 30, 2021, due to the Company’s determination to delay the Annual Meeting of Stockholders, normally held in May, in view of the possibility that COVID-19-related recommendations and directives of the Centers for Disease Control, the California Department of Public Health and local County Health departments might permit an in-person annual meeting at a later date. Except as set forth in this Amendment, no other changes are made to our Form 10-K. Unless expressly stated, this Amendment does not reflect events occurring after the filing of the Form 10-K, nor does it modify or otherwise update in any way the disclosures contained in the Form 10-K. Accordingly, this Amendment should be read in conjunction with our Form 10-K and with our filings with the SEC subsequent to the filing of our Form 10-K. Pursuant to Rule 12b-15 under the Securities Exchange Act of 1934, as amended, this Amendment also contains certifications pursuant to Section 302 of the Sarbanes-Oxley Act of 2002, which are attached hereto. Because no financial statements have been included in this Amendment and this Amendment does not contain or amend any disclosure with respect to Items 307 and 308 of Regulation S-K, paragraphs 3, 4, and 5 of the certifications have been omitted.
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
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 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
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 560,250,536
Entity Common Stock, Shares Outstanding		789,646