| 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 |
