| Document and Entity Information | Oct. 26, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 26, 2020 |
| Entity Registrant Name | FARMERS & MERCHANTS BANCORP |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 000-26099 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001085913 |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | FMCB |
