| Document and Entity Information | Oct. 20, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 20, 2021 |
| Entity Registrant Name | Farmers & Merchants Bancorp, Inc. |
| Entity Central Index Key | 0000792966 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-38084 |
| Entity Incorporation, State Country Code | OH |
| Entity Tax Identification Number | 34-1469491 |
| Entity Address, Address Line One | 307 North Defiance Street |
| Entity Address, City or Town | Archbold |
| Entity Address, State or Province | OH |
| Entity Address, Postal Zip Code | 43502 |
| City Area Code | (419) |
| Local Phone Number | 446-2501 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, No Par Value |
| Trading Symbol | FMAO |
| Security Exchange Name | NASDAQ |
