Document and Entity Information
| Document and Entity Information | Nov. 30, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | MIDDLEFIELD BANC CORP |
| Entity Address, State or Province | OH |
| Amendment Flag | false |
| Entity Central Index Key | 0000836147 |
| Document Type | 8-K |
| Document Period End Date | Nov. 30, 2020 |
| Entity Incorporation State Country Code | OH |
| Entity File Number | 001-36613 |
| Entity Tax Identification Number | 34-1585111 |
| Entity Address, Address Line One | 15985 East High Street |
| Entity Address, City or Town | Middlefield |
| Entity Address, Postal Zip Code | 44062 |
| City Area Code | (440) |
| Local Phone Number | 632-1666 |
| 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 | MBCN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |