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Note 10 - Risks and Uncertainties - Loans at Risk (Details) - Commercial Portfolio Segment [Member] - COVID-19 [Member] $ in Thousands | Jun. 30, 2020 USD ($)
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Number of loans at risk | 1,255
Balance of loans at risk | $ 487,807
% of total loans at risk | 43.90%
Retail Loan [Member] |
Number of loans at risk | 270
Balance of loans at risk | $ 195,550
% of total loans at risk | 17.60%
Multifamily Loan [Member] |
Number of loans at risk | 354
Balance of loans at risk | $ 120,697
% of total loans at risk | 10.90%
Ambulatory Care, Nursing/Rehabilitation and Social Assistance [Member] |
Number of loans at risk | 218
Balance of loans at risk | $ 81,491
% of total loans at risk | 7.30%
Hospitality and Tourism Loan [Member] |
Number of loans at risk | 58
Balance of loans at risk | $ 44,923
% of total loans at risk | 4.10%
Restaurant/food service/bar Loan [Member] |
Number of loans at risk | 136
Balance of loans at risk | $ 24,938
% of total loans at risk | 2.20%
Other Loans [Member] |
Number of loans at risk | 219
Balance of loans at risk | $ 20,208
% of total loans at risk | 1.80%

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- Definition The potential impact of the occurrence of the defined risk or adverse resolution of an uncertainty on the entity's amount of loans, including a range of potential loss if reasonably estimable. + References No definition available. + Details
Name: | mbcn_UnusualRiskOrUncertaintyImpactAmountOfLoans
Namespace Prefix: | mbcn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The potential impact of the occurrence of the defined risk or adverse resolution of an uncertainty on the entity's number of loans, including a range of potential loss if reasonably estimable. + References No definition available. + Details
Name: | mbcn_UnusualRiskOrUncertaintyImpactNumberOfLoans
Namespace Prefix: | mbcn_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition The potential impact of the occurrence of the defined risk or adverse resolution of an uncertainty on the entity's percentage of total loans, including a range of potential loss if reasonably estimable. + References No definition available. + Details
Name: | mbcn_UnusualRiskOrUncertaintyImpactPercentOfTotalLoans
Namespace Prefix: | mbcn_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_UnusualRiskOrUncertaintyByNatureAxis=mbcn_COVID19Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbcn_RetailLoanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbcn_MultifamilyLoanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbcn_AmbulatoryCareNursingrehabilitationAndSocialAssistanceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbcn_HospitalityAndTourismLoanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbcn_RestaurantFoodServiceBarLoanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbcn_OtherLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |