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Note 10 - Risks and Uncertainties - Loans at Risk (Details) - Commercial Portfolio Segment [Member] - COVID-19 [Member] $ in Thousands | Mar. 31, 2020 USD ($)
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Number of loans at risk | 787
Balance of loans at risk | $ 441,731
% of total loans at risk | 44.26%
Residential Non-owner Occupied Loan [Member] |
Number of loans at risk | 337
Balance of loans at risk | $ 142,725
% of total loans at risk | 14.30%
Retail Loan [Member] |
Number of loans at risk | 220
Balance of loans at risk | $ 197,073
% of total loans at risk | 19.75%
Restaurant/food service/bar Loan [Member] |
Number of loans at risk | 48
Balance of loans at risk | $ 16,868
% of total loans at risk | 1.69%
Hospitality and Tourism Loan [Member] |
Number of loans at risk | 32
Balance of loans at risk | $ 45,225
% of total loans at risk | 4.53%
Self-storage Facility Loan [Member] |
Number of loans at risk | 29
Balance of loans at risk | $ 25,622
% of total loans at risk | 2.57%
Other Loans [Member] |
Number of loans at risk | 121
Balance of loans at risk | $ 14,218
% of total loans at risk | 1.42%

X
- 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_ResidentialNonownerOccupiedLoanMember
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_RestaurantFoodServiceBarLoanMember
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_SelfstorageFacilityLoanMember
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: |