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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 ($)
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

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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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

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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:

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- 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: