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Note 5 - Fair Value Measurements - Additional Quantitative Information About Assets Measured at Fair Value on Non-recurring Basis (Details) - Fair Value, Nonrecurring [Member] - Fair Value, Inputs, Level 3 [Member] - Appraisal of Collateral [Member] $ in Thousands | Mar. 31, 2025 USD ($) | Dec. 31, 2024 USD ($)
--- | --- | ---
Collateral-dependent loans |  | $ 3,087 | $ 3,321
Measurement Input, Appraised Value [Member] | Minimum [Member] |  |  |
Collateral-dependent loans, range | [1] | 0 |
Measurement Input, Appraised Value [Member] | Maximum [Member] |  |  |
Collateral-dependent loans, range | [2] | 0.239 |
Measurement Input, Appraised Value [Member] | Weighted Average [Member] |  |  |
Collateral-dependent loans, range |  | 0.239 | 0.239
[1] | Fair value is generally determined through independent appraisals of the underlying collateral, which generally include various level III inputs that are not identifiable, less any associated allowance.
[2] | Appraisals may be adjusted by management for qualitative factors such as economic conditions and estimated liquidation expenses. The range and weighted average of liquidation expenses and other appraisal adjustments are presented as a percent of the appraisal.

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- Definition Value of input used to measure impaired loans. + References No definition available. + Details
Name: | mbcn_ImpairedLoansMeasurementInput
Namespace Prefix: | mbcn_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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- Definition Fair value of impaired loans receivable. + References No definition available. + Details
Name: | mbcn_ImpairedLoansReceivableFairValueDisclosure
Namespace Prefix: | mbcn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputAppraisedValueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |