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Note 18 - Fair Value Disclosure 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 | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
--- | --- | ---
Impaired loans, fair value estimate | [1] | $ 5,166 | $ 1,075
Minimum [Member] | Measurement Input, Discount Rate [Member] |  |  |
Impaired loans, discount rate | [2] | 0.403 | 0
Impaired loans, discount rate | [2] | 0.403 | 0
Maximum [Member] | Measurement Input, Discount Rate [Member] |  |  |
Impaired loans, discount rate | [2] | 0.474 | 1
Impaired loans, discount rate | [2] | 0.474 | 1
Weighted Average [Member] | Measurement Input, Discount Rate [Member] |  |  |
Impaired loans, discount rate | [2] | (0.418) | (0.406)
Impaired loans, discount rate | [2] | (0.418) | (0.406)
[1] | Fair value is generally determined through independent appraisals of the underlying collateral, which generally include various level III inputs which 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 Fair value portion of impaired loans. + References No definition available. + Details
Name: | mbcn_ImpairedLoansFairValue
Namespace Prefix: | mbcn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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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- 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: |
Period Type: |

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- Details
Name: | us-gaap_ValuationTechniqueAxis=mbcn_AppraisalOfCollateralMember
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: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
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
Name: | srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |