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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, Measurements, Nonrecurring [Member] - Fair Value, Inputs, Level 3 [Member] - Appraisal Adjustments [Member] - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Fair value estimate

$ 1,570

Valuation Techniques / Unobservable Input

Appraisal of collateral (1)

Minimum [Member]

Discount rate

0.00%

Maximum [Member]

Discount rate

59.70%

Weighted Average [Member]

Discount rate

28.20%

Impaired Loans [Member]

Fair value estimate

$ 3,072

$ 4,928

Valuation Techniques / Unobservable Input

^[1],[2]

Appraisal of collateral / Appraisal adjustments

Discounted cash flow

Impaired Loans [Member] | Minimum [Member]

Discount rate

0.00%

3.10%

Impaired Loans [Member] | Maximum [Member]

Discount rate

86.10%

7.00%

Impaired Loans [Member] | Weighted Average [Member]

Discount rate

5.10%

Other Real Estate Owned [Member]

Fair value estimate

$ 32

$ 511

Valuation Techniques / Unobservable Input

^[1],[2]

Appraisal of collateral / Appraisal adjustments

Appraisal of collateral / Appraisal adjustments

Other Real Estate Owned [Member] | Minimum [Member]

Discount rate

0.00%

0.00%

Other Real Estate Owned [Member] | Maximum [Member]

Discount rate

10.00%

Other Real Estate Owned [Member] | Weighted Average [Member]

Discount rate

10.00%

[1]	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.
[2]	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.

X

- Definition

This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a nonrecurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

+ References

No definition available.

+ Details

Name:	us-gaap_AssetsFairValueDisclosureNonrecurring
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Description of the inputs and valuation technique(s) used to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementsValuationTechniques
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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

X

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_ValuationTechniqueAxis=mbcn_AppraisalAdjustmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByAssetClassAxis=mbcn_ImpairedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByAssetClassAxis=mbcn_OtherRealEstateOwnedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: