Note 6 - Investment Securities Available for Sale - Amortized Cost and Fair Values of Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Investment securities available for sale, at fair value	$ 104,951	$ 114,376
Amortized cost	102,203	112,557
Gross unrealized gains	2,974	2,702
Gross unrealized losses	(226)	(883)
Investment securities available for sale, at fair value	104,951	114,376
US Government Agencies Debt Securities [Member]
Investment securities available for sale, at fair value	9,514	10,236
Amortized cost	9,383	10,158
Gross unrealized gains	180	174
Gross unrealized losses	(49)	(96)
Investment securities available for sale, at fair value	9,514	10,236
US States and Political Subdivisions Debt Securities [Member]
Investment securities available for sale, at fair value	77,635	81,223
Investment securities available for sale, at fair value	77,635	81,223
US States and Political Subdivisions Debt Securities [Member] \| Taxable [Member]
Investment securities available for sale, at fair value	1,760	1,740
Amortized cost	1,613	1,615
Gross unrealized gains	149	129
Gross unrealized losses	(2)	(4)
Investment securities available for sale, at fair value	1,760	1,740
US States and Political Subdivisions Debt Securities [Member] \| Tax Exempt [Member]
Investment securities available for sale, at fair value	75,875	79,483
Amortized cost	73,582	78,327
Gross unrealized gains	2,333	1,678
Gross unrealized losses	(40)	(522)
Investment securities available for sale, at fair value	75,875	79,483
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investment securities available for sale, at fair value	17,512	20,069
Amortized cost	17,455	20,128
Gross unrealized gains	192	202
Gross unrealized losses	(135)	(261)
Investment securities available for sale, at fair value	17,512	20,069
Debt Securities [Member]
Investment securities available for sale, at fair value	104,661	113,237
Amortized cost	102,033	111,807
Gross unrealized gains	2,854	2,313
Gross unrealized losses	(226)	(883)
Investment securities available for sale, at fair value	104,661	113,237
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Investment securities available for sale, at fair value		1,709
Amortized cost		1,579
Gross unrealized gains		130
Gross unrealized losses
Investment securities available for sale, at fair value		1,709
Equity Securities In Financial Institutions [Member]
Investment securities available for sale, at fair value	290	1,139
Amortized cost	170	750
Gross unrealized gains	120	389
Gross unrealized losses
Investment securities available for sale, at fair value	$ 290	$ 1,139

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

+ Details

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

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=mbcn_TaxableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=mbcn_TaxExemptMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=mbcn_EquitySecuritiesInFinancialInstitutionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: