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Note 21 - Regulatory Matters (Details) (USD $) | 0 Months Ended | 12 Months Ended | 0 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 24, 2014 | Sep. 24, 2014 | Jun. 25, 2014 | May 29, 2014 | Mar. 10, 2014 | Dec. 31, 2014 | Jan. 15, 2015 | Dec. 31, 2013 | Dec. 31, 2009 | Jan. 30, 2015
Note 21 - Regulatory Matters (Details) [Line Items] |  |  |  |  |  |  |  |  |  |
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments without Regulatory Approval |  |  |  |  |  | $ 87,700,000 us-gaap_StatutoryAccountingPracticesStatutoryAmountAvailableForDividendPaymentsWithoutRegulatoryApproval |  |  |  |
Common Stock, Dividends, Per Share, Cash Paid (in Dollars per share) | $ 0.12 us-gaap_CommonStockDividendsPerShareCashPaid | $ 0.12 us-gaap_CommonStockDividendsPerShareCashPaid | $ 0.12 us-gaap_CommonStockDividendsPerShareCashPaid | $ 2.00 us-gaap_CommonStockDividendsPerShareCashPaid | $ 0.12 us-gaap_CommonStockDividendsPerShareCashPaid |  |  |  |  |
Trust Preferred Securities Included in Tier One Capital |  |  |  |  |  | 52,400,000 mbwm_TrustPreferredSecuritiesIncludedInTierOneCapital |  | 32,000,000 mbwm_TrustPreferredSecuritiesIncludedInTierOneCapital |  |
Maximum Restricted Core Element Allowed in Tier One Capital, Percent |  |  |  |  |  | 25.00%mbwm_MaximumRestrictedCoreElementAllowedInTierOneCapitalPercent |  |  |  |
Maximum Level of Consolidated Aggregate Assets Allowing for Inclusion of Trust Preferred Securities in Tier One Capital |  |  |  |  |  |  |  |  | 15,000,000,000 mbwm_MaximumLevelOfConsolidatedAggregateAssetsAllowingForInclusionOfTrustPreferredSecuritiesInTierOneCapital |
Preferred Securities of Subsidiary Trust |  |  |  |  |  | 52,400,000 mbwm_PreferredSecuritiesOfSubsidiaryTrust |  | 32,000,000 mbwm_PreferredSecuritiesOfSubsidiaryTrust |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |
Note 21 - Regulatory Matters (Details) [Line Items] |  |  |  |  |  |  |  |  |  |
Common Stock, Dividends, Per Share, Declared (in Dollars per share) |  |  |  |  |  |  | $ 0.14 us-gaap_CommonStockDividendsPerShareDeclared / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |  |  |
Stock Repurchase Program, Authorized Amount |  |  |  |  |  |  |  |  |  | $ 20,000,000 us-gaap_StockRepurchaseProgramAuthorizedAmount1 / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

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- Definition The maximum level of consolidated aggregate assets allowing for inclusion of trust preferred securities in Tier 1 Capital under the Dodd-Frank Act. + References No definition available. + Details
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- Definition The maximum amount of restricted core elements allowed in tier one capital expressed as a percent. + References No definition available. + Details
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- Definition Trust preferred securities possesses characteristics of both equity and debt issues and are generally issued by bank holding companies. + References No definition available. + Details
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- Definition Trust preferred securities held by the trust that are included in Tier One Capital and subject to certain limitations. + References No definition available. + Details
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- Definition Aggregate dividends paid during the period for each share of common stock outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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- Definition Aggregate dividends declared during the period for each share of common stock outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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- Definition Amount of dividends that can be declared or paid without approval of regulatory agency. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(e)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph e -Article 4 + Details
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- Definition Amount of stock repurchase plan authorized. + References No definition available. + Details
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