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Note 3 - Securities (Details) (USD $) | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2014
Note 3 - Securities (Details) [Line Items] |  |  |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | $ 88,446,000 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue |  | 264,699,000 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 10,100,000 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss |  | 3,576,000 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss |
Available-for-sale Debt Securities, Amortized Cost Basis |  |  | 432,591,000 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis |
Available-for-sale Securities | 131,178,000 us-gaap_AvailableForSaleSecurities | [1] | 432,912,000 us-gaap_AvailableForSaleSecurities | [1]
Debt Securities [Member] |  |  |  |
Note 3 - Securities (Details) [Line Items] |  |  |  |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions |  |  | 341 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_DebtSecuritiesMember |
Michigan Strategic Fund Bonds [Member] |  |  |  |
Note 3 - Securities (Details) [Line Items] |  |  |  |
Available-for-sale Securities, Sold at Par | 10,300,000 us-gaap_AvailableforsaleSecuritiesSoldAtPar / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MichiganStrategicFundBondsMember |  |  |
State of Michigan and all its Political Subdivisions [Member] |  |  |  |
Note 3 - Securities (Details) [Line Items] |  |  |  |
Available-for-sale Debt Securities, Amortized Cost Basis | 17,400,000 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_StateOfMichiganAndAllItsPoliticalSubdivisionsMember |  | 113,100,000 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_StateOfMichiganAndAllItsPoliticalSubdivisionsMember |
Available-for-sale Securities | 17,800,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_StateOfMichiganAndAllItsPoliticalSubdivisionsMember |  | 113,900,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_StateOfMichiganAndAllItsPoliticalSubdivisionsMember |
All Other States and Their Political Subdivisions [Member] |  |  |  |
Note 3 - Securities (Details) [Line Items] |  |  |  |
Available-for-sale Debt Securities, Amortized Cost Basis |  |  | 30,000,000 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_AllOtherStatesAndTheirPoliticalSubdivisionsMember |
Available-for-sale Securities |  |  | 30,000,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_AllOtherStatesAndTheirPoliticalSubdivisionsMember |
Securities Other Than US Government Agencies and the State of Michigan and Political Subdivisions [Member] | Stockholders' Equity, Total [Member] | Securities Issued Concentration Risk [Member] |  |  |  |
Note 3 - Securities (Details) [Line Items] |  |  |  |
Concentration Risk, Percentage |  |  | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_StockholdersEquityTotalMember / us-gaap_ConcentrationRiskByTypeAxis = mbwm_SecuritiesIssuedConcentrationRiskMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_SecuritiesOtherThanUSGovernmentAgenciesAndTheStateOfMichiganAndPoliticalSubdivisionsMember |
US Government Agency Debt Obligations and Mortgage-Backed Securities [Member] |  |  |  |
Note 3 - Securities (Details) [Line Items] |  |  |  |
Available-for-sale Securities, Restricted | $ 94,400,000 us-gaap_AvailableForSaleSecuritiesRestricted / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_USGovernmentAgencyDebtObligationsAndMortgageBackedSecuritiesMember |  | 180,500,000 us-gaap_AvailableForSaleSecuritiesRestricted / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_USGovernmentAgencyDebtObligationsAndMortgageBackedSecuritiesMember |
[1] | See Note 18 for a description of the fair value hierarchy as well as a disclosure of levels for classes of financial assets and liabilities.

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- Definition This item represents the cost of debt 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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