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Note 3 - Securities (Details) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Dec. 31, 2014
Note 3 - Securities (Details) [Line Items] |  |  |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 131,795,000 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue |  | $ 264,699,000 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 1,879,000 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss |  | 3,576,000 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss |
Available-for-sale Debt Securities, Amortized Cost Basis | 410,485,000 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis |  |  |
Available-for-sale Securities | 413,693,000 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 | 147 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_DebtSecuritiesMember |  |  |
State of Michigan and all its Political Subdivisions [Member] |  |  |  |
Note 3 - Securities (Details) [Line Items] |  |  |  |
Available-for-sale Debt Securities, Amortized Cost Basis | 113,700,000 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_StateOfMichiganAndAllItsPoliticalSubdivisionsMember |  | 113,100,000 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_StateOfMichiganAndAllItsPoliticalSubdivisionsMember |
Available-for-sale Securities | 115,000,000 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 | 29,500,000 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_AllOtherStatesAndTheirPoliticalSubdivisionsMember |  | 30,000,000 us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_AllOtherStatesAndTheirPoliticalSubdivisionsMember |
Available-for-sale Securities | 29,500,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_AllOtherStatesAndTheirPoliticalSubdivisionsMember |  | 30,000,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_AllOtherStatesAndTheirPoliticalSubdivisionsMember |
Securities Other Than U.S. Government Agencies and the State of Michigan and Political Subdivisions [Member] | Securities Issued Concentration Risk [Member] | Stockholders' Equity, Total [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 |  |  |
U.S. Government Agency Debt Obligations and Mortgage-Backed Securities [Member] |  |  |  |
Note 3 - Securities (Details) [Line Items] |  |  |  |
Available-for-sale Securities, Restricted | 148,200,000 us-gaap_AvailableForSaleSecuritiesRestricted / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_USGovernmentAgencyDebtObligationsAndMortgageBackedSecuritiesMember |  | $ 167,600,000 us-gaap_AvailableForSaleSecuritiesRestricted / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_USGovernmentAgencyDebtObligationsAndMortgageBackedSecuritiesMember |
[1] | See Note 12 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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- Definition The carrying amount of securities categorized neither as held-to-maturity nor trading which serve as security for borrowings or other arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 9 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5086-111524 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX210.4-08(b)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph b -Article 4 + Details
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