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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 131,795,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue $ 264,699,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 1,879,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss 3,576,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Available-for-sale Debt Securities, Amortized Cost Basis 410,485,000us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis  
Available-for-sale Securities 413,693,000us-gaap_AvailableForSaleSecurities [1] 432,912,000us-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 147us-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 113,700,000us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_StateOfMichiganAndAllItsPoliticalSubdivisionsMember		 113,100,000us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_StateOfMichiganAndAllItsPoliticalSubdivisionsMember
Available-for-sale Securities 115,000,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_StateOfMichiganAndAllItsPoliticalSubdivisionsMember		 113,900,000us-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 29,500,000us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_AllOtherStatesAndTheirPoliticalSubdivisionsMember		 30,000,000us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_AllOtherStatesAndTheirPoliticalSubdivisionsMember
Available-for-sale Securities 29,500,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_AllOtherStatesAndTheirPoliticalSubdivisionsMember		 30,000,000us-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 148,200,000us-gaap_AvailableForSaleSecuritiesRestricted
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_USGovernmentAgencyDebtObligationsAndMortgageBackedSecuritiesMember		 $ 167,600,000us-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.