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Note 6 - Mortgage Loan Servicing (Details) - Mortgage Loans Serviced for Others (USD $)
Dec. 31, 2014
Mortgage loan portfolios serviced for:  
Mortgage loans serviced for others $ 590,820,000us-gaap_ContinuingInvolvementWithTransferredFinancialAssetsPrincipalAmountOutstanding
Federal Home Loan Mortgage Corporation [Member]  
Mortgage loan portfolios serviced for:  
Mortgage loans serviced for others 589,118,000us-gaap_ContinuingInvolvementWithTransferredFinancialAssetsPrincipalAmountOutstanding
/ us-gaap_QualitativeAndQuantitativeInformationTransferorsContinuingInvolvementTransferredFinancialAssetsByTransferOrTransfereeAxis
= mbwm_FederalHomeLoanMortgageCorporationMember
Federal Home Loan Bank [Member]  
Mortgage loan portfolios serviced for:  
Mortgage loans serviced for others $ 1,702,000us-gaap_ContinuingInvolvementWithTransferredFinancialAssetsPrincipalAmountOutstanding
/ us-gaap_QualitativeAndQuantitativeInformationTransferorsContinuingInvolvementTransferredFinancialAssetsByTransferOrTransfereeAxis
= mbwm_FederalHomeLoanBankMember