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Note 2 - Business Combination (Details) - Acquired Impaired and Non-impaired Loans (Firstbank Corporation [Member], USD $)
Jun. 01, 2014
Firstbank Corporation [Member]
 
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period [Line Items]  
Contractually required payments $ 44,936,000us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodContractuallyRequiredPaymentsReceivableAtAcquisition
/ us-gaap_BusinessAcquisitionAxis
= mbwm_FirstbankCorporationMember
Nonaccretable difference 17,057,000us-gaap_CertainLoansAcquiredInTransferNonaccretableDifference
/ us-gaap_BusinessAcquisitionAxis
= mbwm_FirstbankCorporationMember
Expected cash flows 27,879,000us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodCashFlowsExpectedToBeCollectedAtAcquisition
/ us-gaap_BusinessAcquisitionAxis
= mbwm_FirstbankCorporationMember
Accretable yield 2,514,000us-gaap_CertainLoansAcquiredInTransferAccretableYield
/ us-gaap_BusinessAcquisitionAxis
= mbwm_FirstbankCorporationMember
Carrying balance $ 25,365,000us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodAtAcquisitionAtFairValue
/ us-gaap_BusinessAcquisitionAxis
= mbwm_FirstbankCorporationMember