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Note 4 - Loans and Allowance for Loan Losses (Details) - Allowance Related to Troubled Debt Restructurings (USD $)
Dec. 31, 2014
Dec. 31, 2013
Commercial:    
Allowance for losses, troubled debt restructurings $ 5,793,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment $ 10,399,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
Commercial and Industrial [Member]    
Commercial:    
Allowance for losses, troubled debt restructurings 16,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_CommercialAndIndustrialMember		 187,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_CommercialAndIndustrialMember
Commercial Vacant Land and Land Development and Residential Construction Loan [Member]    
Commercial:    
Allowance for losses, troubled debt restructurings 151,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_CommercialVacantLandAndLandDevelopmentAndResidentialConstructionLoanMember		 798,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_CommercialVacantLandAndLandDevelopmentAndResidentialConstructionLoanMember
Commercial Real Estate Owner Occupied Loan [Member]    
Commercial:    
Allowance for losses, troubled debt restructurings 182,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_CommercialRealEstateOwnerOccupiedLoanMember		 528,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_CommercialRealEstateOwnerOccupiedLoanMember
Commercial Real Estate Non Owner Occupied Loan [Member]    
Commercial:    
Allowance for losses, troubled debt restructurings 4,778,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_CommercialRealEstateNonOwnerOccupiedLoanMember		 7,828,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_CommercialRealEstateNonOwnerOccupiedLoanMember
Commercial Real Estate Multi Family and Residential Rental Loan [Member]    
Commercial:    
Allowance for losses, troubled debt restructurings 666,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_CommercialRealEstateMultiFamilyAndResidentialRentalLoanMember		 1,010,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_CommercialRealEstateMultiFamilyAndResidentialRentalLoanMember
Commercial Portfolio Segment [Member]    
Commercial:    
Allowance for losses, troubled debt restructurings 5,793,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember		 10,351,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
Retail Home Equity and Other [Member]    
Commercial:    
Allowance for losses, troubled debt restructurings 0us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_RetailHomeEquityAndOtherMember		 0us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_RetailHomeEquityAndOtherMember
Retail 1-4 Family Mortgages [Member]    
Commercial:    
Allowance for losses, troubled debt restructurings 0us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_Retail1To4FamilyMortgagesMember		 48,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_Retail1To4FamilyMortgagesMember
Retail Loan [Member]    
Commercial:    
Allowance for losses, troubled debt restructurings $ 0us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_RetailLoanMember		 $ 48,000us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= mbwm_RetailLoanMember