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Note 4 - Loans and Allowance for Loan Losses (Details) - Concentrations Within the Loan Portfolio (USD $) | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
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Note 4 - Loans and Allowance for Loan Losses (Details) - Concentrations Within the Loan Portfolio [Line Items] |  |  |
Commercial real estate loans to lessors of non-residential buildings | $ 2,089,277,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome | $ 1,053,243,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome | $ 1,041,189,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Commercial Real Estate Loans to Lessors of Non-Residential Buildings [Member] |  |  |
Note 4 - Loans and Allowance for Loan Losses (Details) - Concentrations Within the Loan Portfolio [Line Items] |  |  |
Commercial real estate loans to lessors of non-residential buildings | $ 480,170,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_CommercialRealEstateLoansToLessorsOfNonResidentialBuildingsMember | $ 299,446,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_CommercialRealEstateLoansToLessorsOfNonResidentialBuildingsMember |
Commercial real estate loans to lessors of non-residential buildings | 23.00%mbwm_LoanPortfolioClassesAndSegmentsAsPercentageOfAggregateLoans / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_CommercialRealEstateLoansToLessorsOfNonResidentialBuildingsMember | 28.40%mbwm_LoanPortfolioClassesAndSegmentsAsPercentageOfAggregateLoans / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_CommercialRealEstateLoansToLessorsOfNonResidentialBuildingsMember |

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- Definition Loan portfolio classes and segments as a percentage of aggregate loans. + References No definition available. + Details
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- Definition Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522 + Details
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