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Note 4 - Loans and Allowance for Loan Losses (Details) - Retail Credit Risk by Collateral Type (USD $) | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
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Financing Receivable, Recorded Investment [Line Items] |  |  |
Total retail | $ 2,089,277,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome | $ 1,053,243,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome | $ 1,041,189,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Originated Loan [Member] | Retail Home Equity and Other [Member] |  |  |
Financing Receivable, Recorded Investment [Line Items] |  |  |
Total retail | 50,059,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailHomeEquityAndOtherMember | 35,080,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailHomeEquityAndOtherMember |
Originated Loan [Member] | Retail 1-4 Family Mortgages [Member] |  |  |
Financing Receivable, Recorded Investment [Line Items] |  |  |
Total retail | 42,868,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_Retail1To4FamilyMortgagesMember | 31,467,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_Retail1To4FamilyMortgagesMember |
Originated Loan [Member] |  |  |
Financing Receivable, Recorded Investment [Line Items] |  |  |
Total retail | 1,246,116,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember | 1,053,243,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember |
Acquired Loan [Member] | Retail Home Equity and Other [Member] |  |  |
Financing Receivable, Recorded Investment [Line Items] |  |  |
Total retail | 109,219,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailHomeEquityAndOtherMember | 0 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailHomeEquityAndOtherMember |
Acquired Loan [Member] | Retail 1-4 Family Mortgages [Member] |  |  |
Financing Receivable, Recorded Investment [Line Items] |  |  |
Total retail | 171,828,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_Retail1To4FamilyMortgagesMember | 0 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_Retail1To4FamilyMortgagesMember |
Acquired Loan [Member] |  |  |
Financing Receivable, Recorded Investment [Line Items] |  |  |
Total retail | 843,161,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember | 0 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember |
Retail Home Equity and Other [Member] |  |  |
Financing Receivable, Recorded Investment [Line Items] |  |  |
Total retail | 159,278,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailHomeEquityAndOtherMember | 35,080,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailHomeEquityAndOtherMember |
Retail 1-4 Family Mortgages [Member] |  |  |
Financing Receivable, Recorded Investment [Line Items] |  |  |
Total retail | $ 214,696,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_Retail1To4FamilyMortgagesMember | $ 31,467,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_Retail1To4FamilyMortgagesMember |

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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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