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Note 4 - Loans and Allowance for Loan Losses (Details) - Activity in Allowance for Loan Losses and the Recorded Investments in Loans (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |
Allowance for loan losses | $ 22,821,000 us-gaap_FinancingReceivableAllowanceForCreditLosses | $ 28,677,000 us-gaap_FinancingReceivableAllowanceForCreditLosses | $ 36,532,000 us-gaap_FinancingReceivableAllowanceForCreditLosses
Allowance for loan losses: individually evaluated for impairment |  | 12,386,000 us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 | 17,189,000 us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Allowance for loan losses: collectively evaluated for impairment |  | 10,435,000 us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment | 11,488,000 us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Total loans: |  |  |
Loans | 2,089,277,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome | 1,053,243,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome | 1,041,189,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Loans: individually evaluated for impairment |  | 36,868,000 us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment | 57,279,000 us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Loans: collectively evaluated for impairment |  | 1,016,375,000 us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment | 983,910,000 us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
Provision for loan losses | (3,000,000)us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet | (7,200,000)us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet | (3,100,000)us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet
Charge-offs |  | (5,290,000)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs | (12,644,000)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs
Recoveries |  | 6,634,000 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery | 7,889,000 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery
Allowance for loan losses |  | 22,821,000 us-gaap_FinancingReceivableAllowanceForCreditLosses | 28,677,000 us-gaap_FinancingReceivableAllowanceForCreditLosses
Originated Loan [Member] | Commercial Portfolio Segment [Member] |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |
Allowance for loan losses | 20,455,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Allowance for loan losses: individually evaluated for impairment | 9,374,000 us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Allowance for loan losses: collectively evaluated for impairment | 8,362,000 us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Total loans: |  |  |
Loans | 1,153,189,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 986,696,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |
Loans: individually evaluated for impairment | 45,021,000 us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Loans: collectively evaluated for impairment | 1,108,168,000 us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Provision for loan losses | (3,140,000)us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Charge-offs | (876,000)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Recoveries | 1,297,000 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Allowance for loan losses | 17,736,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Originated Loan [Member] | Retail Loan [Member] |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |
Allowance for loan losses | 2,358,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Allowance for loan losses: individually evaluated for impairment | 777,000 us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Allowance for loan losses: collectively evaluated for impairment | 710,000 us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Total loans: |  |  |
Loans | 92,927,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | 66,547,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |
Loans: individually evaluated for impairment | 2,835,000 us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Loans: collectively evaluated for impairment | 90,092,000 us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Provision for loan losses | (721,000)us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Charge-offs | (554,000)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Recoveries | 404,000 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Allowance for loan losses | 1,487,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Originated Loan [Member] | Unallocated Financing Receivables [Member] |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |
Allowance for loan losses | 8,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember |  |
Allowance for loan losses: individually evaluated for impairment | 0 us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember |  |
Allowance for loan losses: collectively evaluated for impairment | 76,000 us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember |  |
Total loans: |  |  |
Provision for loan losses | 68,000 us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember |  |
Charge-offs | 0 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember |  |
Recoveries | 0 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember |  |
Allowance for loan losses | 76,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember |  |
Originated Loan [Member] |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |
Allowance for loan losses | 22,821,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember |  |
Allowance for loan losses: individually evaluated for impairment | 10,151,000 us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember |  |
Allowance for loan losses: collectively evaluated for impairment | 9,148,000 us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember |  |
Total loans: |  |  |
Loans | 1,246,116,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember | 1,053,243,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember |
Loans: individually evaluated for impairment | 47,856,000 us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember |  |
Loans: collectively evaluated for impairment | 1,198,260,000 us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember |  |
Provision for loan losses | (3,793,000)us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember |  |
Charge-offs | (1,430,000)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember |  |
Recoveries | 1,701,000 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember |  |
Allowance for loan losses | 19,299,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember |  |
Acquired Loan [Member] | Commercial Portfolio Segment [Member] |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |
Allowance for loan losses | 0 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Total loans: |  |  |
Loans | 562,114,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 0 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |
Provision for loan losses | 734,000 us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Charge-offs | (55,000)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Recoveries | 2,000 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Allowance for loan losses | 681,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember |  |
Acquired Loan [Member] | Retail Loan [Member] |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |
Allowance for loan losses | 0 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Total loans: |  |  |
Loans | 281,047,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | 0 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |
Provision for loan losses | 59,000 us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Charge-offs | (16,000)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Recoveries | 18,000 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Allowance for loan losses | 61,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember |  |
Acquired Loan [Member] | Unallocated Financing Receivables [Member] |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |
Allowance for loan losses | 0 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember |  |
Total loans: |  |  |
Provision for loan losses | 0 us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember |  |
Charge-offs | 0 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember |  |
Recoveries | 0 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember |  |
Allowance for loan losses | 0 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember |  |
Acquired Loan [Member] |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |
Allowance for loan losses | 0 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember |  |
Total loans: |  |  |
Loans | 843,161,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember | 0 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember |
Provision for loan losses | 793,000 us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember |  |
Charge-offs | (71,000)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember |  |
Recoveries | 20,000 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember |  |
Allowance for loan losses | 742,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember |  |
Commercial Portfolio Segment [Member] |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |
Allowance for loan losses |  | 26,043,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 33,431,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Allowance for loan losses: individually evaluated for impairment |  | 11,260,000 us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 16,860,000 us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Allowance for loan losses: collectively evaluated for impairment |  | 9,195,000 us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 9,183,000 us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Total loans: |  |  |
Loans | 1,715,303,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 986,696,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 968,506,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Loans: individually evaluated for impairment |  | 33,240,000 us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 55,138,000 us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Loans: collectively evaluated for impairment |  | 953,456,000 us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 913,368,000 us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Provision for loan losses |  | (6,730,000)us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | (2,800,000)us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Charge-offs |  | (5,120,000)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | (12,075,000)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Recoveries |  | 6,262,000 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 7,487,000 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Allowance for loan losses |  | 20,455,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 26,043,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Retail Loan [Member] |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |
Allowance for loan losses |  | 2,645,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | 3,019,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember
Allowance for loan losses: individually evaluated for impairment |  | 1,126,000 us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | 329,000 us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember
Allowance for loan losses: collectively evaluated for impairment |  | 1,232,000 us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | 2,316,000 us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember
Total loans: |  |  |
Loans | 373,974,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | 66,547,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | 72,683,000 us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember
Loans: individually evaluated for impairment |  | 3,628,000 us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | 2,141,000 us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember
Loans: collectively evaluated for impairment |  | 62,919,000 us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | 70,542,000 us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember
Provision for loan losses |  | (489,000)us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | (207,000)us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember
Charge-offs |  | (170,000)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | (569,000)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember
Recoveries |  | 372,000 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | 402,000 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember
Allowance for loan losses |  | 2,358,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember | 2,645,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_RetailLoanMember
Unallocated Financing Receivables [Member] |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |
Allowance for loan losses |  | (11,000)us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember | 82,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember
Allowance for loan losses: individually evaluated for impairment |  | 0 us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember | 0 us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember
Allowance for loan losses: collectively evaluated for impairment |  | 8,000 us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember | (11,000)us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember
Total loans: |  |  |
Provision for loan losses |  | 19,000 us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember | (93,000)us-gaap_AllowanceForLoanAndLeaseLossesAdjustmentsNet / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember
Charge-offs |  | 0 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember | 0 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember
Recoveries |  | 0 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember | 0 us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember
Allowance for loan losses |  | $ 8,000 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember | $ (11,000)us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_UnallocatedFinancingReceivablesMember

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