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Note 14 - Commitments and Off-balance-sheet Risk (Details) - Exposure to Credit Losses (USD $) | Dec. 31, 2014 | Dec. 31, 2013
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Note 14 - Commitments and Off-balance-sheet Risk (Details) - Exposure to Credit Losses [Line Items] |  |
Contractual amounts of financial instruments with off-balance sheet risk | $ 781,748,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability | $ 374,543,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
Unused lines of Credit [Member] | Commercial Portfolio Segment [Member] |  |
Note 14 - Commitments and Off-balance-sheet Risk (Details) - Exposure to Credit Losses [Line Items] |  |
Contractual amounts of financial instruments with off-balance sheet risk | 554,856,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability / us-gaap_FinancialInstrumentAxis = us-gaap_UnusedLinesOfCreditMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember | 257,937,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability / us-gaap_FinancialInstrumentAxis = us-gaap_UnusedLinesOfCreditMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialPortfolioSegmentMember
Unused lines of Credit [Member] | Retail 1-4 Family Mortgages [Member] |  |
Note 14 - Commitments and Off-balance-sheet Risk (Details) - Exposure to Credit Losses [Line Items] |  |
Contractual amounts of financial instruments with off-balance sheet risk | 60,983,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability / us-gaap_FinancialInstrumentAxis = us-gaap_UnusedLinesOfCreditMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_Retail1To4FamilyMortgagesMember | 23,429,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability / us-gaap_FinancialInstrumentAxis = us-gaap_UnusedLinesOfCreditMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_Retail1To4FamilyMortgagesMember
Unused lines of Credit [Member] | Consumer Credit Card Financing Receivable [Member] |  |
Note 14 - Commitments and Off-balance-sheet Risk (Details) - Exposure to Credit Losses [Line Items] |  |
Contractual amounts of financial instruments with off-balance sheet risk | 11,649,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability / us-gaap_FinancialInstrumentAxis = us-gaap_UnusedLinesOfCreditMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerCreditCardFinancingReceivableMember | 9,013,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability / us-gaap_FinancialInstrumentAxis = us-gaap_UnusedLinesOfCreditMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerCreditCardFinancingReceivableMember
Unused lines of Credit [Member] | Consumer Other Financing Receivable [Member] |  |
Note 14 - Commitments and Off-balance-sheet Risk (Details) - Exposure to Credit Losses [Line Items] |  |
Contractual amounts of financial instruments with off-balance sheet risk | 8,673,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability / us-gaap_FinancialInstrumentAxis = us-gaap_UnusedLinesOfCreditMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember | 5,695,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability / us-gaap_FinancialInstrumentAxis = us-gaap_UnusedLinesOfCreditMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember
Loan Origination Commitments [Member] |  |
Note 14 - Commitments and Off-balance-sheet Risk (Details) - Exposure to Credit Losses [Line Items] |  |
Contractual amounts of financial instruments with off-balance sheet risk | 110,126,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability / us-gaap_FinancialInstrumentAxis = us-gaap_LoanOriginationCommitmentsMember | 58,799,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability / us-gaap_FinancialInstrumentAxis = us-gaap_LoanOriginationCommitmentsMember
Standby Letters of Credit [Member] |  |
Note 14 - Commitments and Off-balance-sheet Risk (Details) - Exposure to Credit Losses [Line Items] |  |
Contractual amounts of financial instruments with off-balance sheet risk | $ 35,461,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability / us-gaap_CreditFacilityAxis = us-gaap_StandbyLettersOfCreditMember | $ 19,670,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability / us-gaap_CreditFacilityAxis = us-gaap_StandbyLettersOfCreditMember

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- Definition The fair value of financial liabilities, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611 + Details
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