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Note 4 - Loans and Allowance for Loan Losses (Details) - Retail Credit Risk by Collateral Type (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 2,120,760,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome	$ 2,089,277,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Home Equity and Other [Member] | Originated Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	53,707,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_HomeEquityAndOtherMember	50,059,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_HomeEquityAndOtherMember
Home Equity and Other [Member] | Acquired Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	99,279,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_HomeEquityAndOtherMember	109,219,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_HomeEquityAndOtherMember
Home Equity and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	152,986,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_HomeEquityAndOtherMember	159,278,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_HomeEquityAndOtherMember
1-4 Family Mortgages [Member] | Originated Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	44,165,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_OneToFourFamilyMortgagesMember	42,868,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_OneToFourFamilyMortgagesMember
1-4 Family Mortgages [Member] | Acquired Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	164,260,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_OneToFourFamilyMortgagesMember	171,828,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_OneToFourFamilyMortgagesMember
1-4 Family Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	208,425,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_OneToFourFamilyMortgagesMember	214,696,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = mbwm_OneToFourFamilyMortgagesMember
Originated Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	1,328,713,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember	1,246,116,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember	1,066,796,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_OriginatedLoanMember
Acquired Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 792,047,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember	$ 843,161,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome / us-gaap_FinancialInstrumentAxis = mbwm_AcquiredLoanMember

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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=49124121&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

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