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Note 2 - Business Combination (Details) - Pro Forma Information (Firstbank Corporation [Member], USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Firstbank Corporation [Member] |  |
Note 2 - Business Combination (Details) - Pro Forma Information [Line Items] |  |
Net interest income | $ 98,607,000 mbwm_BusinessAcquisitionsProFormaNetInterestIncome / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember | $ 93,862,000 mbwm_BusinessAcquisitionsProFormaNetInterestIncome / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember
Noninterest expense | 81,295,000 mbwm_BusinessAcquisitionsProFormaNoninterestExpense / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember | 93,263,000 mbwm_BusinessAcquisitionsProFormaNoninterestExpense / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember
Net income | $ 22,659,000 us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember | $ 16,068,000 us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember
Net income per diluted share (in Dollars per share) | $ 1.33 us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember | $ 0.96 us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember

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- Definition The pro forma net interest Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period. + References No definition available. + Details
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- Definition The pro forma noninterest expense for the period as if the business combination or combinations had been completed at the beginning of a period. + References No definition available. + Details
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- Definition The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)-(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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- Definition The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)-(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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