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Note 2 - Business Combination (Details) (USD $) | 0 Months Ended |  |
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Jun. 01, 2014 | Mar. 31, 2015 | Dec. 31, 2014
Note 2 - Business Combination (Details) [Line Items] |  |  |
Goodwill |  | $ 49,473,000 us-gaap_Goodwill | $ 49,473,000 us-gaap_Goodwill
Minimum [Member] | Firstbank Corporation [Member] | London Interbank Offered Rate (LIBOR) [Member] | Acquired Trusts [Member] |  |  |
Note 2 - Business Combination (Details) [Line Items] |  |  |
Debt Instrument, Basis Spread on Variable Rate | 1.27%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember / us-gaap_DebtInstrumentAxis = mbwm_AcquiredTrustsMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |
Maximum [Member] | Firstbank Corporation [Member] | London Interbank Offered Rate (LIBOR) [Member] | Acquired Trusts [Member] |  |  |
Note 2 - Business Combination (Details) [Line Items] |  |  |
Debt Instrument, Basis Spread on Variable Rate | 1.99%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember / us-gaap_DebtInstrumentAxis = mbwm_AcquiredTrustsMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |
Firstbank Corporation [Member] |  |  |
Note 2 - Business Combination (Details) [Line Items] |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets | 1,500,000,000 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |
Business Combination, Branches Acquired | 46 mbwm_BusinessCombinationBranchesAcquired / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |
Business Acquisition, Conversion of Stock Per Share (in Shares) | 1 mbwm_BusinessAcquisitionConversionOfStockPerShare / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares (in Shares) | 8,087,272 us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |
Goodwill | 49,500,000 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt | 36,100,000 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |
Business Combination, Debt Assumed, Number of Trusts | 4 mbwm_BusinessCombinationDebtAssumedNumberOfTrusts / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |
Estimate of Fair Value Measurement [Member] | Firstbank Corporation [Member] |  |  |
Note 2 - Business Combination (Details) [Line Items] |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt | 21,100,000 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember |  |

X
- Definition The number of shares that each share of the acquiree is converted into shares of the acquirer. + References No definition available. + Details
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- Definition The number of banking branches acquired in a business combination. + References No definition available. + Details
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X
- Definition The number of trusts for which debt assumed in a business combination are payable to. + References No definition available. + Details
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- Definition Number of shares of equity interests issued or issuable to acquire entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b)(4) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
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- Definition Amount of assets acquired at the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=56956791&loc=d3e4845-128472 + Details
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- Definition Amount of long-term debt due after one year or the normal operating cycle, if longer, assumed at the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=56956791&loc=d3e4845-128472 + Details
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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 + Details
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