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Note 2 - Business Combination (Details) (USD $) | 12 Months Ended | 0 Months Ended | 6 Months Ended | 24 Months Ended
--- | --- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2014 | Jun. 01, 2014 | Jun. 30, 2014 | Dec. 31, 2014
Note 2 - Business Combination (Details) [Line Items] |  |  |  |  |  |  |
Goodwill | $ 49,473,000 us-gaap_Goodwill | $ 0 us-gaap_Goodwill |  | $ 49,473,000 us-gaap_Goodwill |  |  | $ 49,473,000 us-gaap_Goodwill
Business Combination, Acquisition Related Costs | 5,447,000 us-gaap_BusinessCombinationAcquisitionRelatedCosts | 1,246,000 us-gaap_BusinessCombinationAcquisitionRelatedCosts | 0 us-gaap_BusinessCombinationAcquisitionRelatedCosts |  |  |  |
Firstbank Corporation [Member] |  |  |  |  |  |  |
Note 2 - Business Combination (Details) [Line Items] |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets |  |  |  |  | 1,452,880,000 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 | 8,087,272 us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |
Goodwill |  |  |  |  | 49,473,000 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |
Goodwill, Purchase Accounting Adjustments |  |  |  | (1,400,000)us-gaap_GoodwillPurchaseAccountingAdjustments / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment Net Deferred Tax Asset |  |  |  | 1,400,000 mbwm_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNetDeferredTaxAsset / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |  |
Business Combination, Acquisition Related Costs | 5,400,000 us-gaap_BusinessCombinationAcquisitionRelatedCosts / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember | 1,200,000 us-gaap_BusinessCombinationAcquisitionRelatedCosts / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |  |  |  |
Business Combination, Aggregate Merger Related Expenses Recorded by Acquirer and Acquiree |  |  |  |  |  |  | 12,200,000 mbwm_BusinessCombinationAggregateMergerRelatedExpensesRecordedByAcquirerAndAcquiree / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember
Business Combination, Acquired Receivables and Certain Loans, Estimated Uncollectible Cash Flows |  |  |  |  | 10,400,000 mbwm_BusinessCombinationAcquiredReceivablesAndCertainLoansEstimatedUncollectible / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |
Business Combination, Acquired Receivables and Certain Loans, Pre-acquisition Charge-offs |  |  |  |  | 2,800,000 mbwm_BusinessCombinationAcquiredReceivablesAndCertainLoansPreacquisitionChargeoffs / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |
Business Combination, Acquired Receivables and Certain Loans, Estimated Uncollectible Interest |  |  |  |  | 3,900,000 mbwm_BusinessCombinationAcquiredReceivablesAndCertainLoansEstimatedUncollectibleInterest / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |
Business Combination, Acquired Receivables, Gross Contractual Amount |  |  |  |  | 926,400,000 us-gaap_BusinessCombinationAcquiredReceivablesGrossContractualAmount / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |
Business Combination, Acquired Receivables, Unaccredted Discount |  |  |  |  | $ 8,100,000 mbwm_BusinessCombinationAcquiredReceivablesUnaccredtedDiscount / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |

X
- Definition The number of shares that each share of the acquiree is converted into shares of the acquirer. + References No definition available. + Details
Name: | mbwm_BusinessAcquisitionConversionOfStockPerShare
Namespace Prefix: | mbwm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition For receivables acquired in a business combination, including certain loans and debt securities acquired in a transfer (as defined), this element represents the best estimate at the acquisition date of the contractual cash flows not expected to be collected, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables. + References No definition available. + Details
Name: | mbwm_BusinessCombinationAcquiredReceivablesAndCertainLoansEstimatedUncollectible
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition For receivables acquired in a business combination, including certain loans and debt securities acquired in a transfer (as defined), this element represents the best estimate at the acquisition date of the future interest not expected to be collected, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables. + References No definition available. + Details
Name: | mbwm_BusinessCombinationAcquiredReceivablesAndCertainLoansEstimatedUncollectibleInterest
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of expense to write-down the amortized cost basis of loans, before acquisition, to fair value. Expressed as a component of nonaccretable difference. + References No definition available. + Details
Name: | mbwm_BusinessCombinationAcquiredReceivablesAndCertainLoansPreacquisitionChargeoffs
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the unaccreted discount. + References No definition available. + Details
Name: | mbwm_BusinessCombinationAcquiredReceivablesUnaccredtedDiscount
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This represents the aggregate in merger related expenses recorded by Mercantile Bank and Firstbank during 2014 and 2013. + References No definition available. + Details
Name: | mbwm_BusinessCombinationAggregateMergerRelatedExpensesRecordedByAcquirerAndAcquiree
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The number of banking branches acquired in a business combination. + References No definition available. + Details
Name: | mbwm_BusinessCombinationBranchesAcquired
Namespace Prefix: | mbwm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to net deferred tax asset account in connection with a business combination for which the initial accounting was incomplete. + References No definition available. + Details
Name: | mbwm_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentNetDeferredTaxAsset
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | mbwm_Note2BusinessCombinationDetailsLineItems
Namespace Prefix: | mbwm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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
Name: | us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the gross contractual amounts receivable, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables. + 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 (b)(2) -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472 + Details
Name: | us-gaap_BusinessCombinationAcquiredReceivablesGrossContractualAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e1043-128460 + Details
Name: | us-gaap_BusinessCombinationAcquisitionRelatedCosts
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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=39739317&loc=d3e4845-128472 + Details
Name: | us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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
Name: | us-gaap_Goodwill
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the 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 -Subparagraph (g) -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 805 -SubTopic 10 -Section 25 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e961-128460 + Details
Name: | us-gaap_GoodwillPurchaseAccountingAdjustments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=mbwm_FirstbankCorporationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |