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Note 1 - Summary of Significant Accounting Policies (Details) (USD $) | 0 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jun. 01, 2014 | Dec. 31, 2014 | Dec. 31, 2013
Note 1 - Summary of Significant Accounting Policies (Details) [Line Items] |  |  |
Entity Incorporation, Date of Incorporation |  | Jul. 15, 1997 |
Entity Incorporation Date of Operations |  | Dec. 15, 1997 |
Business Acquisition, Effective Date of Acquisition | Jun. 01, 2014 |  |
Number of Operating Segments |  | 1 us-gaap_NumberOfOperatingSegments |
Loans and Leases Receivable, Deferred Income |  | $ 1,100,000 us-gaap_LoansAndLeasesReceivableDeferredIncome | $ 500,000 us-gaap_LoansAndLeasesReceivableDeferredIncome
Interest Income on Commercial and Mortgage Loans, Discontinued Period |  | 90 days |
Maximum Delinquency Period for Consumer and Credit Card Loans to be Charged-off |  | 120 days |
Repossessed Assets |  | 2,000,000 us-gaap_ForeclosedAssets | 2,900,000 us-gaap_ForeclosedAssets
Building Improvements [Member] | Minimum [Member] |  |  |
Note 1 - Summary of Significant Accounting Policies (Details) [Line Items] |  |  |
Property, Plant and Equipment, Useful Life |  | 5 years |
Building Improvements [Member] | Maximum [Member] |  |  |
Note 1 - Summary of Significant Accounting Policies (Details) [Line Items] |  |  |
Property, Plant and Equipment, Useful Life |  | 33 years |
Furniture and Fixtures [Member] | Minimum [Member] |  |  |
Note 1 - Summary of Significant Accounting Policies (Details) [Line Items] |  |  |
Property, Plant and Equipment, Useful Life |  | 3 years |
Furniture and Fixtures [Member] | Maximum [Member] |  |  |
Note 1 - Summary of Significant Accounting Policies (Details) [Line Items] |  |  |
Property, Plant and Equipment, Useful Life |  | 7 years |
Core Deposits [Member] |  |  |
Note 1 - Summary of Significant Accounting Policies (Details) [Line Items] |  |  |
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life |  | 10 years |
Firstbank Corporation [Member] |  |  |
Note 1 - Summary of Significant Accounting Policies (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 |  |
Mercantile Real Estate [Member] |  |  |
Note 1 - Summary of Significant Accounting Policies (Details) [Line Items] |  |  |
Business Initiation Date of Company |  | Jul. 21, 2003 |

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