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Note 20 - Subordinated Debentures (Details) - Business Trusts (USD $) | Dec. 31, 2014 | Dec. 31, 2013
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Subordinated Borrowing [Line Items] |  |
Preferred securities outstanding | $ 52,400,000 mbwm_PreferredSecuritiesOfSubsidiaryTrust | $ 32,000,000 mbwm_PreferredSecuritiesOfSubsidiaryTrust
Mercantile Bank Capital Trust I [Member] |  |
Subordinated Borrowing [Line Items] |  |
Preferred securities outstanding | 32,000,000 mbwm_PreferredSecuritiesOfSubsidiaryTrust / us-gaap_SubordinatedBorrowingAxis = mbwm_MercantileBankCapitalTrustIMember |
Firstbank Capital Trust I [Member] |  |
Subordinated Borrowing [Line Items] |  |
Preferred securities outstanding | 10,000,000 mbwm_PreferredSecuritiesOfSubsidiaryTrust / us-gaap_SubordinatedBorrowingAxis = mbwm_FirstbankCapitalTrustIMember |
Firstbank Capital Trust II [Member] |  |
Subordinated Borrowing [Line Items] |  |
Preferred securities outstanding | 10,000,000 mbwm_PreferredSecuritiesOfSubsidiaryTrust / us-gaap_SubordinatedBorrowingAxis = mbwm_FirstbankCapitalTrustIIMember |
Firstbank Capital Trust III [Member] |  |
Subordinated Borrowing [Line Items] |  |
Preferred securities outstanding | 7,500,000 mbwm_PreferredSecuritiesOfSubsidiaryTrust / us-gaap_SubordinatedBorrowingAxis = mbwm_FirstbankCapitalTrustIIIMember |
Firstbank Capital Trust IV [Member] |  |
Subordinated Borrowing [Line Items] |  |
Preferred securities outstanding | $ 7,500,000 mbwm_PreferredSecuritiesOfSubsidiaryTrust / us-gaap_SubordinatedBorrowingAxis = mbwm_FirstbankCapitalTrustIVMember |

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- Definition Trust preferred securities possesses characteristics of both equity and debt issues and are generally issued by bank holding companies. + References No definition available. + Details
Name: | mbwm_PreferredSecuritiesOfSubsidiaryTrust
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_SubordinatedBorrowingLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_SubordinatedBorrowingAxis=mbwm_FirstbankCapitalTrustIIIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_SubordinatedBorrowingAxis=mbwm_FirstbankCapitalTrustIIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubordinatedBorrowingAxis=mbwm_FirstbankCapitalTrustIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubordinatedBorrowingAxis=mbwm_FirstbankCapitalTrustIVMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_SubordinatedBorrowingAxis=mbwm_MercantileBankCapitalTrustIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |