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Note 1 - Summary of Significant Accounting Policies - Year-end Mortgage Loans Held for Sale (Details) - USD ($) | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Mortgage loans held for sale, net | $ 5,000,000 | $ 1,100,000
Mortgage Loans [Member] |  |
Mortgage loans held for sale | 4,978,000 | 1,122,000
Less: Allowance to adjust to lower of cost or market | 0 | 0
Mortgage loans held for sale, net | $ 4,978,000 | $ 1,122,000

X
- Definition Amount of allowance to adjust mortgage loans receivable held for sale to lower of cost or market price. + References No definition available. + Details
Name: | mbwm_AllowanceToAdjustToLowerOfCostOrMarket
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount, before allowance to adjust to lower of cost or market, of mortgage loan receivables held-for-sale that are not part of a disposal group. + References No definition available. + Details
Name: | mbwm_LoansReceivableHeldForSaleAmountNotPartOfDisposalGroupMortgage
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount, after allowance for bad debt, of loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522 + Details
Name: | us-gaap_LoansReceivableHeldForSaleNetNotPartOfDisposalGroup
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mbwm_MortgageLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |