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Note 1 - Summary of Significant Accounting Policies - Year-end Mortgage Loans Held for Sale (Details) - Mortgage Loans [Member] - USD ($) | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Mortgage loans held for sale | $ 22,888,000 | $ 4,978,000
Less: Allowance to adjust to lower of cost or market | 0 | 0
Mortgage loans held for sale, net | $ 22,888,000 | $ 4,978,000

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

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- Definition Amount, after valuation allowance, of financing receivable held for sale and not part of disposal group. Excludes loan covered under loss sharing agreement and loan classified as investment in debt security. + 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=121593590&loc=d3e4428-111522 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 310 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=120238483&loc=d3e48678-111004 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |