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Note 10 - Fair Values of Financial Instruments (Details Textual) | Oct. 21, 2020
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Hastings, Michigan Branch Office [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member] |
Disposal Group, Including Discontinued Operation, Deposit Accounts, Premium of Book Balance | 5.00%

X
- Definition The premium of book balance of deposit accounts paid by the counter party in a disposal group transaction. + References No definition available. + Details
Name: | mbwm_DisposalGroupIncludingDiscontinuedOperationDepositAccountsPremiumOfBookBalance
Namespace Prefix: | mbwm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=mbwm_HastingsMichiganBranchOfficeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DisposalGroupClassificationAxis=us-gaap_DisposalGroupHeldforsaleNotDiscontinuedOperationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |