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Note 8 - Commitments and Off-Balance Sheet Risk (Details Textual) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Reserve or Liability Balance for Financial Instruments With Off Balance Sheet Risk | $ 0 | $ 0

X
- Definition The carrying amount for the reserve or liability for financial instruments with off-balance sheet risk. + References No definition available. + Details
Name: | mbwm_ReserveOrLiabilityBalanceForFinancialInstrumentsWithOffBalanceSheetRisk
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant