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

The carrying amount for the reserve or liability for financial instruments with off-balance sheet risk.

No definition available.

Name:	mbwm_ReserveOrLiabilityBalanceForFinancialInstrumentsWithOffBalanceSheetRisk
Namespace Prefix:	mbwm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant