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Note 8 - Deposits (Details) - USD ($) $ in Millions | Dec. 31, 2015 | Dec. 31, 2014
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Disclosure Text Block [Abstract] |  |
Time Deposits, More Than $250,000 | $ 180.4 | $ 223.0

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- Definition Amount of time deposit liabilities, including certificates of deposit, in denominations of $250,000 or more. + References No definition available. + Details
Name: | mbwm_TimeDepositsMoreThan250000
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_DisclosureTextBlockAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration