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Note 20 - Subordinated Debentures (Details) (USD $) In Millions, unless otherwise specified | 0 Months Ended |  |
--- | --- | --- | ---
Jun. 01, 2014 | Dec. 31, 2014 | Jun. 01, 2014
Debt Disclosure [Abstract] |  |  |
Number of Business Trusts |  | 5 mbwm_NumberOfBusinessTrusts |
Business Combination, Debt Assumed, Number of Trusts |  | 4 mbwm_BusinessCombinationDebtAssumedNumberOfTrusts |
Fair Value Discount of Subordinated Debentures Related to Assumed Business Trusts at Time of Merger |  |  | $ 15.0 mbwm_FairValueDiscountOfSubordinatedDebenturesRelatedToAssumedBusinessTrustsAtTimeOfMerger
Amortization of Fair Value Discount Related to Subordinated Debentures of Assumed Business Trusts | $ 0.7 mbwm_AmortizationOfFairValueDiscountRelatedToSubordinatedDebenturesOfAssumedBusinessTrusts |  |
Amortization Period of Fair Value Discount Related to Subordinated Debentures of Assumed Business Trusts | 21 years 6 months |  |

X
- Definition Represents the annual amortization of the fair value discount related to subordinated debentures of assumed business trusts over a period of time. + References No definition available. + Details
Name: | mbwm_AmortizationOfFairValueDiscountRelatedToSubordinatedDebenturesOfAssumedBusinessTrusts
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the amortization period of the fair value discount related to subordinated debentures of assumed business trusts. + References No definition available. + Details
Name: | mbwm_AmortizationPeriodOfFairValueDiscountRelatedToSubordinatedDebenturesOfAssumedBusinessTrusts
Namespace Prefix: | mbwm_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of trusts for which debt assumed in a business combination are payable to. + References No definition available. + Details
Name: | mbwm_BusinessCombinationDebtAssumedNumberOfTrusts
Namespace Prefix: | mbwm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the fair value discount of subordinated debentures related to assumed business trusts at the time of the merger. + References No definition available. + Details
Name: | mbwm_FairValueDiscountOfSubordinatedDebenturesRelatedToAssumedBusinessTrustsAtTimeOfMerger
Namespace Prefix: | mbwm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Represents the Company's business trusts. + References No definition available. + Details
Name: | mbwm_NumberOfBusinessTrusts
Namespace Prefix: | mbwm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_DebtDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration