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Note 12 - Stock-based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended | 3 Months Ended | 0 Months Ended
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Jun. 30, 2014 | Jun. 01, 2014 | Jun. 26, 2014
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Grants in Period (in Shares) |  | 0 mbwm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityGrantsInPeriod |  | 0 mbwm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityGrantsInPeriod | 0 mbwm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityGrantsInPeriod | 0 mbwm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityGrantsInPeriod |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in Shares) | 277,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in Shares) | 6,488 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross |  |  |  |  |  |
Employee Stock Option [Member] | Firstbank Corporation [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period | 10 years |  |  |  |  |  |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options (in Dollars) | 0.1 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |  |  |  |  |  |  |
Employee Stock Option [Member] | Non-Executive Officers [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 21 days |  |  |  |  |  |  |  |
Employee Stock Option [Member] | Director [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 5 years |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period | 10 years |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent | 125.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember |  |  |  |  |  |  |  |
Employee Stock Option [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 1 year |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period | 10 years |  |  |  |  |  |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition | 2 years |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in Shares) |  | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |  |  |  |
Options Granted During 2006, 2007 and 2008 [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 2 years |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period | 7 years |  |  |  |  |  |  |  |
Restricted Stock Granted During 2006, 2007 and 2008 [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 4 years |  |  |  |  |  |  |  |
Restricted Stock Granted in 2012 [Member] | Directors and Certain Employees [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 2 years |  |  |  |  |  |  |  |
Restricted Stock Granted in 2014 [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 3 years |  |  |  |  |  |  |  |
Stock Options Granted in 2014 [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 2 years |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period | 7 years |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent | 110.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent / us-gaap_AwardTypeAxis = mbwm_StockOptionsGrantedIn2014Member |  |  |  |  |  |  |  |
Restricted Stock [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award Accelerated Compensation Cost (in Dollars) |  |  |  |  |  |  | 0.2 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition | 3 years |  |  |  |  |  |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options (in Dollars) | 2.0 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |  |  |  |  |  |  |  |
Stock Option Grants for 50 Shares or Less [Member] | Firstbank Corporation [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 1 year |  |  |  |  |  |  |  |
Stock Option Grants for Over 50 Shares [Member] | Firstbank Corporation [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 5 years |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | 20.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage / us-gaap_AwardTypeAxis = mbwm_StockOptionGrantsForOver50SharesMember / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |  |  |  |  |  |  |  |
Firstbank Corporation [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in Shares) |  |  |  |  |  |  |  | 282,200 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Number (in Shares) |  |  |  |  |  |  |  | 258,400 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber / us-gaap_BusinessAcquisitionAxis = mbwm_FirstbankCorporationMember |
Maximum [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options (in Dollars) | 0.1 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value (in Dollars) | 0.1 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1 / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |  |  |  |
Director [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, Issued for Services (in Shares) |  |  |  |  |  |  |  |  | 7,375 us-gaap_StockIssuedDuringPeriodSharesIssuedForServices / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember
Allocated Share-based Compensation Expense (in Dollars) |  |  |  |  |  |  |  |  | 0.2 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_TitleOfIndividualAxis = us-gaap_DirectorMember
Stock Issued for Services, Share-based Compensation Expense Period |  |  |  |  |  |  |  |  | 6 months
Firstbank Plans [Member] |  |  |  |  |  |  |  |  |
Note 12 - Stock-based Compensation (Details) [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in Shares) | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value (in Dollars) | 1.3 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1 / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember |  |  |  |  |  |  |  |

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