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Note 15 - Benefit Plans (Details) (USD $) In Millions, except Share data, unless otherwise specified | 0 Months Ended | 12 Months Ended
--- | --- | ---
Jan. 01, 2014 | Oct. 01, 2012 | Jan. 01, 2012 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Note 15 - Benefit Plans (Details) [Line Items] |  |  |  |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay | 4.25%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent | 4.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent | 3.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent |  |  |
Defined Contribution Plan, Cost Recognized |  |  |  | $ 0.9 us-gaap_DefinedContributionPlanCostRecognized | $ 0.5 us-gaap_DefinedContributionPlanCostRecognized | $ 0.4 us-gaap_DefinedContributionPlanCostRecognized
Deferred Compensation Liability, Current and Noncurrent |  |  |  | $ 3.7 us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent | $ 1.6 us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent |
Stock Issued During Period, Shares, Employee Stock Purchase Plans (in Shares) |  |  |  | 1,150 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans | 1,098 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans |
Stock Purchase Plan [Member] |  |  |  |  |  |
Note 15 - Benefit Plans (Details) [Line Items] |  |  |  |  |  |
Common Stock, Capital Shares Reserved for Future Issuance (in Shares) |  |  |  | 250,000 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance / us-gaap_PlanNameAxis = mbwm_StockPurchasePlanMember |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in Shares) |  |  |  | 28,850 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_PlanNameAxis = mbwm_StockPurchasePlanMember |  |

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- Definition Aggregate number of common shares reserved for future issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements. Represents currently earned compensation under compensation arrangements that is not actually paid until a later date. + References No definition available. + Details
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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of shares issued during the period as a result of an employee stock purchase plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5 + Details
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