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Note 12 - Stock-based Compensation (Details) - Summary of Options Outstanding (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |  |
Exercise price range, lower range limit | $ 6.21 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit | $ 6.21 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit | $ 6.21 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit
Exercise price range, upper range limit | $ 35.88 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit | $ 40.28 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit | $ 40.28 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit
Number of Options Outstanding (in Shares) | 35,335 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions |  |
Weighted Average Remaining Contractual Life | 2 years | 1 year 146 days | 1 year 328 days
Weighted Average Exercise Price | $ 31.09 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1 |  |
Number of Options Exercisable (in Shares) | 28,847 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions |  |
Options Exercisable, Weighted Average Exercise Price | $ 33.11 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1 |  |
Exercise Price Range 1 [Member] |  |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |  |
Exercise price range, lower range limit | $ 6.21 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange1Member |  |
Exercise price range, upper range limit | $ 8.00 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange1Member |  |
Number of Options Outstanding (in Shares) | 2,700 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange1Member |  |
Weighted Average Remaining Contractual Life | 328 days |  |
Weighted Average Exercise Price | $ 6.21 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1 / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange1Member |  |
Number of Options Exercisable (in Shares) | 2,700 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange1Member |  |
Options Exercisable, Weighted Average Exercise Price | $ 6.21 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1 / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange1Member |  |
Exercise Price Range 2 [Member] |  |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |  |
Exercise price range, lower range limit | $ 20.01 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange2Member |  |
Exercise price range, upper range limit | $ 24.00 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange2Member |  |
Number of Options Outstanding (in Shares) | 6,488 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange2Member |  |
Weighted Average Remaining Contractual Life | 6 years 328 days |  |
Weighted Average Exercise Price | $ 22.15 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1 / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange2Member |  |
Number of Options Exercisable (in Shares) | 0 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange2Member |  |
Exercise Price Range 3 [Member] |  |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |  |
Exercise price range, lower range limit | $ 32.01 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange3Member |  |
Exercise price range, upper range limit | $ 36.00 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange3Member |  |
Number of Options Outstanding (in Shares) | 26,147 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange3Member |  |
Weighted Average Remaining Contractual Life | 328 days |  |
Weighted Average Exercise Price | $ 35.88 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1 / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange3Member |  |
Number of Options Exercisable (in Shares) | 26,147 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange3Member |  |
Options Exercisable, Weighted Average Exercise Price | $ 35.88 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1 / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange3Member |  |
Exercise Price Range 4 [Member] | Firstbank Plans [Member] |  |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |  |
Exercise price range, lower range limit | $ 4.00 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange4Member |  |
Exercise price range, upper range limit | $ 8.00 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange4Member |  |
Number of Options Outstanding (in Shares) | 63,590 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange4Member |  |
Weighted Average Remaining Contractual Life | 4 years 328 days |  |
Weighted Average Exercise Price | $ 6.53 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1 / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange4Member |  |
Number of Options Exercisable (in Shares) | 56,385 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange4Member |  |
Options Exercisable, Weighted Average Exercise Price | $ 6.70 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1 / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange4Member |  |
Exercise Price Range 5 [Member] | Firstbank Plans [Member] |  |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |  |
Exercise price range, lower range limit | $ 8.01 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange5Member |  |
Exercise price range, upper range limit | $ 12.00 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange5Member |  |
Number of Options Outstanding (in Shares) | 68,430 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange5Member |  |
Weighted Average Remaining Contractual Life | 3 years 255 days |  |
Weighted Average Exercise Price | $ 13.20 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1 / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange5Member |  |
Number of Options Exercisable (in Shares) | 68,430 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange5Member |  |
Options Exercisable, Weighted Average Exercise Price | $ 13.20 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1 / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange5Member |  |
Exercise Price Range 6 [Member] | Firstbank Plans [Member] |  |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |  |
Exercise price range, lower range limit | $ 20.01 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange6Member |  |
Exercise price range, upper range limit | $ 25.00 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange6Member |  |
Number of Options Outstanding (in Shares) | 85,962 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange6Member |  |
Weighted Average Remaining Contractual Life | 1 year 146 days |  |
Weighted Average Exercise Price | $ 22.43 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1 / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange6Member |  |
Number of Options Exercisable (in Shares) | 85,962 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange6Member |  |
Options Exercisable, Weighted Average Exercise Price | $ 22.43 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1 / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbwm_ExercisePriceRange6Member |  |
Firstbank Plans [Member] |  |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |  |
Exercise price range, lower range limit | $ 5.19 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember |  |
Exercise price range, upper range limit | $ 24.46 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember |  |
Number of Options Outstanding (in Shares) | 217,982 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember |  |
Weighted Average Remaining Contractual Life | 3 years 36 days |  |
Weighted Average Exercise Price | $ 14.89 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1 / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember |  |
Number of Options Exercisable (in Shares) | 210,777 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember |  |
Options Exercisable, Weighted Average Exercise Price | $ 15.22 us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1 / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember |  |

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- Definition Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average price as of the balance sheet date at which grantees could acquire the underlying shares with respect to all outstanding stock options which are in the customized range of exercise prices. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average remaining contractual term of outstanding stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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