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Note 12 - Stock-based Compensation (Details) - Summary of Stock Option Grants and Exercises (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Note 12 - Stock-based Compensation (Details) - Summary of Stock Option Grants and Exercises [Line Items] |  |  |
Cash received from stock option exercises | $ 282,000 us-gaap_ProceedsFromStockOptionsExercised | $ 289,000 us-gaap_ProceedsFromStockOptionsExercised | $ 227,000 us-gaap_ProceedsFromStockOptionsExercised
Mercantile Plans [Member] |  |  |
Note 12 - Stock-based Compensation (Details) - Summary of Stock Option Grants and Exercises [Line Items] |  |  |
Aggregate intrinsic value of stock options exercised | 11,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue / us-gaap_PlanNameAxis = mbwm_MercantilePlansMember | 408,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue / us-gaap_PlanNameAxis = mbwm_MercantilePlansMember | 307,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue / us-gaap_PlanNameAxis = mbwm_MercantilePlansMember
Cash received from stock option exercises | 50,000 us-gaap_ProceedsFromStockOptionsExercised / us-gaap_PlanNameAxis = mbwm_MercantilePlansMember | 289,000 us-gaap_ProceedsFromStockOptionsExercised / us-gaap_PlanNameAxis = mbwm_MercantilePlansMember | 227,000 us-gaap_ProceedsFromStockOptionsExercised / us-gaap_PlanNameAxis = mbwm_MercantilePlansMember
Tax benefit realized from stock option exercises | 0 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions / us-gaap_PlanNameAxis = mbwm_MercantilePlansMember | 0 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions / us-gaap_PlanNameAxis = mbwm_MercantilePlansMember | 0 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions / us-gaap_PlanNameAxis = mbwm_MercantilePlansMember
Weighted average per share fair value of stock options granted (in Dollars per share) | $ 2.72 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_PlanNameAxis = mbwm_MercantilePlansMember |  |
Firstbank Plans [Member] |  |  |
Note 12 - Stock-based Compensation (Details) - Summary of Stock Option Grants and Exercises [Line Items] |  |  |
Aggregate intrinsic value of stock options exercised | 333,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember |  |
Cash received from stock option exercises | 232,000 us-gaap_ProceedsFromStockOptionsExercised / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember |  |
Tax benefit realized from stock option exercises | $ 116,000 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions / us-gaap_PlanNameAxis = mbwm_FirstbankPlansMember |  |

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- Definition Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period. + 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 (j) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + 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 (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + 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 (d)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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