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Note 3 - Securities (Details) - Available-for-sale Securities (USD $) | Dec. 31, 2014 | Dec. 31, 2013
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Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Securities available for sale, amortized cost | $ 432,591,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost |  | $ 139,487,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost |
Securities available for sale, gross unrealized gains | 3,897,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax |  | 1,791,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax |
Securities available for sale, gross unrealized losses | (3,576,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax |  | (10,100,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax |
Securities available for sale | 432,912,000 us-gaap_AvailableForSaleSecurities | [1] | 131,178,000 us-gaap_AvailableForSaleSecurities | [1]
US Government Agencies Debt Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Securities available for sale, amortized cost | 194,894,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |  | 108,279,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Securities available for sale, gross unrealized gains | 1,612,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |  | 263,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Securities available for sale, gross unrealized losses | (3,038,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |  | (10,065,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Securities available for sale | 193,468,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |  | 98,477,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember |
Collateralized Mortgage Backed Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Securities available for sale, amortized cost | 92,656,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |  | 12,456,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Securities available for sale, gross unrealized gains | 1,123,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |  | 1,102,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Securities available for sale, gross unrealized losses | (218,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |  | 0 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Securities available for sale | 93,561,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |  | 13,558,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Municipal General Obligation Bonds [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Securities available for sale, amortized cost | 132,347,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalGeneralObligationBondsMember |  | 16,488,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalGeneralObligationBondsMember |
Securities available for sale, gross unrealized gains | 1,042,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalGeneralObligationBondsMember |  | 388,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalGeneralObligationBondsMember |
Securities available for sale, gross unrealized losses | (307,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalGeneralObligationBondsMember |  | (4,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalGeneralObligationBondsMember |
Securities available for sale | 133,082,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalGeneralObligationBondsMember |  | 16,872,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalGeneralObligationBondsMember |
Municipal Revenue Bonds [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Securities available for sale, amortized cost | 10,769,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalRevenueBondsMember |  | 878,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalRevenueBondsMember |
Securities available for sale, gross unrealized gains | 117,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalRevenueBondsMember |  | 38,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalRevenueBondsMember |
Securities available for sale, gross unrealized losses | (13,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalRevenueBondsMember |  | 0 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalRevenueBondsMember |
Securities available for sale | 10,873,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalRevenueBondsMember |  | 916,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MunicipalRevenueBondsMember |
Other Debt Obligations [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Securities available for sale, amortized cost | 1,925,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember |  |  |
Securities available for sale, gross unrealized gains | 3,000 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember |  |  |
Securities available for sale, gross unrealized losses | 0 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember |  |  |
Securities available for sale | 1,928,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember |  |  |
Mutual Funds [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Securities available for sale, amortized cost |  |  | 1,386,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MutualFundsMember |
Securities available for sale, gross unrealized gains |  |  | 0 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MutualFundsMember |
Securities available for sale, gross unrealized losses |  |  | (31,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MutualFundsMember |
Securities available for sale |  |  | $ 1,355,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbwm_MutualFundsMember |
[1] | See Note 18 for a description of the fair value hierarchy as well as a disclosure of levels for classes of financial assets and liabilities.

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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