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Note 3 - Securities (Details) - Available-for-sale Securities (USD $)
Dec. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Securities available for sale, amortized cost $ 432,591,000us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 139,487,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
Securities available for sale, gross unrealized gains 3,897,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 1,791,000us-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 432,912,000us-gaap_AvailableForSaleSecurities [1] 131,178,000us-gaap_AvailableForSaleSecurities [1]
US Government Agencies Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Securities available for sale, amortized cost 194,894,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 108,279,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Securities available for sale, gross unrealized gains 1,612,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 263,000us-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 193,468,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 98,477,000us-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 92,656,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		 12,456,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, gross unrealized gains 1,123,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		 1,102,000us-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		 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale 93,561,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		 13,558,000us-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 132,347,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_MunicipalGeneralObligationBondsMember		 16,488,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_MunicipalGeneralObligationBondsMember
Securities available for sale, gross unrealized gains 1,042,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_MunicipalGeneralObligationBondsMember		 388,000us-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 133,082,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_MunicipalGeneralObligationBondsMember		 16,872,000us-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 10,769,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_MunicipalRevenueBondsMember		 878,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_MunicipalRevenueBondsMember
Securities available for sale, gross unrealized gains 117,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_MunicipalRevenueBondsMember		 38,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_MunicipalRevenueBondsMember
Securities available for sale, gross unrealized losses (13,000)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_MunicipalRevenueBondsMember		 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_MunicipalRevenueBondsMember
Securities available for sale 10,873,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_MunicipalRevenueBondsMember		 916,000us-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 1,925,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember		  
Securities available for sale, gross unrealized gains 3,000us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember		  
Securities available for sale, gross unrealized losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember		  
Securities available for sale 1,928,000us-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   1,386,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbwm_MutualFundsMember
Securities available for sale, gross unrealized gains   0us-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   $ 1,355,000us-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.