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Investment Securities (Narrative) (Details) (USD $)
 12 Months Ended 6 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]        
Carrying value of securities pledged $ 317,220,000us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedOtherDebtSecuritiesAvailableForSaleOrHeldForInvestment $ 315,530,000us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedOtherDebtSecuritiesAvailableForSaleOrHeldForInvestment   $ 315,530,000us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedOtherDebtSecuritiesAvailableForSaleOrHeldForInvestment
Securities classified as trading 0us-gaap_TradingSecurities 0us-gaap_TradingSecurities   0us-gaap_TradingSecurities
Impairment charge 0us-gaap_ImpairmentOfInvestments 166,000us-gaap_ImpairmentOfInvestments 0us-gaap_ImpairmentOfInvestments  
Securities available for sale, at fair value 203,473,000us-gaap_AvailableForSaleSecurities 252,513,000us-gaap_AvailableForSaleSecurities   252,513,000us-gaap_AvailableForSaleSecurities
Securities held to maturity, fair value 139,171,000us-gaap_HeldToMaturitySecuritiesFairValue 137,087,000us-gaap_HeldToMaturitySecuritiesFairValue   137,087,000us-gaap_HeldToMaturitySecuritiesFairValue
Securities transferred from available-for-sale to held-to-maturity category, amortized cost 12,626,000mbvt_TransferOfSecuritiesFromAvailableForSaleToHeldToMaturity 152,887,000mbvt_TransferOfSecuritiesFromAvailableForSaleToHeldToMaturity 0mbvt_TransferOfSecuritiesFromAvailableForSaleToHeldToMaturity  
Securities transferred from available-for-sale to held-to-maturity category, unrealized holding gain 8,000mbvt_OtherComprehensiveIncomeLossTransfersFromAvailableForSaleSecuritiesToHeldToMaturityNetOfTax (3,533,000)mbvt_OtherComprehensiveIncomeLossTransfersFromAvailableForSaleSecuritiesToHeldToMaturityNetOfTax 0mbvt_OtherComprehensiveIncomeLossTransfersFromAvailableForSaleSecuritiesToHeldToMaturityNetOfTax  
Securities transferred from available-for-sale to held-to-maturity category, remaining unamortized loss, net of tax (1,705,000)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax 6,311,000us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax 336,000us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax  
Investment in FHLBB stock 4,378,000us-gaap_FederalHomeLoanBankStock 7,496,000us-gaap_FederalHomeLoanBankStock   7,496,000us-gaap_FederalHomeLoanBankStock
Available for Sale, Fair Value 203,473,000us-gaap_AvailableForSaleSecurities 252,513,000us-gaap_AvailableForSaleSecurities   252,513,000us-gaap_AvailableForSaleSecurities
Transferred From Available-For-Sale to Held-To-Maturity [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Securities transferred from available-for-sale to held-to-maturity category, amortized cost 12,630,000mbvt_TransferOfSecuritiesFromAvailableForSaleToHeldToMaturity
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesMember		     152,890,000mbvt_TransferOfSecuritiesFromAvailableForSaleToHeldToMaturity
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesMember
Securities transferred from available-for-sale to held-to-maturity category, fair value 12,640,000mbvt_FairValueOfTransfersOfSecuritiesFromAvailableForSaleToHeldToMaturity
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesMember		     147,450,000mbvt_FairValueOfTransfersOfSecuritiesFromAvailableForSaleToHeldToMaturity
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesMember
Securities transferred from available-for-sale to held-to-maturity category, unrealized holding gain 3,530,000mbvt_OtherComprehensiveIncomeLossTransfersFromAvailableForSaleSecuritiesToHeldToMaturityNetOfTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesMember		      
Securities transferred from available-for-sale to held-to-maturity category, remaining unamortized loss 4,350,000us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesMember		      
Securities transferred from available-for-sale to held-to-maturity category, remaining unamortized loss, net of tax 2,820,000us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesMember		      
CLOs [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Impairment charge   166,000us-gaap_ImpairmentOfInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember		    
Securities available for sale, at fair value   37,907,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember		   37,907,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember
Available for Sale, Fair Value   37,907,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember		   37,907,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember
Unrealized Gains (Losses) on Securities [Member] | Transferred From Available-For-Sale to Held-To-Maturity [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Securities transferred from available-for-sale to held-to-maturity category, unrealized holding gain 8,000mbvt_OtherComprehensiveIncomeLossTransfersFromAvailableForSaleSecuritiesToHeldToMaturityNetOfTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 (3,533,000)mbvt_OtherComprehensiveIncomeLossTransfersFromAvailableForSaleSecuritiesToHeldToMaturityNetOfTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		    
Securities transferred from available-for-sale to held-to-maturity category, remaining unamortized loss, net of tax $ (480,000)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 $ 3,296,000us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember