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Investment Securities (Summary Of Contractual Maturity Of Debt Securities Classified As Held To Maturity) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]    
Held to Maturity (at fair value), After One But Within Five Years $ 9us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue  
Held to Maturity (at fair value), After Five But Within Ten Years 14,720us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue  
Held to Maturity (at amortized cost), After Ten Years 121,508us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue  
Held-to-maturity Securities, Fair Value 136,237us-gaap_HeldToMaturitySecuritiesFairValue 139,171us-gaap_HeldToMaturitySecuritiesFairValue
Held to Maturity (at amortized cost), After One But Within Five Years 9us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount  
Held to Maturity (at amortized cost), After Five But Within Ten Years 14,336us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount  
Held to Maturity (at amortized cost), After Ten Years 119,900us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount  
Total Held to Maturity Securities, Amortized Cost 134,245us-gaap_HeldToMaturitySecurities 138,421us-gaap_HeldToMaturitySecurities
U.S. Agency Obligations [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Held to Maturity (at amortized cost), After Ten Years 22,059us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Held-to-maturity Securities, Fair Value 22,059us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 22,669us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Held to Maturity (at amortized cost), After Ten Years 21,387us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Total Held to Maturity Securities, Amortized Cost 21,387us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 22,072us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
U.S. GSEs [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Held to Maturity (at fair value), After Five But Within Ten Years 9,707us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		  
Held-to-maturity Securities, Fair Value 9,707us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		 9,557us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Held to Maturity (at amortized cost), After Five But Within Ten Years 9,512us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		  
Total Held to Maturity Securities, Amortized Cost 9,512us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		 9,498us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
FHLB Obligations [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Held to Maturity (at fair value), After Five But Within Ten Years 4,906us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanBankCertificatesAndObligationsFHLBMember		  
Held-to-maturity Securities, Fair Value 4,906us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanBankCertificatesAndObligationsFHLBMember		 4,820us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanBankCertificatesAndObligationsFHLBMember
Held to Maturity (at amortized cost), After Five But Within Ten Years 4,730us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanBankCertificatesAndObligationsFHLBMember		  
Total Held to Maturity Securities, Amortized Cost 4,730us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanBankCertificatesAndObligationsFHLBMember		 4,720us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanBankCertificatesAndObligationsFHLBMember
Agency MBSs [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Held to Maturity (at fair value), After One But Within Five Years 9us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Held to Maturity (at fair value), After Five But Within Ten Years 107us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Held to Maturity (at amortized cost), After Ten Years 7,987us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Held-to-maturity Securities, Fair Value 8,103us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 8,342us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Held to Maturity (at amortized cost), After One But Within Five Years 9us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Held to Maturity (at amortized cost), After Five But Within Ten Years 94us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Held to Maturity (at amortized cost), After Ten Years 7,753us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Total Held to Maturity Securities, Amortized Cost 7,856us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 8,109us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Agency CMOs [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Held to Maturity (at amortized cost), After Ten Years 91,462us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember		  
Held-to-maturity Securities, Fair Value 91,462us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember		 93,783us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
Held to Maturity (at amortized cost), After Ten Years 90,760us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember		  
Total Held to Maturity Securities, Amortized Cost $ 90,760us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember		 $ 94,022us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember