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Investment Securities (Summary Of Gross Unrealized Losses On Investment Securities Available For Sale And Length Of Time) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Schedule Of Available-For-Sale And Held-To-Maturity Securities [Line Items]    
Available-for-sale Securities, Less Than 12 Months, Fair Value $ 34,006us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue $ 24,357us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Available-for-sale Securities, Less Than 12 Months, Loss 148mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments 84mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
Available-for-sale Securities, 12 Months or More, Fair Value 24,737us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 39,203us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Available-for-sale Securities, 12 Months or More, Loss 275mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments 639mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total 58,743us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue 63,560us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total 423mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments 723mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
Held to Maturity Securities, Less Than 12 Months, Fair Value 7,926us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue 14,338us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Held to Maturity Securities, Less Than 12 Months, Loss 44mbvt_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss 108mbvt_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Held to Maturity Securities, 12 Months or More, Fair Value 6,428us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 43,911us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Held to Maturity Securities, 12 Months or More, Loss 71mbvt_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss 411mbvt_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Held to Maturity Securities, Continuous Unrealized Loss Position, Fair Value, Total 14,354us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue 58,249us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Held to Maturity Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total 115mbvt_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss 519mbvt_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
U.S. Treasury Obligations [Member]    
Schedule Of Available-For-Sale And Held-To-Maturity Securities [Line Items]    
Available-for-sale Securities, Less Than 12 Months, Fair Value   5,080us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
Available-for-sale Securities, Less Than 12 Months, Loss   12mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total   5,080us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total   12mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
FHLB Obligations [Member]    
Schedule Of Available-For-Sale And Held-To-Maturity Securities [Line Items]    
Available-for-sale Securities, Less Than 12 Months, Fair Value 5,103us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanBankCertificatesAndObligationsFHLBMember
 
Available-for-sale Securities, Less Than 12 Months, Loss 4mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanBankCertificatesAndObligationsFHLBMember
 
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total 5,103us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanBankCertificatesAndObligationsFHLBMember
 
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total 4mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanBankCertificatesAndObligationsFHLBMember
 
Agency MBSs [Member]    
Schedule Of Available-For-Sale And Held-To-Maturity Securities [Line Items]    
Available-for-sale Securities, Less Than 12 Months, Fair Value 17,239us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
7,893us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available-for-sale Securities, Less Than 12 Months, Loss 84mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
23mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available-for-sale Securities, 12 Months or More, Fair Value   10,763us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available-for-sale Securities, 12 Months or More, Loss   77mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total 17,239us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
18,656us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total 84mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
100mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Agency CMBSs [Member]    
Schedule Of Available-For-Sale And Held-To-Maturity Securities [Line Items]    
Available-for-sale Securities, Less Than 12 Months, Fair Value 5,087us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Available-for-sale Securities, Less Than 12 Months, Loss 25mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Available-for-sale Securities, 12 Months or More, Fair Value 17,479us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
17,478us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Available-for-sale Securities, 12 Months or More, Loss 218mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
372mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total 22,566us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
17,478us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total 243mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
372mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Agency CMOs [Member]    
Schedule Of Available-For-Sale And Held-To-Maturity Securities [Line Items]    
Available-for-sale Securities, Less Than 12 Months, Fair Value 6,577us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
11,384us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
Available-for-sale Securities, Less Than 12 Months, Loss 35mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
49mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
Available-for-sale Securities, 12 Months or More, Fair Value 7,258us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
10,962us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
Available-for-sale Securities, 12 Months or More, Loss 57mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
190mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total 13,835us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
22,346us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total 92mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
239mbvt_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
Held to Maturity Securities, Less Than 12 Months, Fair Value 7,926us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
14,338us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
Held to Maturity Securities, Less Than 12 Months, Loss 44mbvt_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
108mbvt_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
Held to Maturity Securities, 12 Months or More, Fair Value 6,428us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
43,911us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
Held to Maturity Securities, 12 Months or More, Loss 71mbvt_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
411mbvt_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
Held to Maturity Securities, Continuous Unrealized Loss Position, Fair Value, Total 14,354us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
58,249us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
Held to Maturity Securities, Continuous Unrealized Loss Position, Aggregate Losses, Total $ 115mbvt_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember
$ 519mbvt_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mbvt_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEntitiesMember