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SECURITIES AVAILABLE FOR SALE AND HELD TO MATURITY (Details 3) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
security
Dec. 31, 2014
Securities available for sale    
Less Than Twelve Months, Gross Unrealized Losses $ 1,405us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss $ 2,497us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Less Than Twelve Months, Fair Value 68,630us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue 89,203us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Over Twelve Months, Gross Unrealized Losses 2,192us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss 4,753us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Over Twelve Months, Fair Value 167,916us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 201,604us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Total Gross Unrealized Losses 3,597bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments 7,250bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
Total Fair Value 236,546us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue 290,807us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Securities held to maturity    
Over Twelve Months, Gross Unrealized Losses   34us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Over Twelve Months, Fair Value   7,972us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Total Gross Unrealized Losses   34us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Total Fair Value   7,972us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Securities available for sale and held to maturity    
Less Than Twelve Months, Gross Unrealized Losses 1,405bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses 2,497bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses
Less Than Twelve Months, Fair Value 68,630bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue 89,203bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Over Twelve Months, Gross Unrealized Losses 2,192bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses 4,787bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
Over Twelve Months, Fair Value 167,916bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 209,576bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Total Gross Unrealized Losses 3,597bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses 7,284bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses
Total Fair Value 236,546bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue 298,779bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Debt Securities    
Total unrealized losses 3,597bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments 7,250bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Fair value of investment 1,099,656us-gaap_AvailableForSaleSecurities 1,091,818us-gaap_AvailableForSaleSecurities
Level 2    
Debt Securities    
Fair value of investment 1,055,247us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
1,035,686us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Level 3    
Debt Securities    
Fair value of investment 719us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
2,326us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Corporate bonds | One investment | Level 2    
Debt Securities    
Fair value of investment 30,600us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ invest_InvestmentAxis
= bhlb_OneInvestmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
 
Debt securities    
Securities available for sale    
Less Than Twelve Months, Gross Unrealized Losses 769us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,458us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value 61,266us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
79,301us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Gross Unrealized Losses 1,908us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
4,419us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Fair Value 163,340us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
196,849us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses 2,677bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
5,877bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 224,606us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
276,150us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Total unrealized losses 2,677bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
5,877bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 1,054,890us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,036,876us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt securities | Municipal bonds and obligations    
Securities available for sale    
Less Than Twelve Months, Gross Unrealized Losses 62us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
8us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value 19,756us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,001us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Gross Unrealized Losses 333us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
166us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Fair Value 11,796us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
7,206us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses 395bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
174bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 31,552us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
8,207us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Number of securities in the portfolio of AFS 190bhlb_AvailableForSaleSecuritiesPortfolioNumberOfSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Number of AFS investment securities in unrealized loss positions 28us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Aggregate unrealized losses as a percentage of the amortized cost of the securities in unrealized loss positions 1.20%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Total unrealized losses 395bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
174bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 155,450us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
133,699us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt securities | Government guaranteed residential mortgage-backed securities    
Securities available for sale    
Less Than Twelve Months, Gross Unrealized Losses 11us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
46us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value 5,638us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
7,122us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Gross Unrealized Losses 102us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
160us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Fair Value 6,701us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
16,727us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses 113bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
206bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 12,339us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
23,849us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Total unrealized losses 113bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
206bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 65,039us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
69,468us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt securities | Government-sponsored residential mortgage-backed securities    
Securities available for sale    
Less Than Twelve Months, Gross Unrealized Losses   236us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value   30,672us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Gross Unrealized Losses 1,466us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
2,894us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Fair Value 142,786us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
167,473us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses 1,466bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
3,130bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 142,786us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
198,145us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Total unrealized losses 1,466bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
3,130bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 762,421us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
760,184us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt securities | Corporate bonds    
Securities available for sale    
Less Than Twelve Months, Gross Unrealized Losses 614us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,103us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value 33,680us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
39,571us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses 614bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,103bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 33,680us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
39,571us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Number of securities in the portfolio of AFS 6bhlb_AvailableForSaleSecuritiesPortfolioNumberOfSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Number of AFS investment securities in unrealized loss positions 2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Aggregate unrealized losses as a percentage of the amortized cost of the securities in unrealized loss positions 1.80%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Total unrealized losses 614bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,103bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 55,357us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
54,151us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt securities | Corporate bonds | One investment    
Debt Securities    
Total unrealized losses 500bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= bhlb_OneInvestmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Fair value of investment 31,100us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= bhlb_OneInvestmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Debt securities | Trust preferred securities    
Securities available for sale    
Less Than Twelve Months, Gross Unrealized Losses 82us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
65us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value 2,192us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
935us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Gross Unrealized Losses   1,147us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Fair Value   2,408us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses 82bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,212bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 2,192us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
3,343us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Number of securities in the portfolio of AFS 4bhlb_AvailableForSaleSecuritiesPortfolioNumberOfSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Number of AFS investment securities in unrealized loss positions 2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Aggregate unrealized losses as a percentage of the amortized cost of the securities in unrealized loss positions 3.60%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Total unrealized losses 82bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,212bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 13,427us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
16,215us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt securities | Other bonds and obligations    
Securities available for sale    
Over Twelve Months, Gross Unrealized Losses 7us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
52us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Fair Value 2,057us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
3,035us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses 7bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
52bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 2,057us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
3,035us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Number of securities in the portfolio of AFS 8bhlb_AvailableForSaleSecuritiesPortfolioNumberOfSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Number of AFS investment securities in unrealized loss positions 4us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Aggregate unrealized losses as a percentage of the amortized cost of the securities in unrealized loss positions 0.40%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Total unrealized losses 7bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
52bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 3,196us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
3,159us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt securities | Tax advantaged economic development bonds    
Securities held to maturity    
Over Twelve Months, Gross Unrealized Losses   34us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Fair Value   7,972us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses   34us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value   7,972us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt securities | Residential mortgage-backed securities    
Debt Securities    
Number of securities in the portfolio of AFS 198bhlb_AvailableForSaleSecuritiesPortfolioNumberOfSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Number of AFS investment securities in unrealized loss positions 36us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Aggregate unrealized losses as a percentage of the amortized cost of the securities in unrealized loss positions 1.00%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Marketable equity securities    
Securities available for sale    
Less Than Twelve Months, Gross Unrealized Losses 636us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
1,039us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Less Than Twelve Months, Fair Value 7,364us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
9,902us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Over Twelve Months, Gross Unrealized Losses 284us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
334us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Over Twelve Months, Fair Value 4,576us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
4,755us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Total Gross Unrealized Losses 920bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
1,373bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Total Fair Value 11,940us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
14,657us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Debt Securities    
Number of securities in the portfolio of AFS 29bhlb_AvailableForSaleSecuritiesPortfolioNumberOfSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Number of AFS investment securities in unrealized loss positions 6us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Aggregate unrealized losses as a percentage of the amortized cost of the securities in unrealized loss positions 7.20%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Total unrealized losses 920bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
1,373bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Fair value of investment $ 44,766us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
$ 54,942us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember