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SECURITIES (Details 4) (USD $)
12 Months Ended
Dec. 31, 2014
security
Dec. 31, 2013
Securities available for sale    
Less Than Twelve Months, Gross Unrealized Losses $ 2,497,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments $ 15,203,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
Less Than Twelve Months, Fair Value 89,203,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue 510,469,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Over Twelve Months, Gross Unrealized Losses 4,753,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments 2,096,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
Over Twelve Months, Fair Value 201,604,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 5,675,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Total Gross Unrealized Losses 7,250,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments 17,299,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
Total Fair Value 290,807,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue 516,144,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Securities held to maturity    
Less Than Twelve Months, Gross Unrealized Losses   57,000bhlb_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Less Than Twelve Months, Fair Value   9,429,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Over Twelve Months, Gross Unrealized Losses 34,000bhlb_HeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss 357,000bhlb_HeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Over Twelve Months, Fair Value 7,972,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 7,901,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Total Gross Unrealized Losses 34,000bhlb_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss 414,000bhlb_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Total Fair Value 7,972,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue 17,330,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Securities available for sale and held to maturity    
Less Than Twelve Months, Gross Unrealized Losses 2,497,000bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses 15,260,000bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses
Less Than Twelve Months, Fair Value 89,203,000bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue 519,898,000bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Over Twelve Months, Gross Unrealized Losses 4,787,000bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses 2,453,000bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
Over Twelve Months, Fair Value 209,576,000bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 13,576,000bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Total Gross Unrealized Losses 7,284,000bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses 17,713,000bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses
Total Fair Value 298,779,000bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue 533,474,000bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Debt Securities    
Total unrealized losses 7,250,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments 17,299,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Fair value of investment 1,091,818,000us-gaap_AvailableForSaleSecurities 760,048,000us-gaap_AvailableForSaleSecurities
Level 2    
Debt Securities    
Fair value of investment 1,035,686,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
737,193,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Level 3    
Debt Securities    
Fair value of investment 2,326,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
1,964,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Corporate bonds | One investment | Level 2    
Debt Securities    
Fair value of investment 29,700,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ invest_InvestmentAxis
= bhlb_OneInvestmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
 
Trust preferred securities | Level 3    
Debt Securities    
Fair value of investment 1,500,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
 
Debt securities    
Securities available for sale    
Less Than Twelve Months, Gross Unrealized Losses 1,458,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
15,086,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value 79,301,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
508,816,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Gross Unrealized Losses 4,419,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,878,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Fair Value 196,849,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
3,893,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses 5,877,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
16,964,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 276,150,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
512,709,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Total unrealized losses 5,877,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
16,964,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 1,036,876,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
738,075,000us-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 8,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,657,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value 1,001,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
17,776,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Gross Unrealized Losses 166,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
313,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Fair Value 7,206,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,854,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses 174,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,970,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 8,207,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
19,630,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Number of securities in the portfolio of AFS 178bhlb_AvailableForSaleSecuritiesPortfolioNumberOfSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Number of AFS investment securities in unrealized loss positions 14us-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 2.00%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Total unrealized losses 174,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,970,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 133,699,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
77,671,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt securities | Residential mortgage-backed securities    
Debt Securities    
Number of securities in the portfolio of AFS 196bhlb_AvailableForSaleSecuritiesPortfolioNumberOfSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Number of AFS investment securities in unrealized loss positions 46us-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 1.50%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_InvestmentTypeAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ 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 46,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
658,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value 7,122,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
35,631,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Gross Unrealized Losses 160,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Over Twelve Months, Fair Value 16,727,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Total Gross Unrealized Losses 206,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
658,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 23,849,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
35,631,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Total unrealized losses 206,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
658,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 69,468,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
78,771,000us-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 236,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
10,783,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value 30,672,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
423,203,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Gross Unrealized Losses 2,894,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Over Twelve Months, Fair Value 167,473,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Total Gross Unrealized Losses 3,130,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
10,783,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 198,145,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
423,203,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Total unrealized losses 3,130,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
10,783,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 760,184,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
522,658,000us-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 1,103,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,822,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value 39,571,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
29,124,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses 1,103,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,822,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 39,571,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
29,124,000us-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 4us-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 2.70%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Total unrealized losses 1,103,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,822,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 54,151,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
39,280,000us-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 1,000,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= bhlb_OneInvestmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Fair value of investment 30,700,000us-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 65,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Less Than Twelve Months, Fair Value 935,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Over Twelve Months, Gross Unrealized Losses 1,147,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,565,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Fair Value 2,408,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
2,039,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses 1,212,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,565,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 3,343,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
2,039,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Number of securities in the portfolio of AFS 6bhlb_AvailableForSaleSecuritiesPortfolioNumberOfSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Number of AFS investment securities in unrealized loss positions 3us-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 27.00%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Total unrealized losses 1,212,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,565,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 16,215,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
16,611,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Amount of security issued by banking and insurance entities 360,000,000bhlb_AvailableForSaleSecuritiesIssuedByBankingAndInsuranceEntities
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Protection from credit loss afforded in excess subordination above current and projected losses 50,000,000bhlb_AvailableForSaleSecuritiesProtectionFromCreditLossAfforded
/ us-gaap_InvestmentTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Debt securities | Other bonds and obligations    
Securities available for sale    
Less Than Twelve Months, Gross Unrealized Losses   166,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value   3,082,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Gross Unrealized Losses 52,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Over Twelve Months, Fair Value 3,035,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Total Gross Unrealized Losses 52,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
166,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 3,035,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
3,082,000us-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 5us-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 1.70%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Total unrealized losses 52,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
166,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Fair value of investment 3,159,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherDebtSecuritiesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
3,084,000us-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    
Less Than Twelve Months, Gross Unrealized Losses   57,000bhlb_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Less Than Twelve Months, Fair Value   9,429,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Gross Unrealized Losses 34,000bhlb_HeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
357,000bhlb_HeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Over Twelve Months, Fair Value 7,972,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
7,901,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Gross Unrealized Losses 34,000bhlb_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
414,000bhlb_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Total Fair Value 7,972,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
17,330,000us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Debt Securities    
Aggregate unrealized losses as a percentage of the amortized cost of the securities in unrealized loss positions 0.40%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Number of HTM investment securities in unrealized loss positions 1us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Number of securities in the portfolio of HTM 7bhlb_HeldToMaturitySecuritiesPortfolioNumberOfSecurities
/ us-gaap_InvestmentTypeAxis
= bhlb_TaxAdvantagedEconomicDevelopmentBondsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Debt securities | Mezzanine Class B tranche    
Debt Securities    
Total unrealized losses 1,000,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= bhlb_MezzanineClassBTrancheMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Fair value of investment 2,600,000us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= bhlb_MezzanineClassBTrancheMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Marketable equity securities    
Securities available for sale    
Less Than Twelve Months, Gross Unrealized Losses 1,039,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
117,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Less Than Twelve Months, Fair Value 9,902,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
1,653,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Over Twelve Months, Gross Unrealized Losses 334,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
218,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Over Twelve Months, Fair Value 4,755,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
1,782,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Total Gross Unrealized Losses 1,373,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
335,000bhlb_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Total Fair Value 14,657,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
3,435,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Debt Securities    
Number of securities in the portfolio of AFS 30bhlb_AvailableForSaleSecuritiesPortfolioNumberOfSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Number of AFS investment securities in unrealized loss positions 7us-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 9.00%bhlb_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAggregateLossesPercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Total unrealized losses 1,373,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
335,000bhlb_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Fair value of investment $ 54,942,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
$ 21,973,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember