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SUBSEQUENT EVENTS (Details) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Mar. 12, 2015
SUBSEQUENT EVENTS      
Assets $ 6,502,031,000us-gaap_Assets $ 5,672,799,000us-gaap_Assets  
Hampden      
SUBSEQUENT EVENTS      
Merger and acquisition expenses 1,500,000us-gaap_BusinessAcquisitionCostOfAcquiredEntityTransactionCosts
/ us-gaap_BusinessAcquisitionAxis
= bhlb_HampdenBancorpIncMember		    
Hampden | Subsequent event      
SUBSEQUENT EVENTS      
Number of shares into which each outstanding common share is converted in the merger     0.81bhlb_NumberOfSharesInToWhichEachOutstandingCommonShareIsConvertedInMerger
/ us-gaap_BusinessAcquisitionAxis
= bhlb_HampdenBancorpIncMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Hampden      
SUBSEQUENT EVENTS      
Assets $ 711,000,000us-gaap_Assets
/ dei_LegalEntityAxis
= bhlb_HampdenBancorpIncMember		    
Number of Stores 10us-gaap_NumberOfStores
/ dei_LegalEntityAxis
= bhlb_HampdenBancorpIncMember