LOANS - Summary of Impaired Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Business Activities Loans
Recorded Investment
Total	$ 44,770	$ 43,679
Unpaid Principal Balance
Total	52,964	49,702
Related Allowance
With an allowance recorded	609	460
Business Activities Loans \| Commercial Real Estate
Recorded Investment
Total	33,430	34,001
Unpaid Principal Balance
Total	39,127	37,783
Related Allowance
With an allowance recorded	174	229
Business Activities Loans \| Commercial Real Estate \| Construction
Recorded Investment
With an allowance recorded		159
Unpaid Principal Balance
With an allowance recorded		159
Related Allowance
With an allowance recorded		1
Business Activities Loans \| Commercial Real Estate \| Single and multi-family
Recorded Investment
With no related allowance	0	1,077
With an allowance recorded	309	159
Unpaid Principal Balance
With no related allowance	0	3,607
With an allowance recorded	323	171
Related Allowance
With an allowance recorded	1	1
Business Activities Loans \| Commercial Real Estate \| Other
Recorded Investment
With no related allowance	20,286	18,285
With an allowance recorded	12,835	14,321
Unpaid Principal Balance
With no related allowance	23,042	18,611
With an allowance recorded	15,762	15,235
Related Allowance
With an allowance recorded	173	227
Business Activities Loans \| Commercial and industrial loans
Recorded Investment
Total	7,006	5,776
Unpaid Principal Balance
Total	8,340	6,878
Related Allowance
With an allowance recorded	296	66
Business Activities Loans \| Commercial and industrial loans \| Other
Recorded Investment
With no related allowance	3,091	2,060
With an allowance recorded	3,915	3,716
Unpaid Principal Balance
With no related allowance	3,846	2,629
With an allowance recorded	4,494	4,249
Related Allowance
With an allowance recorded	296	66
Business Activities Loans \| Residential Mortgages
Recorded Investment
Total	2,489	2,004
Unpaid Principal Balance
Total	3,013	2,521
Related Allowance
With an allowance recorded	137	130
Business Activities Loans \| Residential Mortgages \| 1-4 Family
Recorded Investment
With no related allowance	1,011	660
With an allowance recorded	1,478	1,344
Unpaid Principal Balance
With no related allowance	1,433	1,075
With an allowance recorded	1,580	1,446
Related Allowance
With an allowance recorded	137	130
Business Activities Loans \| Consumer Loans
Recorded Investment
Total	1,845	1,898
Unpaid Principal Balance
Total	2,484	2,520
Related Allowance
With an allowance recorded	2	35
Business Activities Loans \| Consumer Loans \| Other
Recorded Investment
With no related allowance	0
With an allowance recorded	16	17
Unpaid Principal Balance
With no related allowance	0
With an allowance recorded	16	17
Related Allowance
With an allowance recorded	1	1
Business Activities Loans \| Consumer Loans \| Home equity
Recorded Investment
With no related allowance	1,784	867
With an allowance recorded	45	1,014
Unpaid Principal Balance
With no related allowance	2,415	1,504
With an allowance recorded	53	999
Related Allowance
With an allowance recorded	1	34
Acquired Loans
Recorded Investment
Total	9,980	7,335
Unpaid Principal Balance
Total	14,895	9,178
Related Allowance
With an allowance recorded	580	111
Acquired Loans \| Commercial Real Estate
Recorded Investment
Total	5,433	4,257
Unpaid Principal Balance
Total	8,324	5,625
Related Allowance
With an allowance recorded	41	56
Acquired Loans \| Commercial Real Estate \| Single and multi-family
Recorded Investment
With no related allowance	185	204
With an allowance recorded	766	887
Unpaid Principal Balance
With no related allowance	276	290
With an allowance recorded	763	880
Related Allowance
With an allowance recorded	12	18
Acquired Loans \| Commercial Real Estate \| Other
Recorded Investment
With no related allowance	2,400	1,123
With an allowance recorded	2,082	2,043
Unpaid Principal Balance
With no related allowance	5,192	2,794
With an allowance recorded	2,093	1,661
Related Allowance
With an allowance recorded	29	38
Acquired Loans \| Commercial and industrial loans
Recorded Investment
With no related allowance	574	255
With an allowance recorded	178	165
Total	752	420
Unpaid Principal Balance
With no related allowance	1,618	310
With an allowance recorded	176	166
Total	1,794	476
Related Allowance
With an allowance recorded	3	1
Acquired Loans \| Residential Mortgages
Recorded Investment
Total	2,701	824
Unpaid Principal Balance
Total	3,094	856
Related Allowance
With an allowance recorded	506	9
Acquired Loans \| Residential Mortgages \| 1-4 Family
Recorded Investment
With no related allowance	697	658
With an allowance recorded	2,004	166
Unpaid Principal Balance
With no related allowance	709	671
With an allowance recorded	2,385	185
Related Allowance
With an allowance recorded	506	9
Acquired Loans \| Consumer Loans
Recorded Investment
Total	1,094	1,834
Unpaid Principal Balance
Total	1,683	2,221
Related Allowance
With an allowance recorded	30	45
Acquired Loans \| Consumer Loans \| Other
Recorded Investment
With no related allowance	17	27
Unpaid Principal Balance
With no related allowance	18	27
Acquired Loans \| Consumer Loans \| Home equity
Recorded Investment
With no related allowance	754	1,374
With an allowance recorded	323	433
Unpaid Principal Balance
With no related allowance	1,303	1,654
With an allowance recorded	362	540
Related Allowance
With an allowance recorded	$ 30	$ 45

No definition available.

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossAdditionalInformationAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRecordedInvestmentAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRelatedAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalanceAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=bhlb_LoansFromBusinessActivitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bhlb_SingleAndMultiFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bhlb_OtherReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=bhlb_CommercialandIndustrialLoansPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bhlb_A1to4FamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=bhlb_LoansAcquiredFromBusinessCombinationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: