LOANS AND ALLOWANCE FOR CREDIT LOSSES - Allowance for Credit Loss Activity (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	$ 96,270	$ 106,094
Charge-offs	(7,937)	(6,047)
Recoveries	1,040	3,422
Provision for Credit Losses	9,019	(3,994)
Balance at End of Period	97,991	99,475
Impact of ASU No. 2022-02 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	(401)
Construction and commercial multifamily \| Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	1,227	3,206
Charge-offs	0	0
Recoveries	0	0
Provision for Credit Losses	309	(701)
Balance at End of Period	1,536	2,505
Construction and commercial multifamily \| Construction \| Impact of ASU No. 2022-02 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0
Construction and commercial multifamily \| Commercial multifamily
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	1,810	6,120
Charge-offs	0	0
Recoveries	6	0
Provision for Credit Losses	(118)	(349)
Balance at End of Period	1,698	5,771
Construction and commercial multifamily \| Commercial multifamily \| Impact of ASU No. 2022-02 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0
Commercial real estate \| Commercial real estate owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	10,739	12,752
Charge-offs	(70)	(130)
Recoveries	45	209
Provision for Credit Losses	(460)	(1,333)
Balance at End of Period	10,278	11,498
Commercial real estate \| Commercial real estate owner occupied \| Impact of ASU No. 2022-02 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	24
Commercial real estate \| Commercial real estate non-owner occupied
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	30,724	32,106
Charge-offs	0	(4,884)
Recoveries	95	1,266
Provision for Credit Losses	2,589	(2,674)
Balance at End of Period	33,408	25,814
Commercial real estate \| Commercial real estate non-owner occupied \| Impact of ASU No. 2022-02 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0
Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	18,743	22,584
Charge-offs	(6,033)	(653)
Recoveries	305	1,288
Provision for Credit Losses	7,172	(270)
Balance at End of Period	20,164	22,949
Commercial and industrial \| Impact of ASU No. 2022-02 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	(23)
Residential real estate \| Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	18,666	22,406
Charge-offs	(31)	(164)
Recoveries	387	388
Provision for Credit Losses	(1,434)	(4,814)
Balance at End of Period	17,590	17,816
Residential real estate \| Residential real estate \| Impact of ASU No. 2022-02 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	2
Consumer loans \| Home equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	2,173	4,006
Charge-offs	(10)	0
Recoveries	26	134
Provision for Credit Losses	131	(837)
Balance at End of Period	2,320	3,303
Consumer loans \| Home equity \| Impact of ASU No. 2022-02 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	0
Consumer loans \| Consumer other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	12,188	2,914
Charge-offs	(1,793)	(216)
Recoveries	176	137
Provision for Credit Losses	830	6,984
Balance at End of Period	10,997	$ 9,819
Consumer loans \| Consumer other \| Impact of ASU No. 2022-02 Adoption
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	$ (404)

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount, excluding accrued interest, of increase in allowance for credit loss on financing receivable from recovery. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (e)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable from writeoff. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (d)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
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_CommercialMultifamilyMember
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=bhlb_RealEstateOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bhlb_RealEstateNonOwnerOccupiedMember
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=us-gaap_RealEstateLoanMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bhlb_ConsumerOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: