FAIR VALUE MEASUREMENTS - Schedule of Quantitative Information about the Significant Unobservable Inputs within Level 3 (Details) $ in Thousands	Dec. 31, 2022 USD ($) uSDPerLoan	Dec. 31, 2021 USD ($) uSDPerLoan
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Trading security	$ 6,708	$ 8,354
Securities available for sale, at fair value	1,423,200	1,877,585
Commitments	54,241	79,528
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale, at fair value	4,000	4,030
Level 3 \| Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Trading security	6,708	8,354
Loans held for investment	605	1,200
Total	13,184	15,808
Level 3 \| Recurring \| Commitments to Lend
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Commitments	17	124
Level 3 \| Recurring \| Forward Commitments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Commitments	8	134
Level 3 \| Recurring \| Capitalized Servicing Rights
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Capitalized Servicing Rights	1,846	1,966
Level 3 \| Recurring \| Securities Available for Sale
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities available for sale, at fair value	$ 4,000	$ 4,030
Level 3 \| Recurring \| Discount Rate \| Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Trading security, significant unobservable input value	0.0592	0.0335
Loans held for investment, Significant Unobservable Input Value	0.2500	0.2500
Level 3 \| Recurring \| Discount Rate \| Discounted Cash Flow \| Capitalized Servicing Rights
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Capitalized Servicing Rights, Significant Unobservable Input Value	0.0956	0.0950
Level 3 \| Recurring \| Price \| Indication from Market Maker \| Securities Available for Sale
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Securities Available for Sale, Significant Unobservable Input Value	1.0000	1.0100
Level 3 \| Recurring \| Collateral Value \| Discounted Cash Flow \| Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held for investment, Significant Unobservable Input Value	0	6.3
Level 3 \| Recurring \| Collateral Value \| Discounted Cash Flow \| Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Loans held for investment, Significant Unobservable Input Value	20.4	19.8
Level 3 \| Recurring \| Closing Ratio \| Historical Trend \| Commitments to Lend
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Commitments, Significant Unobservable Input Value	0.8063	0.8209
Level 3 \| Recurring \| Closing Ratio \| Historical Trend \| Forward Commitments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Commitments, Significant Unobservable Input Value	0.8063	0.8209
Level 3 \| Recurring \| Origination Costs, per loan \| Pricing Model \| Commitments to Lend
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Commitments, Significant Unobservable Input Value \| uSDPerLoan	2,000	3,000
Level 3 \| Recurring \| Origination Costs, per loan \| Pricing Model \| Forward Commitments
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Commitments, Significant Unobservable Input Value \| uSDPerLoan	2,000	3,000
Level 3 \| Recurring \| Constant prepayment rate (CPR) \| Discounted Cash Flow \| Capitalized Servicing Rights
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Capitalized Servicing Rights, Significant Unobservable Input Value	0.1107	0.1941

Loans Held for Investment

No definition available.

Name:	bhlb_LoansHeldforInvestment
Namespace Prefix:	bhlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Loans Held for Investment, Measurement Input

+ References

No definition available.

+ Details

Name:	bhlb_LoansHeldforInvestmentMeasurementInput
Namespace Prefix:	bhlb_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

+ Details

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

- Definition

Value of input used to measure investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Value of input used to measure investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

+ Details

Name:	us-gaap_DebtSecuritiesTradingMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Value of input used to measure derivative asset.

+ References

+ Details

Name:	us-gaap_DerivativeAssetMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 22
-URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI https://asc.fasb.org/extlink&oid=123594938&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Fair value of asset after deduction of liability.

+ References

No definition available.

+ Details

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

- Definition

Aggregate amount of servicing assets that are subsequently measured at fair value and servicing assets that are subsequently measured using the amortization method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

+ Details

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

- Definition

Value of input used to measure servicing asset.

+ References

+ Details

Name:	us-gaap_ServicingAssetMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=124260329&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124260329&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (f)
-URI https://asc.fasb.org/extlink&oid=124098289&loc=d3e6676-107765

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommitmentsToExtendCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=bhlb_ForwardCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=bhlb_CapitalizedMortgageServicingRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_ValuationTechniqueDiscountedCashFlowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=bhlb_MeasurementInputPriceIndicationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_MarketApproachValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=bhlb_CollateralValueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=bhlb_MeasurementInputClosingRatioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=bhlb_HistoricalTrendValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=bhlb_MeasurementInputOriginationCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=bhlb_PricingModelValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputConstantPrepaymentRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: