DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Offsetting Assets [Line Items]
Notional Amount	$ 2,199,862	$ 1,366,667
Estimated Fair Value Asset (Liability)	(2,804)	(10,676)
Cash flow hedges \| Interest rate swap \| FHLBB borrowings
Offsetting Assets [Line Items]
Notional Amount	300,000
Economic hedges \| Interest rate swap \| Tax advantaged economic development bonds
Offsetting Assets [Line Items]
Notional Amount	11,400
Hedging derivatives \| Cash flow hedges
Offsetting Assets [Line Items]
Notional Amount	300,000	300,000
Estimated Fair Value Asset (Liability)	(6,573)	(8,532)
Hedging derivatives \| Cash flow hedges \| Interest rate swap
Offsetting Assets [Line Items]
Notional Amount	300,000
Hedging derivatives \| Cash flow hedges \| Interest rate swap \| FHLBB borrowings
Offsetting Assets [Line Items]
Notional Amount	$ 300,000
Weighted Average Maturity	2 years 3 months 18 days
Derivative, weighted average rate, received (percent)	0.63%
Derivative, weighted average rate, paid (percent)	2.29%
Estimated Fair Value Asset (Liability)	$ (6,573)
Hedging derivatives \| Cash flow hedges \| Forward starting interest rate swap \| FHLBB borrowings
Offsetting Assets [Line Items]
Notional Amount		$ 300,000
Weighted Average Maturity		3 years 3 months 18 days
Derivative, weighted average rate, received (percent)		0.14%
Derivative, weighted average rate, paid (percent)		2.29%
Estimated Fair Value Asset (Liability)		$ (8,532)
Hedging derivatives \| Economic hedges
Offsetting Assets [Line Items]
Notional Amount	1,691,717	1,030,624
Estimated Fair Value Asset (Liability)	(969)	(2,467)
Hedging derivatives \| Economic hedges \| Interest rate swap
Offsetting Assets [Line Items]
Notional Amount	1,300,000
Hedging derivatives \| Economic hedges \| Interest rate swap \| Tax advantaged economic development bonds
Offsetting Assets [Line Items]
Notional Amount	$ 11,386	$ 11,984
Weighted Average Maturity	12 years 10 months 24 days	13 years 10 months 24 days
Derivative, weighted average rate, received (percent)	0.98%	0.61%
Derivative, weighted average rate, paid (percent)	5.09%	5.09%
Estimated Fair Value Asset (Liability)	$ (2,021)	$ (2,450)
Hedging derivatives \| Economic hedges \| Interest rate swap \| Commercial loan
Offsetting Assets [Line Items]
Notional Amount	$ 668,541	$ 457,392
Weighted Average Maturity	6 years 2 months 5 days	6 years 8 months 6 days
Derivative, weighted average rate, received (percent)	2.43%	2.18%
Derivative, weighted average rate, paid (percent)	4.21%	4.49%
Estimated Fair Value Asset (Liability)	$ (6,752)	$ (17,143)
Hedging derivatives \| Economic hedges \| Reverse interest rate swaps \| Commercial loan
Offsetting Assets [Line Items]
Notional Amount	$ 668,541	$ 457,392
Weighted Average Maturity	6 years 2 months 5 days	6 years 8 months 6 days
Derivative, weighted average rate, received (percent)	4.21%	4.49%
Derivative, weighted average rate, paid (percent)	2.43%	2.18%
Estimated Fair Value Asset (Liability)	$ 7,077	$ 17,129
Hedging derivatives \| Economic hedges \| Risk participation agreements
Offsetting Assets [Line Items]
Notional Amount	$ 83,360	$ 59,016
Weighted Average Maturity	11 years 6 months 20 days	15 years 18 days
Estimated Fair Value Asset (Liability)	$ 5	$ (56)
Hedging derivatives \| Economic hedges \| Forward sale commitments
Offsetting Assets [Line Items]
Notional Amount	$ 259,889	$ 44,840
Weighted Average Maturity	2 months 12 days	2 months 12 days
Estimated Fair Value Asset (Liability)	$ 722	$ 53
Non-hedging derivatives
Offsetting Assets [Line Items]
Notional Amount	208,145	36,043
Estimated Fair Value Asset (Liability)	4,738	323
Non-hedging derivatives \| Commitments to lend
Offsetting Assets [Line Items]
Notional Amount	$ 208,145	$ 36,043
Weighted Average Maturity	2 months 12 days	2 months 12 days
Estimated Fair Value Asset (Liability)	$ 4,738	$ 323

Represents information pertaining to the derivative weighted average rate which is paid.

No definition available.

Name:	bhlb_DerivativeWeightedAverageRatePaid
Namespace Prefix:	bhlb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents information pertaining to the derivative weighted average rate which is received.

+ References

No definition available.

+ Details

Name:	bhlb_DerivativeWeightedAverageRateReceived
Namespace Prefix:	bhlb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

+ Details

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

- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageRemainingMaturity1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

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

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=us-gaap_FederalHomeLoanBankBorrowingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=bhlb_EconomicHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=us-gaap_NontaxableMunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=bhlb_ForwardStartingInterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=bhlb_ReverseInterestRateSwapsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=bhlb_RiskParticipationAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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