Represents the closing ratio used as an input to measure fair value.

No definition available.

Name:	bhlb_FairValueInputsClosingRatio
Namespace Prefix:	bhlb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value Inputs, Median Peer Price and Tangible Book Value Percentage Multiple

+ References

No definition available.

+ Details

Name:	bhlb_FairValueInputsMedianPeerPriceandTangibleBookValuePercentageMultiple
Namespace Prefix:	bhlb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the origination costs used as an input to measure fair value.

+ References

No definition available.

+ Details

Name:	bhlb_FairValueInputsOriginationCosts
Namespace Prefix:	bhlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a recurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

+ References

No definition available.

+ Details

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

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.

+ References

+ Details

Name:	us-gaap_FairValueInputsPrepaymentRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
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Balance Type:
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- Details

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

- Details

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

- Details

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

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_CommitmentsToExtendCreditMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_FairValueByAssetClassAxis=bhlb_CapitalizedMortgageServicingRightsMember
Namespace Prefix:
Data Type:	na
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FAIR VALUE MEASUREMENTS - Significant Unobservable Inputs for Recurring Assets (Details) - Level 3 - Recurring - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured on recurring basis	$ 18,865	$ 15,229
Trading Security
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured on recurring basis	$ 13,229	$ 14,189
Trading Security \| Discounted Cash Flow
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount Rate (percent)	2.62%	2.49%
Securities Available for Sale
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured on recurring basis		$ 708
Securities Available for Sale \| Pricing Model
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Median Peer Price/ Tangible Book Value Percentage Multiple (percent)		88.52%
Forward Commitments
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured on recurring basis	$ 100	$ 9
Forward Commitments \| Pricing Model
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Origination Costs, per loan	$ 3,692	$ 2,500
Forward Commitments \| Historical Trend
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Closing Ratio (percent)	80.36%	92.57%
Commitments to lend
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured on recurring basis	$ 4,738	$ 323
Commitments to lend \| Pricing Model
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Origination Costs, per loan	$ 3,692	$ 2,500
Commitments to lend \| Historical Trend
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Closing Ratio (percent)	80.36%	92.57%
Capitalized servicing rights
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets measured on recurring basis			$ 798
Capitalized servicing rights \| Discounted Cash Flow
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount Rate (percent)	11.00%
Constant prepayment rate (percent)	10.40%