LOANS (Details 7) (USD $) In Thousands, unless otherwise specified	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011
TDR activity
Balance at beginning of the period	$ 1,264	$ 7,829
Principal Payments	6	72
TDR Status Change	1,125	7,041
Other Reductions		189
Newly Identified TDRs	2,043	177
Balance at end of the period	2,176	704
1-4 family \| Residential mortgages
Troubled Debt Restructuring
Number of Modifications	1
Pre-Modification Outstanding Recorded Investment	70
Post-Modification Outstanding Recorded Investment	$ 70

Name:	bhlb_FinancingReceivableTroubledDebtRestructuringActivityRollForward
Namespace Prefix:	bhlb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Represents the amount of additions of financing receivables which were newly identified as troubled debt restructurings.

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of other reductions which consist of transfer to other real estate owned and charge-offs to loans.

+ References

No definition available.

+ Details

Name:	bhlb_FinancingReceivableTroubledDebtRestructuringOtherReductions
Namespace Prefix:	bhlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the amount of principal payment of financing receivables modified by troubled debt restructurings.

+ References

No definition available.

+ Details

Name:	bhlb_FinancingReceivableTroubledDebtRestructuringPrincipalPayments
Namespace Prefix:	bhlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the amount of troubled debt restructurings status change in which troubled debt restructuring loans with a specified interest rate equal to or greater than the rate that the entity was willing to accept at the time of the restructuring for a new loan with comparable risk and the loan was on current payment status and not impaired based on the terms specified by the restructuring agreement.

+ References

No definition available.

+ Details

Name:	bhlb_FinancingReceivableTroubledDebtRestructuringStatusChange
Namespace Prefix:	bhlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of investment in financing receivables (after modification) modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in financing receivables (before modification) modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 40

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6951948&loc=d3e18845-111554

+ Details

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