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BASIS OF PRESENTATION - Additional Information (Details) - USD ($) $ in Thousands | Jun. 30, 2019 | Jan. 01, 2019
--- | --- | ---
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Lease, right-of-use asset | $ 88,572 |
Lease, liability | $ 96,168 |
ASU 2016-02 |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Lease, right-of-use asset |  | $ 79,600
Lease, liability |  | $ 82,800

X
- Definition Lease, Liability + References No definition available. + Details
Name: | bhlb_LeaseLiability
Namespace Prefix: | bhlb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Lease, Right-of-Use Asset + References No definition available. + Details
Name: | bhlb_LeaseRightofUseAsset
Namespace Prefix: | bhlb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201602Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |