SECURITIES - Reclassifications out of AOCI into Earnings (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Securities available for sale
Less Than Twelve Months, Gross Unrealized Losses	$ 8,044	$ 3,838
Less Than Twelve Months, Fair Value	523,307	352,256
Over Twelve Months, Gross Unrealized Losses	2,255	6,574
Over Twelve Months, Fair Value	28,874	206,258
Total Gross Unrealized Losses	10,299	10,412
Total Fair Value	552,181	558,514
Securities held to maturity
Less Than Twelve Months, Gross Unrealized Losses	3,504	0
Less Than Twelve Months, Fair Value	103,002	0
Over Twelve Months, Gross Unrealized Losses	0	34
Over Twelve Months, Fair Value	0	2,143
Total Gross Unrealized Losses	3,504	34
Total Fair Value	103,002	2,143
Securities available for sale and held to maturity
Less Than Twelve Months, Gross Unrealized Losses	11,548	3,838
Less Than Twelve Months, Fair Value	626,309	352,256
Over Twelve Months, Gross Unrealized Losses	2,255	6,608
Over Twelve Months, Fair Value	28,874	208,401
Total Gross Unrealized Losses	13,803	10,446
Total Fair Value	655,183	560,657
Debt securities
Securities available for sale
Less Than Twelve Months, Gross Unrealized Losses	7,887	3,304
Less Than Twelve Months, Fair Value	516,707	349,348
Over Twelve Months, Gross Unrealized Losses	799	5,222
Over Twelve Months, Fair Value	22,947	200,529
Total Gross Unrealized Losses	8,686	8,526
Total Fair Value	539,654	549,877
Debt securities \| Municipal bonds and obligations
Securities available for sale
Less Than Twelve Months, Gross Unrealized Losses	1,049	9
Less Than Twelve Months, Fair Value	13,839	1,587
Over Twelve Months, Gross Unrealized Losses	0	115
Over Twelve Months, Fair Value	0	3,400
Total Gross Unrealized Losses	1,049	124
Total Fair Value	13,839	4,987
Securities held to maturity
Less Than Twelve Months, Gross Unrealized Losses	2,416	0
Less Than Twelve Months, Fair Value	69,308	0
Over Twelve Months, Gross Unrealized Losses	0	34
Over Twelve Months, Fair Value	0	2,143
Total Gross Unrealized Losses	2,416	34
Total Fair Value	69,308	2,143
Debt securities \| Agency collateralized mortgage obligations
Securities available for sale
Less Than Twelve Months, Gross Unrealized Losses	3,291	2,958
Less Than Twelve Months, Fair Value	319,448	304,907
Over Twelve Months, Gross Unrealized Losses	0	2,596
Over Twelve Months, Fair Value	0	136,988
Total Gross Unrealized Losses	3,291	5,554
Total Fair Value	319,448	441,895
Securities held to maturity
Less Than Twelve Months, Gross Unrealized Losses	411
Less Than Twelve Months, Fair Value	14,724
Over Twelve Months, Gross Unrealized Losses	0
Over Twelve Months, Fair Value	0
Total Gross Unrealized Losses	411
Total Fair Value	14,724
Debt securities \| Agency residential mortgage-backed securities
Securities available for sale
Less Than Twelve Months, Gross Unrealized Losses	2,153	306
Less Than Twelve Months, Fair Value	130,766	34,543
Over Twelve Months, Gross Unrealized Losses	28	681
Over Twelve Months, Fair Value	2,061	35,522
Total Gross Unrealized Losses	2,181	987
Total Fair Value	132,827	70,065
Securities held to maturity
Less Than Twelve Months, Gross Unrealized Losses	243
Less Than Twelve Months, Fair Value	8,859
Over Twelve Months, Gross Unrealized Losses	0
Over Twelve Months, Fair Value	0
Total Gross Unrealized Losses	243
Total Fair Value	8,859
Debt securities \| Agency commercial mortgage-backed securities
Securities available for sale
Less Than Twelve Months, Gross Unrealized Losses	1,368
Less Than Twelve Months, Fair Value	44,860
Over Twelve Months, Gross Unrealized Losses	0
Over Twelve Months, Fair Value	0
Total Gross Unrealized Losses	1,368
Total Fair Value	44,860
Securities held to maturity
Less Than Twelve Months, Gross Unrealized Losses	434
Less Than Twelve Months, Fair Value	10,111
Over Twelve Months, Gross Unrealized Losses	0
Over Twelve Months, Fair Value	0
Total Gross Unrealized Losses	434
Total Fair Value	10,111
Debt securities \| Corporate bonds
Securities available for sale
Less Than Twelve Months, Gross Unrealized Losses	11	30
Less Than Twelve Months, Fair Value	4,780	6,934
Over Twelve Months, Gross Unrealized Losses	711	1,796
Over Twelve Months, Fair Value	19,655	21,587
Total Gross Unrealized Losses	722	1,826
Total Fair Value	24,435	28,521
Debt securities \| Trust preferred securities
Securities available for sale
Less Than Twelve Months, Gross Unrealized Losses	0	1
Less Than Twelve Months, Fair Value	0	1,269
Over Twelve Months, Gross Unrealized Losses	59	0
Over Twelve Months, Fair Value	1,204	0
Total Gross Unrealized Losses	59	1
Total Fair Value	1,204	1,269
Debt securities \| Other bonds and obligations
Securities available for sale
Less Than Twelve Months, Gross Unrealized Losses	15	0
Less Than Twelve Months, Fair Value	3,014	108
Over Twelve Months, Gross Unrealized Losses	1	34
Over Twelve Months, Fair Value	27	3,032
Total Gross Unrealized Losses	16	34
Total Fair Value	3,041	3,140
Marketable equity securities
Securities available for sale
Less Than Twelve Months, Gross Unrealized Losses	157	534
Less Than Twelve Months, Fair Value	6,600	2,908
Over Twelve Months, Gross Unrealized Losses	1,456	1,352
Over Twelve Months, Fair Value	5,927	5,729
Total Gross Unrealized Losses	1,613	1,886
Total Fair Value	$ 12,527	$ 8,637

No definition available.

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

- Definition

Represents the excess of amortized cost basis over the fair value of securities that have been in a loss position for twelve months or longer for those securities that are categorized as held-to-maturity and available-for-sale.

+ References

No definition available.

+ Details

Name:	bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
Namespace Prefix:	bhlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the excess of amortized cost basis over the fair value of securities that have been in a loss position, which are categorized as held-to-maturity and available-for-sale.

+ References

No definition available.

+ Details

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

- Definition

Represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized as held-to-maturity and available-for-sale securities.

+ References

No definition available.

+ Details

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

- Definition

Represents the excess of amortized cost basis over the fair value of securities that have been in a loss position for less than twelve months for those securities that are categorized as held-to-maturity and available-for-sale.

+ References

No definition available.

+ Details

Name:	bhlb_HeldToMaturityAndAvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses
Namespace Prefix:	bhlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the aggregate fair value of investments in debt and equity securities categorized as held-to-maturity and available-for-sale that have been in a continuous unrealized loss position for less than twelve months.

+ References

No definition available.

+ Details

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

- Definition

+ References

No definition available.

+ Details

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

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563

+ Details

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

- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

+ Details

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

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563

+ Details

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

- Definition

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563

+ Details

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

- Definition

Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

+ Details

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

- Definition

Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity that have been in a continuous loss position for twelve months or longer.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

This item represents the fair value of securities categorized as held-to-maturity which are in an unrealized loss position.

+ References

+ Details

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

- Definition

Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity that have been in a continuous loss position for less than twelve months.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the fair value of securities categorized as held-to-maturity that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563

+ Details

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

- Definition

This item represents the fair value of securities categorized as held-to-maturity securities that have been in a continuous unrealized loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563

+ Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CollateralizedDebtObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=bhlb_ResidentialMortgageBackedSecuritiesGuaranteedByUSGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_OtherDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: