Fair Value of Financial Instruments (Assets and Liabilities Recorded at Fair Value on a Recurring Basis) (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Assets:
AFS, debt securities, estimated fair value	$ 498,995,000	$ 502,793,000
Investment securities available-for-sale		502,793,000
Equity securities held-for-trading	3,581,000	4,207,000
Derivatives	60,648,000	22,488,000
Liabilities:
Derivatives	59,701,000	22,220,000
GSE debentures
Assets:
AFS, debt securities, estimated fair value	185,803,000	181,079,000
GSE CMOs
Assets:
AFS, debt securities, estimated fair value	85,932,000	103,130,000
GSE MBSs
Assets:
AFS, debt securities, estimated fair value	153,343,000	165,089,000
SBA commercial loan asset-backed securities
Assets:
AFS, debt securities, estimated fair value	34,000.0	51,000.0
Corporate debt obligations
Assets:
AFS, debt securities, estimated fair value	28,986,000	39,708,000
U.S. treasury bonds
Assets:
AFS, debt securities, estimated fair value	44,897,000	13,736,000
Recurring basis
Assets:
AFS, debt securities, estimated fair value	498,995,000
Investment securities available-for-sale		502,793,000
Equity securities held-for-trading	3,581,000	4,207,000
Liabilities:
Derivatives	59,365,000
Recurring basis \| GSE debentures
Assets:
AFS, debt securities, estimated fair value	185,803,000	181,079,000
Recurring basis \| GSE CMOs
Assets:
AFS, debt securities, estimated fair value	85,932,000	103,130,000
Recurring basis \| GSE MBSs
Assets:
AFS, debt securities, estimated fair value	153,343,000	165,089,000
Recurring basis \| SBA commercial loan asset-backed securities
Assets:
AFS, debt securities, estimated fair value	34,000	51,000
Recurring basis \| Corporate debt obligations
Assets:
AFS, debt securities, estimated fair value	28,986,000	39,708,000
Recurring basis \| U.S. treasury bonds
Assets:
AFS, debt securities, estimated fair value	44,897,000	13,736,000
Recurring basis \| Loan level derivatives
Assets:
Derivatives	59,365,000	22,013,000
Liabilities:
Derivatives		22,013,000
Recurring basis \| Risk participation-out agreements
Assets:
Derivatives	1,229,000	344,000
Recurring basis \| Risk participation-in agreements
Liabilities:
Derivatives	283,000	84,000
Recurring basis \| Foreign exchange contracts
Assets:
Derivatives	54,000	131,000
Liabilities:
Derivatives	53,000	123,000
Recurring basis \| Level 1
Assets:
AFS, debt securities, estimated fair value	0
Investment securities available-for-sale		0
Equity securities held-for-trading	2,569,000	3,235,000
Liabilities:
Derivatives	0
Recurring basis \| Level 1 \| GSE debentures
Assets:
AFS, debt securities, estimated fair value	0	0
Recurring basis \| Level 1 \| GSE CMOs
Assets:
AFS, debt securities, estimated fair value	0	0
Recurring basis \| Level 1 \| GSE MBSs
Assets:
AFS, debt securities, estimated fair value	0	0
Recurring basis \| Level 1 \| SBA commercial loan asset-backed securities
Assets:
AFS, debt securities, estimated fair value	0	0
Recurring basis \| Level 1 \| Corporate debt obligations
Assets:
AFS, debt securities, estimated fair value	0	0
Recurring basis \| Level 1 \| U.S. treasury bonds
Assets:
AFS, debt securities, estimated fair value	0	0
Recurring basis \| Level 1 \| Loan level derivatives
Assets:
Derivatives	0	0
Liabilities:
Derivatives		0
Recurring basis \| Level 1 \| Risk participation-out agreements
Assets:
Derivatives	0	0
Recurring basis \| Level 1 \| Risk participation-in agreements
Liabilities:
Derivatives	0	0
Recurring basis \| Level 1 \| Foreign exchange contracts
Assets:
Derivatives	0	0
Liabilities:
Derivatives	0	0
Recurring basis \| Level 2
Assets:
AFS, debt securities, estimated fair value	498,995,000
Investment securities available-for-sale		502,793,000
Equity securities held-for-trading	1,012,000	972,000
Liabilities:
Derivatives	59,365,000
Recurring basis \| Level 2 \| GSE debentures
Assets:
AFS, debt securities, estimated fair value	185,803,000	181,079,000
Recurring basis \| Level 2 \| GSE CMOs
Assets:
AFS, debt securities, estimated fair value	85,932,000	103,130,000
Recurring basis \| Level 2 \| GSE MBSs
Assets:
AFS, debt securities, estimated fair value	153,343,000	165,089,000
Recurring basis \| Level 2 \| SBA commercial loan asset-backed securities
Assets:
AFS, debt securities, estimated fair value	34,000	51,000
Recurring basis \| Level 2 \| Corporate debt obligations
Assets:
AFS, debt securities, estimated fair value	28,986,000	39,708,000
Recurring basis \| Level 2 \| U.S. treasury bonds
Assets:
AFS, debt securities, estimated fair value	44,897,000	13,736,000
Recurring basis \| Level 2 \| Loan level derivatives
Assets:
Derivatives	59,365,000	22,013,000
Liabilities:
Derivatives		22,013,000
Recurring basis \| Level 2 \| Risk participation-out agreements
Assets:
Derivatives	1,229,000	344,000
Recurring basis \| Level 2 \| Risk participation-in agreements
Liabilities:
Derivatives	283,000	84,000
Recurring basis \| Level 2 \| Foreign exchange contracts
Assets:
Derivatives	54,000	131,000
Liabilities:
Derivatives	53,000	123,000
Recurring basis \| Level 3
Assets:
AFS, debt securities, estimated fair value	0
Investment securities available-for-sale		0
Equity securities held-for-trading	0	0
Liabilities:
Derivatives	0
Recurring basis \| Level 3 \| GSE debentures
Assets:
AFS, debt securities, estimated fair value	0	0
Recurring basis \| Level 3 \| GSE CMOs
Assets:
AFS, debt securities, estimated fair value	0	0
Recurring basis \| Level 3 \| GSE MBSs
Assets:
AFS, debt securities, estimated fair value	0	0
Recurring basis \| Level 3 \| SBA commercial loan asset-backed securities
Assets:
AFS, debt securities, estimated fair value	0	0
Recurring basis \| Level 3 \| Corporate debt obligations
Assets:
AFS, debt securities, estimated fair value	0	0
Recurring basis \| Level 3 \| U.S. treasury bonds
Assets:
AFS, debt securities, estimated fair value	0	0
Recurring basis \| Level 3 \| Loan level derivatives
Assets:
Derivatives	0	0
Liabilities:
Derivatives		0
Recurring basis \| Level 3 \| Risk participation-out agreements
Assets:
Derivatives	0	0
Recurring basis \| Level 3 \| Risk participation-in agreements
Liabilities:
Derivatives	0	0
Recurring basis \| Level 3 \| Foreign exchange contracts
Assets:
Derivatives	0	0
Liabilities:
Derivatives	$ 0	$ 0

No definition available.

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

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

No definition available.

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=118948506&loc=SL51823488-111719

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

+ Details

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

- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=118948506&loc=SL51823488-111719

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

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

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=brkl_CollateralizedMortgageObligationsIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=brkl_SBACommercialLoanAssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=brkl_LoanLevelDerivativeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=brkl_RiskParticipationoutAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=brkl_RiskParticipationinAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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