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Derivatives and Hedging Activities - Schedule of Gain (Loss) on Derivatives (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives designated as hedges	$ 7	$ (2,033)
Gain (loss) in OCI on derivatives (effective portion), net of tax	50	(1,324)
Gain (loss) reclassified from OCI into interest income or interest expense (effective portion)	$ (1,875)	$ (4,036)

X

- Definition

Gain (Loss) in OCI on Derivatives, Effective Portion, Net of Tax

+ References

No definition available.

+ Details

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Namespace Prefix:	brkl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Gain (Loss) Reclassified From OCI Into Interest Income or Interest Expense, Effective Portion

+ References

No definition available.

+ Details

Name:	brkl_GainLossReclassifiedFromOCIIntoInterestIncomeOrInterestExpenseEffectivePortion
Namespace Prefix:	brkl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- References

No definition available.

+ Details

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

X

- Definition

Net fair value of all derivative instruments designated as fair value hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4B
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4B

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	instant