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Derivatives and Hedging Activities - Schedule of Gain (Loss) on Derivatives (Details) - USD ($) $ in Thousands | 12 Months Ended
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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)

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- Definition Gain (Loss) in OCI on Derivatives, Effective Portion, Net of Tax + References No definition available. + Details
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- Definition Gain (Loss) Reclassified From OCI Into Interest Income or Interest Expense, Effective Portion + References No definition available. + Details
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Namespace Prefix: | brkl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
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- 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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