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Business Combinations - Schedule of Business Combination, Consideration Transferred (Details)
$ / shares in Units, $ in Thousands
Sep. 01, 2025
USD ($)
shares
Aug. 31, 2025
shares
$ / shares
Business Combination [Line Items]    
Brookline market price per share (in USD per share) | $ / shares   $ 26.14
Brookline Bancorp, Inc.    
Business Combination [Line Items]    
Brookline market price per share (in USD per share) | $ / shares   $ 10.95
Holdco Merger    
Business Combination [Line Items]    
Value of consideration transferred, equity interest $ 1,147  
Fraction share payments 49  
Purchase price consideration 1,210,647  
Holdco Merger | Brookline Bancorp, Inc.    
Business Combination [Line Items]    
Number of hypothetical Brookline shares issued to Berkshire Stockholders (in shares) | shares   200,150,309
Value of consideration transferred, equity interest $ 1,209,451  
Company Stockholders | Holdco Merger | Brookline Bancorp, Inc.    
Business Combination [Line Items]    
Number of hypothetical Brookline shares issued to Berkshire Stockholders (in shares) | shares 110,452,183 110,452,183