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Business Combinations - Business Combination, Consideration Transferred (Details) - USD ($)
$ / shares in Units, $ in Thousands
Sep. 01, 2025
Aug. 31, 2025
Business Combination [Line Items]    
Brookline market price (in USD per share)   $ 26.14
Brookline Bancorp, Inc.    
Business Combination [Line Items]    
Brookline market price (in USD per share)   $ 10.95
Holdco Merger    
Business Combination [Line Items]    
Value of consideration transferred, equity interest $ 1,147  
Cash paid for fractional shares 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)   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) 110,452,183 110,452,183