Business Combinations - Schedule Of Ownership And Market Capitalization Of Combined Company (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 01, 2025	Aug. 31, 2025
Business Combination [Line Items]
Number of Berkshire Outstanding (in shares)		84,063,130
Brookline market price per share (in USD per share)		$ 26.14
Market Value at $26.14 Company Share Price (in thousands)		$ 2,196,570
Company Stockholders
Business Combination [Line Items]
Number of Berkshire Outstanding (in shares)		46,389,917
Market Value at $26.14 Company Share Price (in thousands)		$ 1,212,169
Legacy Brookline Stockholders
Business Combination [Line Items]
Number of Berkshire Outstanding (in shares)		37,673,213
Market Value at $26.14 Company Share Price (in thousands)		$ 984,401
Holdco Merger
Business Combination [Line Items]
Percentage Ownership	45.00%	100.00%
Holdco Merger | Company Stockholders
Business Combination [Line Items]
Percentage Ownership		55.18%
Holdco Merger | Legacy Brookline Stockholders
Business Combination [Line Items]
Percentage Ownership		44.82%