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Business Combinations - Schedule Of Ownership And Market Capitalization Of Combined Company (Details) - USD ($) $ / shares in Units, $ in Thousands | Sep. 01, 2025 | Aug. 31, 2025
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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%

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