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BUSINESS COMBINATION - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Sep. 02, 2025 | Sep. 30, 2025 | Sep. 30, 2025 | Sep. 30, 2024 | Jun. 30, 2025 | Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024 | Dec. 31, 2023
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Bargain purchase gain |  |  | $ 90,363 | $ 0 |  | $ 90,363 | $ 0 |  |
Net income (loss) |  |  | $ (55,161) | $ (39,944) |  | $ (141,437) | $ 22,664 |  |
Mechanics Bank Acquisition |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Bargain purchase gain | $ 90,363 |  |  |  |  |  |  |  |
Principal of PCD loans acquired | 2,956,577 |  |  |  |  |  |  |  |
PCD ACL at acquisition | 63,494 |  |  |  |  |  |  |  |
Revenue of acquiree since acquisition date |  | $ 20,000 |  |  |  |  |  |  |
Deferred tax assets | 81,420 |  |  |  |  |  |  |  |
Mechanics Bank Acquisition | Commercial Real Estate [Member] | COVID |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Principal of PCD loans acquired | 2,400,000 |  |  |  |  |  |  |  |
PCD ACL at acquisition | $ 29,500 |  |  |  |  |  |  |  |
Mechanics Bank Acquisition | Mechanics Bancorp |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Voting equity interest after merger | 100.00% |  |  |  |  |  |  |  |
Mechanics Bank Acquisition | Financial Asset Acquired with Credit Deterioration |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Principal of PCD loans acquired | $ 3,000,000 |  |  |  |  |  |  |  |
Mechanics Bank Acquisition | Common Class A |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Business combination, fixed exchange ratio | 3,301.092 |  |  |  |  |  |  |  |
Mechanics Bank Acquisition | Common Class B |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Business combination, fixed exchange ratio | 330.1092 |  |  |  |  |  |  |  |
Mechanics Bank Acquisition | Nonvoting Common Stock |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Business combination, fixed exchange ratio | 330.1092 |  |  |  |  |  |  |  |
Mechanics Bank Acquisition | Voting Common Stock |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Business combination, fixed exchange ratio | 3,301.092 |  |  |  |  |  |  |  |
Mechanics Bank | Common Class A |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Common stock, par value (in dollars per share) | $ 50 |  |  |  |  |  |  |  |
Mechanics Bank | Common Class B |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Common stock, par value (in dollars per share) | $ 50 |  |  |  |  |  |  |  |
Legacy Mechanics Bank Shareholders | Mechanics Bancorp |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Economic equity interest after merger | 91.70% |  |  |  |  |  |  |  |
Voting equity interest after merger | 91.34% |  |  |  |  |  |  |  |
Legacy Homestreet, Inc. Shareholders | Mechanics Bancorp |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Economic equity interest after merger | 8.30% |  |  |  |  |  |  |  |
Voting equity interest after merger | 8.66% |  |  |  |  |  |  |  |
Homestreet, Inc. |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Net income (loss) |  |  |  |  | $ 8,900 |  |  | $ 144,300 | $ 27,500

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