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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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