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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions	Sep. 02, 2025 USD ($)	Mar. 31, 2026 branch
Business Combination [Line Items]
Number of banking branches | branch		166
Adoption Adjustment | Mechanics Bank Acquisition
Business Combination [Line Items]
Purchased seasoned loan, allowance for credit losses | $	$ 20.3
Mechanics Bancorp | Legacy Mechanics Bank Shareholders
Business Combination [Line Items]
Economic equity interest after merger	91.70%
Voting equity interest after merger	91.30%
Mechanics Bancorp | Legacy Homestreet, Inc. Shareholders
Business Combination [Line Items]
Economic equity interest after merger	8.30%
Voting equity interest after merger	8.70%

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