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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	Sep. 30, 2025 USD ($) branch shares	Sep. 02, 2025	Dec. 31, 2024 USD ($)
Business Combination [Line Items]
Number of banking branches \| branch	166
Intangible assets amortization period (in years)	8 years
Accrued contingent liability \| $	$ 4,200		$ 3,100
Mechanics Bancorp \| Legacy Mechanics Bank Shareholders
Business Combination [Line Items]
Economic equity interest after merger		91.70%
Voting equity interest after merger		91.34%
Mechanics Bancorp \| Legacy Homestreet, Inc. Shareholders
Business Combination [Line Items]
Economic equity interest after merger		8.30%
Voting equity interest after merger		8.66%
Mechanics Bancorp 2025 Equity Incentive Plan
Business Combination [Line Items]
Equity incentive plan shares (in shares) \| shares	7,750,000
Mechanics Bank Acquisition \| Mechanics Bancorp
Business Combination [Line Items]
Voting equity interest after merger		100.00%
Minimum \| Core Deposits
Business Combination [Line Items]
Intangible assets amortization period (in years)	6 years
Maximum \| Core Deposits
Business Combination [Line Items]
Intangible assets amortization period (in years)	10 years