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Summary of Significant Accounting Policies (Detail) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 USD ($) investmentSecurity | Sep. 30, 2025 USD ($) segment county | Sep. 30, 2024 USD ($)
Significant Accounting Policies [Line Items] |  |  |
Number of subsidiary business trusts (in trusts) | investmentSecurity | 2 |  |
Company's investments in the trusts | $ | $ 1,200,000 | $ 1,200,000 |
Number of operating segments (in segments) | segment |  | 1 |
Number of reportable segments (in segments) | segment |  | 1 |
Loans contractual past due |  | 90 days |
Security credit losses recognized | $ |  | $ 0 | $ 0
California |  |  |
Significant Accounting Policies [Line Items] |  |  |
Number of counties (in counties) | county |  | 31 |

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- Definition Number Of Counties Bank Operates + References No definition available. + Details
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- Definition Amount of credit loss expense on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) transferred into available-for-sale out of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 8B -Publisher FASB -URI https://asc.fasb.org/1943274/2147481830/320-10-45-8B + Details
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- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 49 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482785/280-10-55-49 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-18 + Details
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