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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

Nonaccrual status loans contractual past due.

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- Definition

Number Of Counties Bank Operates

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Number of subsidiary business trust.

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Significant Accounting Policies [Line Items]

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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

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- Definition

This element represents the aggregate cost of investments accounted for under the equity method of accounting.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
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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.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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