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v3.25.3

General - Additional Information (Details) | 9 Months Ended
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Sep. 30, 2025
Organization Consolidation And Presentation Of Financial Statements [Line Items] |
Number of states | 17

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- Definition Number of states conducting lending of loan portfolio. + References No definition available. + Details
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Namespace Prefix: | fusb_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Organization consolidation and presentation of financial statements. + References No definition available. + Details
Name: | fusb_OrganizationConsolidationAndPresentationOfFinancialStatementsLineItems
Namespace Prefix: | fusb_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration