XML 48 R42.htm IDEA: XBRL DOCUMENT
v3.25.2

General - Additional Information (Details) | 6 Months Ended
--- | ---
Jun. 30, 2025
Organization Consolidation And Presentation Of Financial Statements [Line Items] |
Number of states | 17

X
- Definition Number of states conducting lending of loan portfolio. + References No definition available. + Details
Name: | fusb_NumberOfStates
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