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General - Additional Information (Details)	9 Months Ended
	Sep. 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