XML 84 R78.htm IDEA: XBRL DOCUMENT
v3.24.1.1.u2

Other Operating Income and Expense - Additional Information (Details) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2024 USD ($)
Other Income and Expenses [Abstract] |
Reduce of other expense associated with recovery of check fraud losses | $ 0.2

X
- Definition Increase decrease In other expense. + References No definition available. + Details
Name: | fusb_IncreaseDecreaseInOtherExpense
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_OtherIncomeAndExpensesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration