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Restructuring Charges - Additional Information (Details) - Branch |  | 3 Months Ended
--- | --- | ---
Sep. 03, 2021 | Sep. 30, 2021
ALC [Member] |  |
Restructuring Cost And Reserve [Line Items] |  |
Number of office closed | 20 | 20

X
- Definition Represents number of office closed during period for restructuring and related cost. + References No definition available. + Details
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Namespace Prefix: | fusb_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_RestructuringCostAndReserveLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_RestructuringCostAndReserveAxis=fusb_AcceptanceLoanCompanyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |