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Loans and Allowance for Credit Losses - Schedule of Loan Portfolio (Parenthetical) (Details) - USD ($) $ in Thousands | Mar. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Loans | $ 775,889 | $ 773,873
Equipment Financing Leases [Member] |  |
Loans | $ 11,200 | $ 10,300

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- Definition Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI https://asc.fasb.org/extlink&oid=124259787&loc=d3e4428-111522 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=fusb_EquipmentFinancingLeasesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |