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Other Real Estate Owned and Repossessions - Additional Information (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Other Real Estate [Abstract] |  |
Foreclosed real estate owned, fair value less disposal costs | $ 28 | $ 100
Mortgage loans in process of foreclosure, amount | $ 0 | $ 700

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- Definition The fair value less estimated costs to sell of foreclosed residential real estate held by the Company as of the balance sheet date. + References No definition available. + Details
Name: | fusb_ForeclosedRealEstateOwnedFairValueLessDisposalCosts
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 35 -URI http://asc.fasb.org/extlink&oid=121611835&loc=SL49126937-111524 + Details
Name: | us-gaap_MortgageLoansInProcessOfForeclosureAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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