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Other Real Estate Owned and Repossessed Assets - Additional Information (Details) - USD ($) | Sep. 30, 2024 | Sep. 30, 2023
--- | --- | ---
Other Real Estate [Abstract] |  |
Foreclosed real estate owned, fair value less disposal costs | $ 0 | $ 0
Mortgage loans in process of foreclosure, amount | 0 | 0
Repossessed assets | $ 49,000 | $ 300,000

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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://www.xbrl.org/2003/role/disclosureRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 35 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481962/310-10-50-35 + Details
Name: | us-gaap_MortgageLoansInProcessOfForeclosureAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Carrying amount as of the balance sheet date of assets other than real property that were received in full or partial satisfaction of a debt arrangement through foreclosure proceedings or defeasance. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481962/310-10-50-11 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481990/310-10-45-3 + Details
Name: | us-gaap_OtherForeclosedAssets
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