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Other Real Estate Owned and Repossessed Assets - Additional Information (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022
Other Real Estate [Abstract]
Foreclosed real estate owned, fair value less disposal costs	$ 20	$ 0
Mortgage loans in process of foreclosure, amount	$ 0	$ 19

X

- 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

X

- 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
-Name Accounting Standards Codification
-Section 50
-Paragraph 35
-SubTopic 10
-Topic 310
-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

X

- References

No definition available.

+ Details

Name:	us-gaap_OtherRealEstateAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration