Number of financing receivables that have been modified by troubled debt restructurings.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI https://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fusb_RealEstate1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fusb_SecuredBy1To4FamilyResidentialPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fusb_SecuredByNonfarmNonresidentialPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fusb_CommercialAndIndustrialLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans and Allowance for Loan and Lease Losses - Loans Modified in a Troubled Debt Restructuring (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2022 Loan	Dec. 31, 2022 DebtSecurityNumber	Dec. 31, 2022 USD ($)	Dec. 31, 2021 Loan	Dec. 31, 2021 DebtSecurityNumber	Dec. 31, 2021 USD ($)
Number of loans	0	4		0	4
Pre- Modification Outstanding Principal Balance			$ 715			$ 715
Post- Modification Principal Balance			646			660
Real Estate [Member] \| Secured by 1-4 Family Residential Properties [Member]
Number of loans \| DebtSecurityNumber		1			1
Pre- Modification Outstanding Principal Balance			23			23
Post- Modification Principal Balance			12			12
Real Estate [Member] \| Secured By Non-farm Non residential Properties [Member]
Number of loans \| DebtSecurityNumber		2			2
Pre- Modification Outstanding Principal Balance			621			621
Post- Modification Principal Balance			612			617
Commercial and Industrial Loans [Member]
Number of loans \| DebtSecurityNumber		1			1
Pre- Modification Outstanding Principal Balance			71			71
Post- Modification Principal Balance			$ 22			$ 31