Loans And Leases - Allowance for Credit Losses on Loans and Leases (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Beginning balance	$ 10,184	$ 10,507
Charge-offs	(2,213)	(806)
Recoveries	252	414
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	3,165	112
Ending balance	11,388	10,227
Loans	871,431		$ 823,039
Credit Losses on Unfunded Lending Commitments [Member]
Beginning balance	376	569
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	80	(112)
Ending balance	456	457
Construction, Land Development and Other Land Loans [Member]
Beginning balance	352	565
Recoveries		20
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	(69)	(50)
Ending balance	283	535
Construction, Land Development and Other Land Loans [Member] \| Credit Losses on Unfunded Lending Commitments [Member]
Beginning balance	280	450
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	82	(100)
Ending balance	362	350
Secured by Real Estate 1-4 Family Residential Properties [Member]
Beginning balance	406	591
Charge-offs		(2)
Recoveries	11	35
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	(5)	(78)
Ending balance	412	546
Secured by Real Estate 1-4 Family Residential Properties [Member] \| Credit Losses on Unfunded Lending Commitments [Member]
Beginning balance	1	1
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments		(1)
Ending balance	1
Secured By Multi family Residential Properties [Member]
Beginning balance	546	415
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	168	343
Ending balance	714	758
Secured By Multi family Residential Properties [Member] \| Credit Losses on Unfunded Lending Commitments [Member]
Beginning balance	43	9
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	2	(1)
Ending balance	45	8
Secured by Non-residential Commercial Real Estate [Member]
Beginning balance	1,428	1,425
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	177	280
Ending balance	1,605	1,705
Secured by Non-residential Commercial Real Estate [Member] \| Credit Losses on Unfunded Lending Commitments [Member]
Beginning balance	16	2
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	(9)	4
Ending balance	7	6
Reserve for Unfunded Commitments [Member]
Beginning balance	400
Ending balance	500
Commercial and Industrial Loans [Member]
Beginning balance	1,531	513
Charge-offs	(1,191)	(97)
Recoveries	16
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	647	(23)
Ending balance	1,003	393
Loans	40,986		44,238
Commercial and Industrial Loans [Member] \| Credit Losses on Unfunded Lending Commitments [Member]
Beginning balance	34	102
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	5	(14)
Ending balance	39	88
Consumer Portfolio Segment [Member]
Loans	448,502		384,456
Consumer Portfolio Segment [Member] \| Secured by 1-4 Family Residential Properties [Member]
Loans	67,587		69,999
Consumer Portfolio Segment [Member] \| Direct [Member]
Beginning balance	49	64
Charge-offs	(5)	(28)
Recoveries	107	178
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	(68)	(162)
Ending balance	83	52
Loans	4,836		4,774
Consumer Portfolio Segment [Member] \| Direct [Member] \| Credit Losses on Unfunded Lending Commitments [Member]
Beginning balance	2	5
Ending balance	2	5
Consumer Portfolio Segment [Member] \| Indirect Consumer [Member]
Beginning balance	5,872	6,934
Charge-offs	(1,017)	(679)
Recoveries	118	181
Provision for (recovery of) credit losses on loans and leases/unfunded lending commitments	2,315	(198)
Ending balance	7,288	$ 6,238
Loans	$ 376,079		$ 309,683

Amount of direct write-downs of accounts receivable charged against the allowance.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

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

- Definition

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

+ References

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11B

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11B

Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481696/310-30-50-2

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Accounting Standards Codification
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-SubTopic 10
-Topic 310
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480418/310-10-S99-4

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-2

+ Details

Name:	us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of credit loss expense (reversal of expense) for financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(11))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

+ Details

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

- Details

Name:	us-gaap_CreditLossStatusAxis=fusb_CreditLossesOnUnfundedLendingCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fusb_SecuredByRealEstate1To4FamilyResidentialPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_UnfundedLoanCommitmentMember
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:

- Details

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

- Details

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