Loans And Leases - Carrying Amount of Loans And Leases by Credit Quality Indicator and Year of Origination (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2025	Dec. 31, 2024
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 871,431	$ 823,039
Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	15,525	47,502
2024	45,064	49,085
2023	46,764	125,939
2022	109,127	72,076
2021	71,145	63,529
Prior	135,304	80,452
Total	422,929	438,583
Current period gross charge-offs, 2023	1,191	54
Current period gross charge-offs, 2021		43
Current period gross charge-offs, Prior		272
Current period gross charge-offs	1,191	369
Commercial [Member] \| Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	15,525	47,502
2024	45,064	43,312
2023	42,494	125,593
2022	97,746	71,470
2021	70,577	63,048
Prior	134,259	79,766
Total	405,665	430,691
Commercial [Member] \| Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	544	346
2022	11,381	121
2021	103	334
Prior	329
Total	12,357	801
Commercial [Member] \| Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		2,489
2023	2,322
2022		485
2021	465	147
Prior	716	686
Total	3,503	3,807
Commercial [Member] \| Doubtful [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		3,284
2023	1,404
Total	1,404	3,284
Commercial [Member] \| Construction, Land Development and Other Land Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	958	852
2024	5,140	7,115
2023	14,698	45,087
2022	19,114	11,992
2021	8,009	41
Prior	182	450
Total	48,101	65,537
Commercial [Member] \| Construction, Land Development and Other Land Loans [Member] \| Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	958	852
2024	5,140	4,626
2023	12,376	45,087
2022	19,114	11,931
2021	7,957	41
Prior	182	450
Total	45,727	62,987
Commercial [Member] \| Construction, Land Development and Other Land Loans [Member] \| Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022		61
2021	52
Total	52	61
Commercial [Member] \| Construction, Land Development and Other Land Loans [Member] \| Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		2,489
2023	2,322
Total	2,322	2,489
Commercial [Member] \| Secured By Multi family Residential Properties [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	594	7,941
2024	201	8,218
2023	8,927	42,077
2022	62,134	17,557
2021	22,620	5,592
Prior	24,331	19,672
Total	118,807	101,057
Commercial [Member] \| Secured By Multi family Residential Properties [Member] \| Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	594	7,941
2024	201	8,218
2023	8,927	42,077
2022	53,635	17,557
2021	22,620	5,592
Prior	24,331	19,672
Total	110,308	101,057
Commercial [Member] \| Secured By Multi family Residential Properties [Member] \| Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022	8,499
Total	8,499
Commercial [Member] \| Secured by Non-residential Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	11,245	25,251
2024	26,039	24,906
2023	16,796	35,246
2022	25,015	37,276
2021	35,982	55,008
Prior	99,958	50,064
Total	215,035	227,751
Current period gross charge-offs, Prior		248
Current period gross charge-offs		248
Commercial [Member] \| Secured by Non-residential Commercial Real Estate [Member] \| Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	11,245	25,251
2024	26,039	24,906
2023	16,252	34,930
2022	22,156	36,793
2021	35,517	54,527
Prior	99,121	49,681
Total	210,330	226,088
Commercial [Member] \| Secured by Non-residential Commercial Real Estate [Member] \| Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	544	316
2022	2,859
2021		334
Prior	329
Total	3,732	650
Commercial [Member] \| Secured by Non-residential Commercial Real Estate [Member] \| Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022		483
2021	465	147
Prior	508	383
Total	973	1,013
Commercial [Member] \| Commercial and Industrial Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		13,458
2024		8,846
2023		3,529
2022		5,251
2021		2,888
Prior		10,266
Total		44,238
Current period gross charge-offs, 2023	1,191	54
Current period gross charge-offs, 2021		43
Current period gross charge-offs, Prior		24
Current period gross charge-offs	1,191	121
Commercial [Member] \| Commercial and Industrial Loans [Member] \| Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		13,458
2024		5,562
2023		3,499
2022		5,189
2021		2,888
Prior		9,963
Total		40,559
Commercial [Member] \| Commercial and Industrial Loans [Member] \| Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023		30
2022		60
Total		90
Commercial [Member] \| Commercial and Industrial Loans [Member] \| Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022		2
Prior		303
Total		305
Commercial [Member] \| Commercial and Industrial Loans [Member] \| Doubtful [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		3,284
Total		3,284
Commercial [Member] \| Commercial and Industrial Loans and Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	2,728
2024	13,684
2023	6,343
2022	2,864
2021	4,534
Prior	10,833
Total	40,986
Commercial [Member] \| Commercial and Industrial Loans and Leases [Member] \| Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	2,728
2024	13,684
2023	4,939
2022	2,841
2021	4,483
Prior	10,625
Total	39,300
Commercial [Member] \| Commercial and Industrial Loans and Leases [Member] \| Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022	23
2021	51
Total	74
Commercial [Member] \| Commercial and Industrial Loans and Leases [Member] \| Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Prior	208
Total	208
Commercial [Member] \| Commercial and Industrial Loans and Leases [Member] \| Doubtful [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	1,404
Total	1,404
Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	101,645	61,502
2024	57,484	77,063
2023	68,557	93,004
2022	83,939	71,137
2021	64,512	48,075
Prior	72,365	33,675
Total	448,502	384,456
Current period gross charge-offs, 2025		21
Current period gross charge-offs, 2024	87	95
Current period gross charge-offs, 2023	128	279
Current period gross charge-offs, 2022	396	573
Current period gross charge-offs, 2021	164	332
Current period gross charge-offs, Prior	247	145
Current period gross charge-offs	1,022	1,445
Consumer [Member] \| Performing [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	101,645	61,502
2024	57,484	77,063
2023	68,557	93,004
2022	83,939	71,137
2021	64,512	48,075
Prior	71,765	32,062
Total	447,902	382,843
Consumer [Member] \| Non-performing [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Prior	600	1,613
Total	600	1,613
Consumer [Member] \| Secured by 1-4 Family Residential Properties [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1,832	4,559
2024	4,542	3,525
2023	3,224	18,060
2022	16,882	14,746
2021	14,214	5,884
Prior	26,893	23,225
Total	67,587	69,999
Current period gross charge-offs, Prior		2
Current period gross charge-offs		2
Consumer [Member] \| Secured by 1-4 Family Residential Properties [Member] \| Performing [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1,832	4,559
2024	4,542	3,525
2023	3,224	18,060
2022	16,882	14,746
2021	14,214	5,884
Prior	26,298	21,612
Total	66,992	68,386
Consumer [Member] \| Secured by 1-4 Family Residential Properties [Member] \| Non-performing [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Prior	595	1,613
Total	595	1,613
Consumer [Member] \| Direct Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1,850	2,397
2024	1,561	1,077
2023	729	430
2022	250	487
2021	279	283
Prior	167	100
Total	4,836	4,774
Current period gross charge-offs, 2024	4
Current period gross charge-offs, 2023		1
Current period gross charge-offs, 2022		42
Current period gross charge-offs, 2021	1	3
Current period gross charge-offs, Prior		16
Current period gross charge-offs	5	62
Consumer [Member] \| Direct Consumer [Member] \| Performing [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1,850	2,397
2024	1,561	1,077
2023	729	430
2022	250	487
2021	279	283
Prior	167	100
Total	4,836	4,774
Consumer [Member] \| Indirect [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	97,963
2024	51,381
2023	64,604
2022	66,807
2021	50,019
Prior	45,305
Total	376,079	309,683
Current period gross charge-offs, 2024	83
Current period gross charge-offs, 2023	128
Current period gross charge-offs, 2022	396
Current period gross charge-offs, 2021	163
Current period gross charge-offs, Prior	247
Current period gross charge-offs	1,017
Consumer [Member] \| Indirect [Member] \| Performing [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	97,963
2024	51,381
2023	64,604
2022	66,807
2021	50,019
Prior	45,300
Total	376,074
Consumer [Member] \| Indirect [Member] \| Non-performing [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Prior	5
Total	$ 5
Consumer [Member] \| Indirect Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		54,546
2024		72,461
2023		74,514
2022		55,904
2021		41,908
Prior		10,350
Total		309,683
Current period gross charge-offs, 2025		21
Current period gross charge-offs, 2024		95
Current period gross charge-offs, 2023		278
Current period gross charge-offs, 2022		531
Current period gross charge-offs, 2021		329
Current period gross charge-offs, Prior		127
Current period gross charge-offs		1,381
Consumer [Member] \| Indirect Consumer [Member] \| Performing [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		54,546
2024		72,461
2023		74,514
2022		55,904
2021		41,908
Prior		10,350
Total		$ 309,683

Allowance for loan and lease losses write offs current fiscal year.

No definition available.

Name:	fusb_AllowanceForLoanAndLeaseLossesWriteOffsCurrentFiscalYear
Namespace Prefix:	fusb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Allowance for loan and lease losses write offs fiscal year before latest fiscal year.

+ References

No definition available.

+ Details

Name:	fusb_AllowanceForLoanAndLeaseLossesWriteOffsFiscalYearBeforeLatestFiscalYear
Namespace Prefix:	fusb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Allowance for loan and lease losses write offs five or more years before latest fiscal year.

+ References

No definition available.

+ Details

Name:	fusb_AllowanceForLoanAndLeaseLossesWriteOffsFiveOrMoreYearsBeforeLatestFiscalYear
Namespace Prefix:	fusb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Allowance for loan and lease losses write offs four years before latest fiscal year.

+ References

No definition available.

+ Details

Name:	fusb_AllowanceForLoanAndLeaseLossesWriteOffsFourYearsBeforeLatestFiscalYear
Namespace Prefix:	fusb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Allowance for loan and lease losses write offs three years before latest fiscal year.

+ References

No definition available.

+ Details

Name:	fusb_AllowanceForLoanAndLeaseLossesWriteOffsThreeYearsBeforeLatestFiscalYear
Namespace Prefix:	fusb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Allowance for loan and lease losses write offs two years before latest fiscal year

+ References

No definition available.

+ Details

Name:	fusb_AllowanceForLoanAndLeaseLossesWriteOffsTwoYearsBeforeLatestFiscalYear
Namespace Prefix:	fusb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11B

+ Details

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

- Definition

Amortized cost of financing receivable originated more than five years prior to current fiscal year. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-79

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6

+ Details

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

- Definition

Amortized cost of financing receivable originated four years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable originated in current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable originated in fiscal year prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable originated three years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost of financing receivable originated two years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 29
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-29

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-79

+ Details

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

- 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

- Details

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

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_PassMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_DoubtfulMember
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_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=fusb_CommercialAndIndustrialLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fusb_CommercialAndIndustrialLoansAndLeasesMember
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_FinancialInstrumentPerformanceStatusAxis=us-gaap_PerformingFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_NonperformingFinancingReceivableMember
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:

- Details

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