Loans And Related Allowances For Credit Losses (Narrative) (Details) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2024 USD ($) loan	Mar. 31, 2023 USD ($) loan	Dec. 31, 2023 USD ($)
Financing Receivable, Modifications [Line Items]
Allowance for loan losses	$ 17,982	$ 16,871	$ 17,480
Non-Accrual	16,007		3,956
Threshold commitment amount	$ 100
Number of loan modifications made to borrowers facing financial difficulties \| loan	0	0
Partial Charge Off [Member]
Financing Receivable, Modifications [Line Items]
Non-Accrual	$ 100		100
Commercial real estate- non owner-occupied [Member]
Financing Receivable, Modifications [Line Items]
Non-Accrual	219		227
Commercial real estate- all other CRE [Member]
Financing Receivable, Modifications [Line Items]
Non-Accrual	$ 599		$ 599
Acquisition and development- 1-4 family residential construction [Member]
Financing Receivable, Modifications [Line Items]
Percentage of loan portfolio accruing loan past due 30 days or more	0.40%	0.17%	0.24%
Financing receivable excluding accrued interest subject to foreclosure	$ 1,800		$ 1,800
Acquisition and development- All other A&D [Member]
Financing Receivable, Modifications [Line Items]
Non-Accrual	105		113
Commercial and industrial [Member]
Financing Receivable, Modifications [Line Items]
Allowance for loan losses	4,002	$ 3,755	3,717
Non-Accrual	12,093
Residential mortgage- term [Member]
Financing Receivable, Modifications [Line Items]
Non-Accrual	2,528		2,720
Residential mortgage- home equity [Member]
Financing Receivable, Modifications [Line Items]
Non-Accrual	335		268
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Allowance for loan losses	987	$ 827	929
Non-Accrual	$ 128		$ 29

Number of loan modifications made to borrowers facing financial difficulties.

No definition available.

Name:	func_FinancingReceivableExcludingAccruedInterestModificationsMadeToBorrowersFacingFinancialDifficultiesNumberOfContracts
Namespace Prefix:	func_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of financing receivable excluding accrued interest subject to foreclosure.

+ References

No definition available.

+ Details

Name:	func_FinancingReceivableExcludingAccruedInterestSubjectToForeclosure
Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of loan portfolio accruing loan past due thirty days or more.

+ References

No definition available.

+ Details

Name:	func_PercentageOfLoanPortfolioAccruingLoanPastDueThirtyDaysOrMore
Namespace Prefix:	func_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Threshold commitment amount for individually evaluated restructured financing receivable.

+ References

No definition available.

+ Details

Name:	func_RestructuredFinancingReceivableIndividuallyEvaluatedLoanThresholdCommitment
Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable on nonaccrual status. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-16

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrual
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://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-42

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-42

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481933/310-10-55-12A

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-33

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-34

+ Details

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

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=func_PartialChargeOffMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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