Commercial amount benchmark minimum criticized relationships for annual review by independent reviewer.

No definition available.

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Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Commercial amount benchmark minimum for internal annual review.

+ References

No definition available.

+ Details

Name:	func_CommercialAmountBenchmarkMinimumForAnnualReview
Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Commercial amount benchmark minimum for annual review by independent reviewer.

+ References

No definition available.

+ Details

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Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The minimum period has to be taken in past appraisal for conducting the review by external team.

+ References

No definition available.

+ Details

Name:	func_CommercialLoanAppraisalByExternalTeamConditionForMinimumPeriodPastAppraisal
Namespace Prefix:	func_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Commercial amount benchmark for appraisal by internal generated discounted appraisal grid.

+ References

No definition available.

+ Details

Name:	func_CommercialLoanBenchmarkMaximumForInternalDiscountedAppraisalGrid
Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Commercial amount benchmark minimum for review by external appraisal team.

+ References

No definition available.

+ Details

Name:	func_CommercialLoanBenchmarkMinimumForExternalAppraisal
Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Increase In Financing Receivable Recorded Investment Nonaccrual Status

+ References

No definition available.

+ Details

Name:	func_IncreaseInFinancingReceivableRecordedInvestmentNonaccrualStatus
Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Non Consumer loan amount benchmark minimum for annual review by independent reviewer.

+ References

No definition available.

+ Details

Name:	func_NonConsumerLoanAmountBenchmarkMinimumForAnnualReviewByIndependentReviewer
Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number Of Commercial Accounts In Forbearance

+ References

No definition available.

+ Details

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Namespace Prefix:	func_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number Of Geographic Regions Representing Of Lending Footprint

+ References

No definition available.

+ Details

Name:	func_NumberOfGeographicRegionsRepresentingOfLendingFootprint
Namespace Prefix:	func_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number Of Geographic Regions Utilized

+ References

No definition available.

+ Details

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Namespace Prefix:	func_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number Of Loans That Decreased Non Accrual Balance

+ References

No definition available.

+ Details

Name:	func_NumberOfLoansThatDecreasedNonAccrualBalance
Namespace Prefix:	func_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number Of Loans That Increased Non Accrual Balance

+ References

No definition available.

+ Details

Name:	func_NumberOfLoansThatIncreasedNonAccrualBalance
Namespace Prefix:	func_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number Of Well Collateralized Loans That Increased Balance Of Substandard Category

+ References

No definition available.

+ Details

Name:	func_NumberOfWellCollateralizedLoansThatIncreasedBalanceOfSubstandardCategory
Namespace Prefix:	func_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- 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

No definition available.

+ Details

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

- Definition

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

+ References

+ Details

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

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

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

- Definition

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953682-111524

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

- Definition

Amount of financing receivable on nonaccrual status.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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-Section 50
-Paragraph 16
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-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919258-210447

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before allowance and fees, of loans and leases receivable. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4428-111522

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

- Definition

Reflects the aggregate value of loans, trade receivables, securities and financial instruments that serve as collateral for obligations arising from a transfer of financial assets recorded as a secured borrowing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Paragraph 1A
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-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

+ Details

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

- Definition

Change in carrying amount of other real estate not separately disclosed. The change includes for example, but is not limited to, additions, improvements, disposals and valuation adjustments.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherRealEstatePeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Unused amount of commitments to extend credit to customers.

+ References

No definition available.

+ Details

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

- Details

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Balance Type:
Period Type:

- Details

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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Balance Type:
Period Type:

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Balance Type:
Period Type:

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Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
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Balance Type:
Period Type:

- Details

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Balance Type:
Period Type:

- Details

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Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Loans And Related Allowances For Loan Losses (Narrative) (Details)		3 Months Ended		12 Months Ended
	Feb. 28, 2021 USD ($) contract	Dec. 31, 2020 USD ($) loan	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) loan contract region	Dec. 31, 2019 USD ($) loan
Financing Receivable, Modifications [Line Items]
OREO increase (decrease)		$ 8,000,000.0	$ 8,000,000.0
Nonaccrual loans		3,339,000		$ 3,339,000	$ 10,849,000
Commercial amount benchmark minimum for internal annual review				1,000,000
Commercial amount benchmark minimum for annual review by independent reviewer				1,000,000
Non consumer loan amount benchmark minimum for annual review by independent reviewer				750,000
Commercial relationship amount benchmark for evaluation				750,000
Maximum commercial loan benchmark for external appraisal				$ 500,000
Past appraisal period benchmark for external appraisal				18 months
Threshold limit for commercial loan appraisal by internal discounted appraisal grid				$ 500,000
Financing receivable, modifications, number of contracts \| contract				10,891
Financing receivable, modifications, recorded investment		1,053,821,000		$ 1,053,821,000
Financing receivable, modifications, subsequent default, recorded investment				$ 0
Pass [Member]
Financing Receivable, Modifications [Line Items]
Number of geographic regions utilized \| region				7
Number of geographic regions that represent lending footprint \| region				6
COVID Modifications [Member] \| Subsequent Event [Member]
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts \| contract	40
Financing receivable, modifications, recorded investment	$ 18,240,000
Forbearance Granted [Member] \| COVID-19 Pandemic [Member]
Financing Receivable, Modifications [Line Items]
Number of commercial accounts in forbearance \| loan				19
Amount of commercial accounts in forberance		16,000,000.0		$ 16,000,000.0
Forbearance Granted [Member] \| COVID-19 Pandemic [Member] \| Substandard [Member]
Financing Receivable, Modifications [Line Items]
Amount of commercial accounts in forberance		4,600,000		4,600,000
Forbearance Granted [Member] \| COVID-19 Pandemic [Member] \| Special Mention [Member]
Financing Receivable, Modifications [Line Items]
Amount of commercial accounts in forberance		12,300,000		12,300,000
Partial Charge Off [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans		400,000		$ 400,000	$ 100,000
New TDRs [Member]
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts \| loan				0
Existing TDRs loans Remodified After Maturity [Member]
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts \| loan				9
Troubled Debt Restructuring [Member]
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts \| loan				14	15
Financing receivable, modifications, recorded investment		4,000,000.0		$ 4,000,000.0	$ 4,200,000
Unused commitments to extend credit		0		0	0
Foreclosure 1-4 Family Real Estate Properties [Member]
Financing Receivable, Modifications [Line Items]
Loans secured by real estate properties in process of foreclosure		400,000		400,000	100,000
Commercial real estate- non owner-occupied [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans		8,000		8,000	33,000
Commercial real estate- non owner-occupied [Member] \| Special Mention [Member]
Financing Receivable, Modifications [Line Items]
Increase in non-accrual status				$ 7,900,000
Number of loans that increased non accrual balance in Special Mention \| loan				1
Commercial real estate- all other CRE [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans		890,000		$ 890,000	647,000
Commercial real estate- all other CRE [Member] \| Substandard [Member]
Financing Receivable, Modifications [Line Items]
Increase in non-accrual status				$ 10,100,000
Number of well collateralized loans that increased balance of substandard category \| loan				3
Acquisition and development- All other A&D [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans		$ 366,000		$ 366,000	8,058,000
Commercial and industrial [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans					30,000
Number of loans that decreased non accrual balance \| loan		1
Commercial and industrial [Member] \| Medical Profession Loan [Member]
Financing Receivable, Modifications [Line Items]
Number of loans that increased non accrual balance in Special Mention \| loan				1
Commercial and industrial [Member] \| Relationship With Chartered Airline Company [Member]
Financing Receivable, Modifications [Line Items]
Number of loans that increased non accrual balance in Special Mention \| loan				1
Residential mortgage- term [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans		$ 1,634,000		$ 1,634,000	1,219,000
Residential mortgage- home equity [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans		414,000		414,000	858,000
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans		$ 27,000		$ 27,000	$ 4,000