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

No definition available.

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

- Definition

Commerical amount benchmark minimum for internal annual review.

+ References

No definition available.

+ Details

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

- Definition

Commerical 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

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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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of financing receivable balance that is past due.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

+ Details

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

- Definition

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL49126937-111524

+ Details

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

- Details

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Namespace Prefix:
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- Details

Name:	us-gaap_LoanRestructuringModificationAxis=func_NewTroubledDebtRestructingLoansMember
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Data Type:	na
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Period Type:

- Details

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Namespace Prefix:
Data Type:	na
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- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
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Loans and Related Allowance for Loan Losses (Narrative) (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) contract	Mar. 31, 2017 contract	Dec. 31, 2017 USD ($)
Financing Receivable, Modifications [Line Items]
Commercial amount benchmark minimum for internal annual review	$ 500,000
Commercial amount benchmark minimum for annual review by independent reviewer	1,000,000
Commercial amount benchmark minimum criticized relationships for annual review by independent reviewer	500,000
Nonaccrual loans	4,301,000		$ 7,118,000
Loans in process of foreclosure	$ 300,000		$ 400,000
Accruing loans past due 30 days or greater as percent of loan portfolio	1.14%		0.63%
Partial Charge Off [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans	$ 1,900,000		$ 2,100,000
New TDR [Member]
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts \| contract	0	0
Pre Existing TDR Loans Remodified After Maturity [Member]
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts \| contract		2
Commercial real estate- non owner-occupied [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans	$ 1,577,000		3,248,000
Commercial and industrial [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans	$ 26,000		$ 378,000