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

No definition available.

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Namespace Prefix:	func_
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Period Type:	duration

- Definition

Commerical amount benchmark minimum for internal annual review.

+ References

No definition available.

+ Details

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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
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- 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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- 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

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- Definition

Percentage of financing receivable balance that is past due.

+ References

No definition available.

+ Details

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

- Definition

Reflects the amount of the write-down related to a troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

+ Details

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

- 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

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

- Definition

Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Cash received from principal payments made on loans related to operating activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585

+ Details

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

- Details

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- Details

Name:	us-gaap_LoanRestructuringModificationAxis=func_NewTroubledDebtRestructingLoansMember
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- Details

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- Details

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- Details

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- Details

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Loans and Related Allowance for Loan Losses (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) contract	Jun. 30, 2017 contract	Jun. 30, 2018 USD ($) contract	Jun. 30, 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	$ 5,016,000		5,016,000		$ 7,118,000
Decrease in non-accrual balance due to charge-offs			800,000
Loan recovery			2,500,000
Loans in process of foreclosure	$ 100,000		$ 100,000		$ 400,000
Accruing loans past due 30 days or greater as percent of loan portfolio	0.31%		0.31%		0.63%
Partial Charge Off [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans	$ 900,000		$ 900,000		$ 2,100,000
New TDR [Member]
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts \| contract		0	0	0
Pre Existing TDR Loans Remodified After Maturity [Member]
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts \| contract	2		2	2
One Large Commercial Real Estate Relationship [Member]
Financing Receivable, Modifications [Line Items]
Additional nonaccrual credit added	$ 1,900,000		$ 1,900,000
Commercial real estate- non owner-occupied [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans	689,000		689,000		3,248,000
Commercial and industrial [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans	$ 16,000		$ 16,000		$ 378,000