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

Commercial amount benchmark to require external appraisal.

+ References

No definition available.

+ Details

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

- Definition

Commerical amount benchmark internal impairment review.

+ References

No definition available.

+ Details

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

- Definition

Commerical amount benchmark internal impairment review minimum relationship amount.

+ References

No definition available.

+ Details

Name:	func_CommericalAmountBenchmarkInternalImpairmentReviewMinimumRelationshipAmount
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

Name:	func_CommericalAmountBenchmarkMinimumForAnnualReviewByIndependentReviewer
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

Name:	us-gaap_FinancingReceivableModificationsLineItems
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

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=49124121&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Percent of financing receivables past due.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivablePercentPastDue
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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 (d)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5111-111524

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
Namespace Prefix:	us-gaap_
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=25495931&loc=d3e18845-111554

+ Details

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 6
-Section L
-Subsection 1

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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=49124121&loc=SL49126937-111524

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ResidentialPortfolioSegmentMember
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:

Loans and Related Allowance for Loan Losses (Narrative) (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($) contract	Jun. 30, 2015 USD ($) contract	Jun. 30, 2016 USD ($) contract	Jun. 30, 2015 USD ($) contract	Dec. 31, 2015 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
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
Commerical amount benchmark internal impairment review	500,000
Commerical amount benchmark internal impairment review minimum relationship amount	750,000
Commercial amount benchmark to require external appraisal	500,000
Nonaccrual loans	11,318,000		$ 11,318,000		$ 15,498,000
Reduction of the ALL resulting from change in evaluation method of loans	486		2,873	$ 4,324
Loans in process of foreclosure	$ 2,100,000		2,100,000		$ 1,800,000
Accruing loans past due 30 days or greater as percent of loan portfolio	0.47%				0.76%
Total allowance for loan losses	$ 12,678,000	$ 11,809,000	12,678,000	$ 11,809,000	$ 11,922,000	$ 12,256,000	$ 11,751,000	$ 12,065,000
Partial Charge Off [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans	$ 5,900,000		$ 5,900,000		4,100,000
New TDR [Member]
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts \| contract	1		3	7
Pre Existing TDR [Member]
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts \| contract		1
Pre Existing TDR Loans Remodified After Maturity [Member]
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts \| contract			1	8
Unallocated [Member]
Financing Receivable, Modifications [Line Items]
Total allowance for loan losses	$ 500,000		$ 500,000
Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans		$ 70,000		$ 70,000
Commercial real estate- non owner-occupied [Member]
Financing Receivable, Modifications [Line Items]
Nonaccrual loans	$ 1,026,000		$ 1,026,000		$ 1,308,000