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

Amount of gain (loss) recognized in earnings from transfer of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) to investment in debt security measured at amortized cost (held-to-maturity).

No definition available.

Name:	func_DebtSecuritiesAvailableForSaleTransferToHeldToMaturityGainLoss
Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Financing Receivable Recovery Percentage Benchmark.

No definition available.

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

Interest Rate Post None Prepayment Benchmark.

No definition available.

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

None Prepayment Benchmark.

No definition available.

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

Recovery Benchmark Lag Period.

No definition available.

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

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=124260329&loc=SL6284422-111562

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

Amount, after tax and adjustment, of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and unrealized gain (loss) on investment in debt security measured at amortized cost (held-to-maturity) from transfer to available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=124509347&loc=SL7669646-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=124509347&loc=d3e637-108580

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=124509347&loc=SL7669646-108580

Name:	us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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.

No definition available.