Other Real Estate Owned, Net (Schedule of Components of OREO Expenses, Net) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Other Real Estate Owned, Net [Abstract]
Gains on sale of real estate, net		$ (491)
Fair value write-down		4
Expenses, net	96	123
Rental and other income	(1)	(48)
Total OREO expenses, net	$ 95	$ (412)

Foreclosed Real Estate Expense Income Net

No definition available.

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

- Definition

The charge (credit) against earnings in the period to reduce (increase) the carrying amount of real property to fair value.

+ References

No definition available.

+ Details

Name:	func_ImpairmentRecoveryOfRealEstate
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Balance Type:	debit
Period Type:	duration

- Definition

Other expenses, net of other income related to real estate owned activities.

+ References

No definition available.

+ Details

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

- Definition

The net gain (loss) resulting from sales and other disposals of other real estate owned, increases (decreases) in the valuation allowance for foreclosed real estate, and write-downs of other real estate owned after acquisition or physical possession.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.14(d))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

+ Details

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

- References

No definition available.

+ Details

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

Income earned by providing the use of assets to an outside party in exchange for a payment or series of payments that is nonoperating in nature.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7)
-URI http://asc.fasb.org/extlink&oid=123367319&loc=SL114868664-224227

+ Details

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Balance Type:	credit
Period Type:	duration