Deposits (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Deposits [Abstract]
Time Deposits, $250,000 or More	$ 48.8	$ 37.5
Deposit Liabilities Reclassified as Loans Receivable	0.4	0.3
Related Party Deposit Liabilities	$ 17.2	$ 14.0

The aggregate amount of domestic and foreign time deposit liabilities (including certificates of deposits, individual retirement accounts and open accounts) in denominations of $200,000 or more

No definition available.

+ Details

Name:	func_TimeDeposits250000OrMore
Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of deposits held by the entity that have been reclassified as loan balances receivable, for example, but not limited to, overdrafts receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
-SubTopic 405
-Topic 942
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481047/942-405-50-1

+ Details

Name:	us-gaap_DepositLiabilitiesReclassifiedAsLoansReceivable1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_DepositsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of deposits held by the entity for a related party (entity, shareholder, employee).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(k)(1))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

+ Details

Name:	us-gaap_RelatedPartyDepositLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant