Fair Value of Financial Instruments (Narrative) (Details) - USD ($)	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Fair Value of Financial Instruments [Abstract]
Gains or losses included in earnings attributable to the change in realized/unrealized gains or losses related to the assets	$ 0	$ 0
Allowance for credit losses	(17,982,000)	$ (16,871,000)	$ (17,480,000)
Loans	1,412,327,000		1,406,667,000
Financing receivables minimum benchmark for evaluation for specific reserves	100,000
Financing receivable individually evaluated with no allowance carrying value	$ 14,900,000		$ 3,000,000.0

Financing receivable individually evaluated with no allowance carrying value.

No definition available.

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- Definition

Financing receivables minimum benchmark for evaluation for specific reserves.

+ References

No definition available.

+ Details

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Namespace Prefix:	func_
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- References

No definition available.

+ Details

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- Definition

For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482907/825-10-50-30

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

+ Details

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

- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, fee, and loan in process, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481598/310-20-45-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1

+ Details

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