Loans And Related Allowance for Credit Losses (Narrative) (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2026 USD ($) loan	Dec. 31, 2025 USD ($)
Loans and Related Allowance for Credit Losses
Non-Accrual	$ 4,695,000	$ 4,192,000
Threshold commitment amount	$ 100,000
Number of new loans modified \| loan	1
Number of existing loans modified \| loan	1
Modified loan, subsequent defaults	$ 100,000
Partial Charge Off
Loans and Related Allowance for Credit Losses
Non-Accrual	100,000	200,000
Acquisition and development- 1-4 family residential construction
Loans and Related Allowance for Credit Losses
Financing receivable excluding accrued interest subject to foreclosure	$ 0	$ 500,000
Percentage of loan portfolio accruing loan past due 30 days or more	0.35%	0.32%
Commercial and industrial
Loans and Related Allowance for Credit Losses
Non-Accrual	$ 1,026,000	$ 1,068,000

The amount of financing receivable excluding accrued interest subject to foreclosure.

No definition available.

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- Definition

Number of existing loans modified during the period.

+ References

No definition available.

+ Details

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- Definition

Number of new loans modified during the period.

+ References

No definition available.

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- Definition

Percentage of loan portfolio accruing loan past due thirty days or more.

+ References

No definition available.

+ Details

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- Definition

Threshold commitment amount for individually evaluated restructured financing receivable.

+ References

No definition available.

+ Details

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Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

Amortized cost excluding accrued interest, of financing receivable with payment default in current period and modified within previous 12 months preceding payment default for debtor experiencing financial difficulty.

+ References

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-Paragraph 44
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- Definition

Amortized cost excluding accrued interest, of financing receivable on nonaccrual status. Excludes net investment in lease.

+ References

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- 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.

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- Details

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