Loans and Related Allowance for Credit Losses (Impaired Loans and Related Interest Income by Loan Portfolio Class) (Details) $ in Thousands	Dec. 31, 2022 USD ($)
Financing Receivable, Impaired [Line Items]
Impaired Loans with Specific Allowance: Recorded Investment	$ 345
Impaired Loans with Specific Allowance: Related Allowance	26
Impaired Loans with No Specific Allowance: Recorded Investment	6,153
Total Impaired Loans: Recorded Investment	6,498
Unpaid Principal Balance	150
Commercial real estate- non owner-occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired Loans with No Specific Allowance: Recorded Investment	187
Total Impaired Loans: Recorded Investment	187
Commercial real estate- all other CRE [Member]
Financing Receivable, Impaired [Line Items]
Impaired Loans with No Specific Allowance: Recorded Investment	2,075
Total Impaired Loans: Recorded Investment	2,075
Acquisition and development- 1-4 family residential construction [Member]
Financing Receivable, Impaired [Line Items]
Impaired Loans with No Specific Allowance: Recorded Investment	210
Total Impaired Loans: Recorded Investment	210
Acquisition and development- All other A&D [Member]
Financing Receivable, Impaired [Line Items]
Impaired Loans with No Specific Allowance: Recorded Investment	146
Total Impaired Loans: Recorded Investment	146
Unpaid Principal Balance	109
Residential mortgage- term [Member]
Financing Receivable, Impaired [Line Items]
Impaired Loans with Specific Allowance: Recorded Investment	345
Impaired Loans with Specific Allowance: Related Allowance	26
Impaired Loans with No Specific Allowance: Recorded Investment	3,225
Total Impaired Loans: Recorded Investment	3,570
Unpaid Principal Balance	41
Residential mortgage- home equity [Member]
Financing Receivable, Impaired [Line Items]
Impaired Loans with No Specific Allowance: Recorded Investment	310
Total Impaired Loans: Recorded Investment	$ 310

- Definition

Amount of investment of impaired financing receivables excluding accrued interest with related allowance for credit losses and without a related allowance for credit losses.

+ References

No definition available.

+ Details

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Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance for credit losses related to impaired financing receivable excluding accrued interest.

+ References

No definition available.

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables excluding accrued with related allowance for credit losses and without related allowance for credit losses.

+ References

No definition available.

+ Details

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Namespace Prefix:	func_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in impaired financing receivables excluding accrued interest for which there is no related allowance for credit losses.

+ References

No definition available.

+ Details

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- Definition

Amount of investment in impaired financing receivables excluding accrued interest for which there is a related allowance for credit losses.

+ References

No definition available.

+ Details

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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.

+ References

No definition available.

+ Details

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- Details

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- Details

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- Details

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- Details

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