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Contractual Obligations, Commitments and Contingent Liabilities (Schedule of Commitments) (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Other Commitments [Line Items] |  |
Other Commitment | $ 219,118 | $ 156,926
Residential mortgage- home equity [Member] |  |
Other Commitments [Line Items] |  |
Other Commitment | 59,615 | 53,827
Residential Mortgage - Construction [Member] |  |
Other Commitments [Line Items] |  |
Other Commitment | 12,220 | 4,995
Commercial Loan [Member] |  |
Other Commitments [Line Items] |  |
Other Commitment | 125,294 | 85,089
Consumer Loan - Personal Credit Lines [Member] |  |
Other Commitments [Line Items] |  |
Other Commitment | 4,314 | 3,903
Standby Letters of Credit [Member] |  |
Other Commitments [Line Items] |  |
Other Commitment | $ 17,675 | $ 9,112

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- Definition Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions. + References No definition available. + Details
Name: | us-gaap_OtherCommitment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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- Details
Name: | us-gaap_OtherCommitmentsAxis=us-gaap_HomeEquityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_OtherCommitmentsAxis=func_ResidentialMortgageConstructionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_OtherCommitmentsAxis=us-gaap_CommercialLoanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_OtherCommitmentsAxis=func_ConsumerLoanPersonalCreditLinesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_OtherCommitmentsAxis=us-gaap_StandbyLettersOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |