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Loans and Related Allowance for Credit Losses (Collateral-dependent individually evaluated loans) (Details) $ in Thousands | Sep. 30, 2023 USD ($)
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Accounts, Notes, Loans and Financing Receivable [Line Items] |
Amortized cost basis of collateral-dependent individually evaluated loans | $ 2,314
Non-Accrual loans with no allowance | 2,314
Residential Mortgage [Member] |
Accounts, Notes, Loans and Financing Receivable [Line Items] |
Amortized cost basis of collateral-dependent individually evaluated loans | 2,314
Non-Accrual loans with no allowance | $ 2,314

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- Definition The amortized cost of collateral dependent loans excluding accrued interest. + References No definition available. + Details
Name: | func_FinancingReceivableExcludingAccruedInterestCollateralDependentLoansAmortizedCost
Namespace Prefix: | func_
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_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amortized cost excluding accrued interest, of financing receivable on nonaccrual status with no allowance for credit loss. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 16 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-16 + Details
Name: | us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrualNoAllowance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |