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v3.21.1

401(k) Profit Sharing Plan (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Defined Contribution Plan, Employer Matching Contribution, Percent of Match, First One Percent of Contributions | 100.00% |
Defined Contribution Plan, Employer Matching Contribution, Percent of Match, Next Five Percent of Contributions | 50.00% |
Defined Contribution Plan, Cost Recognized | $ 1.1 | $ 1.2
Other Than SERP Hired Prior to 2010 [Member] |  |
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent | 0.05% |
Other Than SERP Hired Since Jan 1 2010 [Member] |  |
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent | 4.00% |

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- Definition Percentage employer matches of the employee's first one percentage contribution matched. + References No definition available. + Details
Name: | func_DefinedContributionPlanEmployerMatchingContributionPercentOfMatchFirstOnePercentOfContributions
Namespace Prefix: | func_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage employer matches of the employee's next five percentage contribution matched. + References No definition available. + Details
Name: | func_DefinedContributionPlanEmployerMatchingContributionPercentOfMatchNextFivePercentOfContributions
Namespace Prefix: | func_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
Name: | us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_RetirementPlanNameAxis=func_OtherThanSerpHiredPriorTo2010Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RetirementPlanNameAxis=func_OtherThanSerpHiredSinceJan12010Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |