Investments Securities (Gross Unrealized Losses and Fair Values of Securities) (Details) $ in Thousands	Dec. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Schedule of Investments [Line Items]
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 118,172	$ 34,430
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	$ 459	$ 134
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Number of Investments \| security	14	6
Available-for-Sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 13,260	$ 70,605
Available-for-Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	$ 5,284	$ 4,266
Available-for-Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Investments \| security	9	16
Held-to-Maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 2,973	$ 2,722
Held-to-Maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	$ 38	$ 6
Held-to-Maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Number Of Investments \| security	1	3
Held-to-Maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		$ 9,486
Held-to-Maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses		$ 149
Held-to-Maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Number Of Investments \| security		12
U.S. government agencies [Member]
Schedule of Investments [Line Items]
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 39,611	$ 24,907
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	$ 322	$ 80
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Number of Investments \| security	7	3
Available-for-Sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		$ 14,987
Available-for-Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses		$ 13
Available-for-Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Investments \| security		3
Residential mortgage-backed agencies [Member]
Schedule of Investments [Line Items]
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 22,899	$ 4,900
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	$ 100	$ 17
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Number of Investments \| security	2	1
Held-to-Maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 2,973	$ 2,722
Held-to-Maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	$ 38	$ 6
Held-to-Maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Number Of Investments \| security	1	3
Held-to-Maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		$ 9,486
Held-to-Maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses		$ 149
Held-to-Maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Number Of Investments \| security		12
Commercial mortgage-backed agencies [Member]
Schedule of Investments [Line Items]
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 16,034	$ 4,623
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	$ 36	$ 37
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Number of Investments \| security	1	2
Available-for-Sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		$ 5,793
Available-for-Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses		$ 55
Available-for-Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Investments \| security		3
Collateralized mortgage obligations [Member]
Schedule of Investments [Line Items]
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 39,628
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses	$ 1
Available-for-Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Number of Investments \| security	4
Available-for-Sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		$ 35,472
Available-for-Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses		$ 109
Available-for-Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Investments \| security		1
Collateralized debt obligations [Member]
Schedule of Investments [Line Items]
Available-for-Sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 13,260	$ 14,353
Available-for-Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Unrealized Losses	$ 5,284	$ 4,089
Available-for-Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Investments \| security	9	9

Held To Maturity Securities In Unrealized Loss Positions 12 Months Or Longer Number Of Positions

No definition available.

Name:	func_HeldToMaturitySecuritiesInUnrealizedLossPositions12MonthsOrLongerNumberOfPositions
Namespace Prefix:	func_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Held To Maturity Securities In Unrealized Loss Positions Less Than 12 Months Number Of Positions

+ References

No definition available.

+ Details

Name:	func_HeldToMaturitySecuritiesInUnrealizedLossPositionsLessThan12MonthsNumberOfPositions
Namespace Prefix:	func_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922888-210455

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for 12 months or longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27337-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27290-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for less than 12 months.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27290-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27337-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for less than 12 months.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for 12 months or longer.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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_ScheduleOfInvestmentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedDebtObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: