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Consolidated Statement of Income (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Consolidated Statements of Income [Abstract]
Provision for income tax expense, tax reform impact	$ 3,226

X

- Definition

Amount of reasonable estimate for income tax expense (benefit) from effect of Tax Cuts and Jobs Act of 2017 for which accounting for tax effect is incomplete.

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

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