Regulatory Capital Requirements (Narrative) (Details)	12 Months Ended
Dec. 31, 2022
Regulatory Capital Requirements [Abstract]
Percentage of Capital Stock and Surplus on Secured Basis	10.00%

Percentage of Capital Stock and Surplus on Secured Basis.

No definition available.

Name:	func_PercentageOfCapitalStockAndSurplusOnSecuredBasis
Namespace Prefix:	func_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration