Organization consolidation and presentation of financial statements.

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Namespace Prefix:	fusb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Description Of Business - Additional Information (Details) - Branch		3 Months Ended	12 Months Ended
	Sep. 03, 2021	Sep. 30, 2021	Dec. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of states			12
ALC [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of office closed	20	20