Derivative Financial Instruments - Schedule of Cumulative Basis Adjustments for Fair Value Hedges (Parenthetical) (Details) $ in Millions	Dec. 31, 2020 USD ($)
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Amortized cost basis of closed portfolios used in hedging relationships	$ 41.9
Cumulative basis adjustments associated with hedging relationships	0.8
Designated hedged items	$ 20.0

Amortized cost basis of closed portfolios used in hedging relationships.

No definition available.

Name:	fusb_AmortizedCostBasisOfClosedPortfoliosUsedInHedgingRelationships
Namespace Prefix:	fusb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant