XML 112 R100.htm IDEA: XBRL DOCUMENT
v3.20.4

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

X
- Definition Amortized cost basis of closed portfolios used in hedging relationships. + References No definition available. + Details
Name: | fusb_AmortizedCostBasisOfClosedPortfoliosUsedInHedgingRelationships
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Cumulative basis adjustments associated with hedging relationships. + References No definition available. + Details
Name: | fusb_CumulativeBasisAdjustmentsAssociatedWithHedgingRelationships
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of hedged item in hedging relationship. Excludes hedged asset or liability. + References No definition available. + Details
Name: | us-gaap_DerivativeAmountOfHedgedItem
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration