XML 63 R51.htm IDEA: XBRL DOCUMENT
v3.20.4

Description Of Business - Additional Information (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Abstract] |
Transfer of indirect loan portfolio to bank | $ 45.5
Number of states | 11

X
- Definition Number of states conducting lending of loan portfolio. + References No definition available. + Details
Name: | fusb_NumberOfStates
Namespace Prefix: | fusb_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Transfer of indirect loan portfolio to bank. + References No definition available. + Details
Name: | fusb_TransferOfIndirectLoanPortfolioToBank
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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