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Loans and Allowance for Loan Losses - Schedule of Loan Portfolio (Parenthetical) (Details) $ in Millions | 3 Months Ended
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Dec. 31, 2019 USD ($)
Receivables [Abstract] |
Transfer of indirect loan portfolio to bank | $ 45.5

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- Definition Transfer Of Indirect Sales Portfolio To Bank. + References No definition available. + Details
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Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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