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Fair Value of Financial Instruments - Assets Measured at Fair Value on Non-recurring Basis (Details) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
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Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Impaired loans | $ 5 |
OREO and other assets held-for-sale | 846 |
Fair Value, Recurring [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Impaired loans | 5 | $ 28
OREO and other assets held-for-sale | 846 | 949
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Impaired loans | 5 | 28
OREO and other assets held-for-sale | $ 846 | $ 949

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- Definition Fair value portion of impaired loans by an entity as a result of past transactions or events. + References No definition available. + Details
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Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Other real estate and other assets held-for-sale. + References No definition available. + Details
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Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Balance Type: | na
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- Details
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |