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Leases - Additional Information (Details) | 3 Months Ended
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Mar. 31, 2021
Lessee Lease Description [Line Items] |
Lessee, Operating Lease, Renewal Term | 5 years
Lessee, Operating Lease, Period of Option to Terminate | 1 year
Minimum [Member] |
Lessee Lease Description [Line Items] |
Lessee, Operating Lease, Remaining Term of Contract | 1 year
Maximum [Member] |
Lessee Lease Description [Line Items] |
Lessee, Operating Lease, Remaining Term of Contract | 13 years

X
- Definition The period of time in which the lessee holds the option to terminate an operating lease, in 'PnYnMnDTnHnMnS' format. For example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | fusb_LesseeOperatingLeasePeriodOfOptionToTerminate
Namespace Prefix: | fusb_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The remaining term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | fusb_LesseeOperatingLeaseRemainingTermOfContract
Namespace Prefix: | fusb_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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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
Name: | us-gaap_LesseeLeaseDescriptionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918673-209980 + Details
Name: | us-gaap_LesseeOperatingLeaseRenewalTerm
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |