Leases - Additional Information (Details)	12 Months Ended
Dec. 31, 2020
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	0 years
Maximum [Member]
Lessee Lease Description [Line Items]
Lessee, Operating Lease, Remaining Term of Contract	13 years

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.

No definition available.

+ Details

Name:	fusb_LesseeOperatingLeasePeriodOfOptionToTerminate
Namespace Prefix:	fusb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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

- 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

- 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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: