XML 90 R84.htm IDEA: XBRL DOCUMENT
v3.25.3

Segment Reporting - Additional Information (Details) | 9 Months Ended
--- | ---
Sep. 30, 2025 Segment
Segment Reporting [Abstract] |
Number Of States | 17
Number of reportable segments | 1
Segment Reporting, CODM, Individual Title and Position or Group Name [Extensible Enumeration] | srt:ChiefExecutiveOfficerMember, srt:ChiefFinancialOfficerMember, Chief commercial lending officer [Member], Chief consumer lending officer [Member], Chief information and consumer banking officer [Member], Chief risk officer [Member]
Segment Reporting, CODM, Profit (Loss) Measure, How Used, Description | The CODM uses net income to evaluate income generated from segment assets in deciding whether to reinvest profits into the Bank segment or into other parts of the entity, such as for acquisitions or to pay dividends. Net income is used to monitor budget versus actual results. The CODM also uses net income in competitive analysis by benchmarking to the Company’s competitors. The competitive analysis along with the monitoring of budgeted versus actual results are used in assessing performance of the segment and in establishing management’s compensation.

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 Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 47 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482785/280-10-55-47 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 54 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482785/280-10-55-54 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-18 + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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

X
- Definition Indicates title and position of individual or name of group identified as chief operating decision maker (CODM) for segment reporting. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 54 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482785/280-10-55-54 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 47 -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482785/280-10-55-47 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 21 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-21 + Details
Name: | us-gaap_SegmentReportingCodmIndividualTitleAndPositionOrGroupOrCommitteeNameExtensibleEnumeration
Namespace Prefix: | us-gaap_
Data Type: | enum2:enumerationSetItemType
Balance Type: | na
Period Type: | duration

X
- Definition Description of how chief operating decision maker (CODM) uses reported segment profit (loss) measure to assess performance and allocate resource. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 47 -Subparagraph (bb) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482785/280-10-55-47 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 54 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482785/280-10-55-54 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 29 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482810/280-10-50-29 + Details
Name: | us-gaap_SegmentReportingCodmProfitLossMeasureHowUsedDescription
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration