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Employee Benefit Plans - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Defined Contribution Plan, Employer Matching Contribution, Percent of Match | 100.00% |
Defined Contribution Plan, Discretionary Contribution Percent | 2.00% |
Defined Contribution Plan, Employer Discretionary Contribution Amount | $ 0.5 | $ 0.5
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent | 20.00% |
Defined Benefit Plan, Plan Assets, Employer, Related Party, Number of Shares | 214,056 | 221,357
Maximum [Member] |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay | 4.00% |

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- Definition Percentage of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | fusb_DefinedContributionPlanDiscretionaryContributionPercent
Namespace Prefix: | fusb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of shares of employer stock in which defined benefit plan asset is invested. Includes, but is not limited to, stock issued or managed by related party of employer. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (n) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanNumberOfSharesOfEquitySecuritiesIssuedByEmployerAndRelatedPartiesIncludedInPlanAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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