<?xml version="1.0" encoding="utf-8"?>
<FilingSummary xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <Version>2.4.0.8</Version>
  <ProcessingTime />
  <ReportFormat>Html</ReportFormat>
  <ContextCount>422</ContextCount>
  <ElementCount>437</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>78</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>12</UnitCount>
  <MyReports>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>101 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DocumentandEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>103 - Statement - Consolidated Statements of Financial Condition</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/StatementOfFinancialPositionUnclassified-DepositBasedOperations</Role>
      <ShortName>Consolidated Statements of Financial Condition</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>104 - Statement - Consolidated Statements of Financial Condition (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/StatementOfFinancialPositionUnclassified-DepositBasedOperationsParenthetical</Role>
      <ShortName>Consolidated Statements of Financial Condition (Parenthetical)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>105 - Statement - Consolidated Statements of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/StatementOfIncomeInterestBasedRevenue</Role>
      <ShortName>Consolidated Statements of Operations</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>106 - Statement - Consolidated Statements of Shareholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/StatementOfShareholdersEquityAndOtherComprehensiveIncome</Role>
      <ShortName>Consolidated Statements of Shareholders' Equity</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>107 - Statement - Consolidated Statements of Shareholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/StatementOfShareholdersEquityAndOtherComprehensiveIncomeParenthetical</Role>
      <ShortName>Consolidated Statements of Shareholders' Equity (Parenthetical)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>108 - Statement - Consolidated Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/StatementOfOtherComprehensiveIncome</Role>
      <ShortName>Consolidated Statements of Comprehensive Income</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>109 - Statement - Consolidated Statements of Comprehensive Income (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/StatementOfOtherComprehensiveIncomeParenthetical</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Parenthetical)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>110 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/StatementOfCashFlowsIndirectDepositBasedOperations</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>111 - Disclosure - Description of Business</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsNatureOfOperations</Role>
      <ShortName>Description of Business</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>112 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsSignificantAccountingPoliciesTextBlock</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>113 - Disclosure - Investment Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsInvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock</Role>
      <ShortName>Investment Securities</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>114 - Disclosure - Loans and Allowance for Loan Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsLoansAndAllowanceForLoanLossesTextBlock</Role>
      <ShortName>Loans and Allowance for Loan Losses</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>115 - Disclosure - Other Real Estate Owned</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsRealEstateOwnedTextBlock</Role>
      <ShortName>Other Real Estate Owned</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>116 - Disclosure - Premises and Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsPropertyPlantAndEquipmentDisclosureTextBlock</Role>
      <ShortName>Premises and Equipment</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>117 - Disclosure - Investment in Limited Partnerships</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsInvestmentInVariableInterestEntitiesDisclosureTextBlock</Role>
      <ShortName>Investment in Limited Partnerships</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>118 - Disclosure - Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsDepositLiabilitiesDisclosuresTextBlock</Role>
      <ShortName>Deposits</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>119 - Disclosure - Short-Term Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsShortTermDebtTextBlock</Role>
      <ShortName>Short-Term Borrowings</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>120 - Disclosure - Long-Term Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsLongTermDebtTextBlock</Role>
      <ShortName>Long-Term Debt</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>121 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsIncomeTaxDisclosureTextBlock</Role>
      <ShortName>Income Taxes</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>122 - Disclosure - Employee Benefit Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsDisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock</Role>
      <ShortName>Employee Benefit Plans</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>123 - Disclosure - Deferred Compensation Plan</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsCompensationAndEmployeeBenefitPlansTextBlock</Role>
      <ShortName>Deferred Compensation Plan</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>124 - Disclosure - Shareholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsStockholdersEquityNoteDisclosureTextBlock</Role>
      <ShortName>Shareholders' Equity</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>125 - Disclosure - Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsSegmentReportingDisclosureTextBlock</Role>
      <ShortName>Segment Reporting</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>126 - Disclosure - Other Operating Expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsOtherIncomeAndOtherExpenseDisclosureTextBlock</Role>
      <ShortName>Other Operating Expenses</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>127 - Disclosure - Operating Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsOperatingLeasesOfLessorDisclosureTextBlock</Role>
      <ShortName>Operating Leases</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>128 - Disclosure - Guarantees, Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsCommitmentsAndContingenciesDisclosureTextBlock</Role>
      <ShortName>Guarantees, Commitments and Contingencies</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>129 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsFairValueDisclosuresTextBlock</Role>
      <ShortName>Fair Value Measurements</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>130 - Disclosure - United Security Bancshares, Inc. (Parent Company Only) Financial Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsCondensedFinancialInformationOfParentCompanyOnlyDisclosureTextBlock</Role>
      <ShortName>United Security Bancshares, Inc. (Parent Company Only) Financial Information</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>131 - Disclosure - Quarterly Data (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsQuarterlyFinancialInformationTextBlock</Role>
      <ShortName>Quarterly Data (Unaudited)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>132 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsSignificantAccountingPoliciesTextBlockPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>133 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsSignificantAccountingPoliciesTextBlockTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>134 - Disclosure - Investment Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsInvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlockTables</Role>
      <ShortName>Investment Securities (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>135 - Disclosure - Loans and Allowance for Loan Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsLoansAndAllowanceForLoanLossesTextBlockTables</Role>
      <ShortName>Loans and Allowance for Loan Losses (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>136 - Disclosure - Other Real Estate Owned (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsRealEstateOwnedTextBlockTables</Role>
      <ShortName>Other Real Estate Owned (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>137 - Disclosure - Premises and Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsPropertyPlantAndEquipmentDisclosureTextBlockTables</Role>
      <ShortName>Premises and Equipment (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>138 - Disclosure - Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsDepositLiabilitiesDisclosuresTextBlockTables</Role>
      <ShortName>Deposits (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>139 - Disclosure - Long-Term Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsLongTermDebtTextBlockTables</Role>
      <ShortName>Long-Term Debt (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>140 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsIncomeTaxDisclosureTextBlockTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>141 - Disclosure - Shareholders' Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsStockholdersEquityNoteDisclosureTextBlockTables</Role>
      <ShortName>Shareholders' Equity (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>142 - Disclosure - Segment Reporting (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsSegmentReportingDisclosureTextBlockTables</Role>
      <ShortName>Segment Reporting (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>143 - Disclosure - Other Operating Expenses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsOtherIncomeAndOtherExpenseDisclosureTextBlockTables</Role>
      <ShortName>Other Operating Expenses (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>144 - Disclosure - Operating Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsOperatingLeasesOfLessorDisclosureTextBlockTables</Role>
      <ShortName>Operating Leases (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>145 - Disclosure - Guarantees, Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsCommitmentsAndContingenciesDisclosureTextBlockTables</Role>
      <ShortName>Guarantees, Commitments and Contingencies (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>146 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsFairValueDisclosuresTextBlockTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>147 - Disclosure - United Security Bancshares, Inc. (Parent Company Only) Financial Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsCondensedFinancialInformationOfParentCompanyOnlyDisclosureTextBlockTables</Role>
      <ShortName>United Security Bancshares, Inc. (Parent Company Only) Financial Information (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>148 - Disclosure - Quarterly Data (Unaudited) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/NotesToFinancialStatementsQuarterlyFinancialInformationTextBlockTables</Role>
      <ShortName>Quarterly Data (Unaudited) (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>149 - Disclosure - Description of Business - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureDescriptionOfBusinessAdditionalInformation</Role>
      <ShortName>Description of Business - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>150 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformation</Role>
      <ShortName>Summary of Significant Accounting Policies - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>151 - Disclosure - Summary of Significant Accounting Policies - Disclosures of Cash Flow Information and Non-cash Transactions (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesDisclosuresOfCashFlowInformationAndNoncashTransactions</Role>
      <ShortName>Summary of Significant Accounting Policies - Disclosures of Cash Flow Information and Non-cash Transactions (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>152 - Disclosure - Summary of Significant Accounting Policies - Basic and Diluted Net Income Share Calculations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesBasicAndDilutedNetIncomeShareCalculations</Role>
      <ShortName>Summary of Significant Accounting Policies - Basic and Diluted Net Income Share Calculations (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>153 - Disclosure - Investment Securities - Investment Securities Available-for-Sale and Held-to-Maturity (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureInvestmentSecuritiesInvestmentSecuritiesAvailableforSaleAndHeldtoMaturity</Role>
      <ShortName>Investment Securities - Investment Securities Available-for-Sale and Held-to-Maturity (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>154 - Disclosure - Investment Securities - Scheduled Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureInvestmentSecuritiesScheduledMaturitiesOfInvestmentSecuritiesAvailableforSaleAndHeldtoMaturity</Role>
      <ShortName>Investment Securities - Scheduled Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>155 - Disclosure - Investment Securities - Investments' Gross Unrealized Losses and Fair Value (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureInvestmentSecuritiesInvestmentsGrossUnrealizedLossesAndFairValue</Role>
      <ShortName>Investment Securities - Investments' Gross Unrealized Losses and Fair Value (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>156 - Disclosure - Investment Securities - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureInvestmentSecuritiesAdditionalInformation</Role>
      <ShortName>Investment Securities - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>157 - Disclosure - Loans and Allowance for Loan Losses - Schedule of Loan Portfolio by Reporting Segment and Portfolio Segment (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureLoansAndAllowanceForLoanLossesScheduleOfLoanPortfolioByReportingSegmentAndPortfolioSegment</Role>
      <ShortName>Loans and Allowance for Loan Losses - Schedule of Loan Portfolio by Reporting Segment and Portfolio Segment (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>158 - Disclosure - Loans and Allowance for Loan Losses - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureLoansAndAllowanceForLoanLossesAdditionalInformation</Role>
      <ShortName>Loans and Allowance for Loan Losses - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>159 - Disclosure - Loans and Allowance for Loan Losses - Schedule of Changes in Allowance for Loan Losses and Recorded Investment in Loans (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureLoansAndAllowanceForLoanLossesScheduleOfChangesInAllowanceForLoanLossesAndRecordedInvestmentInLoans</Role>
      <ShortName>Loans and Allowance for Loan Losses - Schedule of Changes in Allowance for Loan Losses and Recorded Investment in Loans (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>160 - Disclosure - Loans and Allowance for Loan Losses - Schedule of Carrying Amount of Loans by Credit Quality Indicator (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureLoansAndAllowanceForLoanLossesScheduleOfCarryingAmountOfLoansByCreditQualityIndicator</Role>
      <ShortName>Loans and Allowance for Loan Losses - Schedule of Carrying Amount of Loans by Credit Quality Indicator (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>161 - Disclosure - Loans and Allowance for Loan Losses - Schedule of Aging Analysis of Past Due Loans (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureLoansAndAllowanceForLoanLossesScheduleOfAgingAnalysisOfPastDueLoans</Role>
      <ShortName>Loans and Allowance for Loan Losses - Schedule of Aging Analysis of Past Due Loans (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>162 - Disclosure - Loans and Allowance for Loan Losses - Schedule of Analysis of Non-Accruing Loans (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureLoansAndAllowanceForLoanLossesScheduleOfAnalysisOfNonAccruingLoans</Role>
      <ShortName>Loans and Allowance for Loan Losses - Schedule of Analysis of Non-Accruing Loans (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>163 - Disclosure - Loans and Allowance for Loan Losses - Schedule of Carrying Amount of Impaired Loans (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureLoansAndAllowanceForLoanLossesScheduleOfCarryingAmountOfImpairedLoans</Role>
      <ShortName>Loans and Allowance for Loan Losses - Schedule of Carrying Amount of Impaired Loans (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>164 - Disclosure - Loans and Allowance for Loan Losses - Schedule of Average Net Investment Impaired Loans and Interest Income Recognized and Received on Impaired Loans (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureLoansAndAllowanceForLoanLossesScheduleOfAverageNetInvestmentImpairedLoansAndInterestIncomeRecognizedAndReceivedOnImpairedLoans</Role>
      <ShortName>Loans and Allowance for Loan Losses - Schedule of Average Net Investment Impaired Loans and Interest Income Recognized and Received on Impaired Loans (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>165 - Disclosure - Loans and Allowance for Loan Losses - Schedule of Number of Loans Modified Troubled Debt Restructuring by Loan Portfolio (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureLoansAndAllowanceForLoanLossesScheduleOfNumberOfLoansModifiedTroubledDebtRestructuringByLoanPortfolio</Role>
      <ShortName>Loans and Allowance for Loan Losses - Schedule of Number of Loans Modified Troubled Debt Restructuring by Loan Portfolio (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>166 - Disclosure - Loans and Allowance for Loan Losses - Schedule of Troubled Debt Restructuring (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureLoansAndAllowanceForLoanLossesScheduleOfTroubledDebtRestructuring</Role>
      <ShortName>Loans and Allowance for Loan Losses - Schedule of Troubled Debt Restructuring (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>167 - Disclosure - Other Real Estate Owned - Summary of Foreclosed Property Activity (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureOtherRealEstateOwnedSummaryOfForeclosedPropertyActivity</Role>
      <ShortName>Other Real Estate Owned - Summary of Foreclosed Property Activity (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>168 - Disclosure - Other Real Estate Owned - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureOtherRealEstateOwnedAdditionalInformation</Role>
      <ShortName>Other Real Estate Owned - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>169 - Disclosure - Premises and Equipment - Summary of Premises and Equipment and their Depreciable Lives (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosurePremisesAndEquipmentSummaryOfPremisesAndEquipmentAndTheirDepreciableLives</Role>
      <ShortName>Premises and Equipment - Summary of Premises and Equipment and their Depreciable Lives (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>170 - Disclosure - Premises and Equipment - Summary of Premises and Equipment and their Depreciable Lives (Parenthetical) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosurePremisesAndEquipmentSummaryOfPremisesAndEquipmentAndTheirDepreciableLivesParenthetical</Role>
      <ShortName>Premises and Equipment - Summary of Premises and Equipment and their Depreciable Lives (Parenthetical) (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>171 - Disclosure - Premises and Equipment - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosurePremisesAndEquipmentAdditionalInformation</Role>
      <ShortName>Premises and Equipment - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>172 - Disclosure - Investment in Limited Partnerships - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureInvestmentInLimitedPartnershipsAdditionalInformation</Role>
      <ShortName>Investment in Limited Partnerships - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>173 - Disclosure - Deposit - Scheduled Maturities of the Bank's Time Deposits (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureDepositScheduledMaturitiesOfTheBanksTimeDeposits</Role>
      <ShortName>Deposit - Scheduled Maturities of the Bank's Time Deposits (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>174 - Disclosure - Deposits - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureDepositsAdditionalInformation</Role>
      <ShortName>Deposits - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>175 - Disclosure - Short-Term Borrowings - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureShortTermBorrowingsAdditionalInformation</Role>
      <ShortName>Short-Term Borrowings - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>176 - Disclosure - Long-Term Debt - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureLongTermDebtAdditionalInformation</Role>
      <ShortName>Long-Term Debt - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>177 - Disclosure - Long-Term Debt - Summary of Information Concerning FHLB Advances and Other Borrowings (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureLongTermDebtSummaryOfInformationConcerningFHLBAdvancesAndOtherBorrowings</Role>
      <ShortName>Long-Term Debt - Summary of Information Concerning FHLB Advances and Other Borrowings (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>178 - Disclosure - Income Taxes - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureIncomeTaxesAdditionalInformation</Role>
      <ShortName>Income Taxes - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>179 - Disclosure - Income Taxes - Consolidated Provisions for (Benefits from) Income Taxes (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureIncomeTaxesConsolidatedProvisionsForBenefitsFromIncomeTaxes</Role>
      <ShortName>Income Taxes - Consolidated Provisions for (Benefits from) Income Taxes (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>180 - Disclosure - Income Taxes - Schedule of Effective Income Tax Rate (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureIncomeTaxesScheduleOfEffectiveIncomeTaxRate</Role>
      <ShortName>Income Taxes - Schedule of Effective Income Tax Rate (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>181 - Disclosure - Income Taxes - Effects of Temporary Differences of Deferred Tax Assets and Deferred Tax Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureIncomeTaxesEffectsOfTemporaryDifferencesOfDeferredTaxAssetsAndDeferredTaxLiabilities</Role>
      <ShortName>Income Taxes - Effects of Temporary Differences of Deferred Tax Assets and Deferred Tax Liabilities (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>182 - Disclosure - Employee Benefit Plans - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureEmployeeBenefitPlansAdditionalInformation</Role>
      <ShortName>Employee Benefit Plans - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>183 - Disclosure - Deferred Compensation Plan - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureDeferredCompensationPlanAdditionalInformation</Role>
      <ShortName>Deferred Compensation Plan - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>184 - Disclosure - Shareholders' Equity - Summary of Actual Capital Amount and Well Capitalized Total Risk-Based, Tier I Risk-Based, and Tier I Leverage Ratios (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureShareholdersEquitySummaryOfActualCapitalAmountAndWellCapitalizedTotalRiskBasedTierIRiskBasedAndTierILeverageRatios</Role>
      <ShortName>Shareholders' Equity - Summary of Actual Capital Amount and Well Capitalized Total Risk-Based, Tier I Risk-Based, and Tier I Leverage Ratios (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>185 - Disclosure - Segment Reporting - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureSegmentReportingAdditionalInformation</Role>
      <ShortName>Segment Reporting - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>186 - Disclosure - Segment Reporting - Summary of Results for Reportable Segments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureSegmentReportingSummaryOfResultsForReportableSegments</Role>
      <ShortName>Segment Reporting - Summary of Results for Reportable Segments (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>187 - Disclosure - Other Operating Expenses - Components of Other Operating Expenses (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureOtherOperatingExpensesComponentsOfOtherOperatingExpenses</Role>
      <ShortName>Other Operating Expenses - Components of Other Operating Expenses (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>188 - Disclosure - Operating Leases - Future Minimum Rental Payments Required Under Operating Leases (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureOperatingLeasesFutureMinimumRentalPaymentsRequiredUnderOperatingLeases</Role>
      <ShortName>Operating Leases - Future Minimum Rental Payments Required Under Operating Leases (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>189 - Disclosure - Operating Leases - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureOperatingLeasesAdditionalInformation</Role>
      <ShortName>Operating Leases - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>190 - Disclosure - Guarantees, Commitments and Contingencies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureGuaranteesCommitmentsAndContingenciesAdditionalInformation</Role>
      <ShortName>Guarantees, Commitments and Contingencies - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>191 - Disclosure - Guarantees, Commitments and Contingencies - Summary of Commitments and Contingent Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureGuaranteesCommitmentsAndContingenciesSummaryOfCommitmentsAndContingentLiabilities</Role>
      <ShortName>Guarantees, Commitments and Contingencies - Summary of Commitments and Contingent Liabilities (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>192 - Disclosure - Fair Value Measurements - Balances of Available-for-Sale Securities Measured at Fair Value on Recurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureFairValueMeasurementsBalancesOfAvailableforSaleSecuritiesMeasuredAtFairValueOnRecurringBasis</Role>
      <ShortName>Fair Value Measurements - Balances of Available-for-Sale Securities Measured at Fair Value on Recurring Basis (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>193 - Disclosure - Fair Value Measurements - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureFairValueMeasurementsAdditionalInformation</Role>
      <ShortName>Fair Value Measurements - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>194 - Disclosure - Fair Value Measurements - Balances of Impaired Loans and Foreclosed Assets Measured at Fair Value on Nonrecurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureFairValueMeasurementsBalancesOfImpairedLoansAndForeclosedAssetsMeasuredAtFairValueOnNonrecurringBasis</Role>
      <ShortName>Fair Value Measurements - Balances of Impaired Loans and Foreclosed Assets Measured at Fair Value on Nonrecurring Basis (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>195 - Disclosure - Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value Using Significant Unobservable Inputs (Level 3) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureFairValueMeasurementsSummaryOfAssetsAndLiabilitiesMeasuredAtFairValueUsingSignificantUnobservableInputsLevel3</Role>
      <ShortName>Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value Using Significant Unobservable Inputs (Level 3) (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>196 - Disclosure - Fair Value Measurements - Schedule of Estimated Fair Value and Related Carrying or Notional Amounts of Company's Financial Instruments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureFairValueMeasurementsScheduleOfEstimatedFairValueAndRelatedCarryingOrNotionalAmountsOfCompanysFinancialInstruments</Role>
      <ShortName>Fair Value Measurements - Schedule of Estimated Fair Value and Related Carrying or Notional Amounts of Company's Financial Instruments (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>197 - Disclosure - United Security Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Condition (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureUnitedSecurityBancsharesIncParentCompanyOnlyFinancialInformationStatementsOfCondition</Role>
      <ShortName>United Security Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Condition (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>198 - Disclosure - United Security Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Income (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureUnitedSecurityBancsharesIncParentCompanyOnlyFinancialInformationStatementsOfIncome</Role>
      <ShortName>United Security Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Income (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>199 - Disclosure - United Security Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Cash Flows (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureUnitedSecurityBancsharesIncParentCompanyOnlyFinancialInformationStatementsOfCashFlows</Role>
      <ShortName>United Security Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Cash Flows (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>200 - Disclosure - Quarterly Data (Unaudited) - Summary of Quarterly Data (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/taxonomy/role/DisclosureQuarterlyDataUnauditedSummaryOfQuarterlyData</Role>
      <ShortName>Quarterly Data (Unaudited) - Summary of Quarterly Data (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>true</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Info">Element us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing had a mix of decimals attribute values: -5 -3.</Log>
    <Log type="Info">Element us-gaap_ProvisionForLoanAndLeaseLosses had a mix of decimals attribute values: -5 -3.</Log>
    <Log type="Info">'Monetary' elements on report '150 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '156 - Disclosure - Investment Securities - Additional Information (Detail)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '158 - Disclosure - Loans and Allowance for Loan Losses - Additional Information (Detail)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '176 - Disclosure - Long-Term Debt - Additional Information (Detail)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">Process Flow-Through: 103 - Statement - Consolidated Statements of Financial Condition</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Dec. 31, 2011'</Log>
    <Log type="Info">Process Flow-Through: 104 - Statement - Consolidated Statements of Financial Condition (Parenthetical)</Log>
    <Log type="Info">Process Flow-Through: 105 - Statement - Consolidated Statements of Operations</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Dec. 31, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Sep. 30, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Jun. 30, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Mar. 31, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Dec. 31, 2012'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Sep. 30, 2012'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Jun. 30, 2012'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Mar. 31, 2012'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '12 Months Ended
Dec. 31, 2011'</Log>
    <Log type="Info">Process Flow-Through: 107 - Statement - Consolidated Statements of Shareholders' Equity (Parenthetical)</Log>
    <Log type="Info">Process Flow-Through: 108 - Statement - Consolidated Statements of Comprehensive Income</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Dec. 31, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Sep. 30, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Jun. 30, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Mar. 31, 2013'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Dec. 31, 2012'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Sep. 30, 2012'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Jun. 30, 2012'</Log>
    <Log type="Info">	Process Flow-Through: Removing column '3 Months Ended
Mar. 31, 2012'</Log>
    <Log type="Info">Process Flow-Through: 109 - Statement - Consolidated Statements of Comprehensive Income (Parenthetical)</Log>
    <Log type="Info">Process Flow-Through: 110 - Statement - Consolidated Statements of Cash Flows</Log>
    <Log type="Info">	Process Flow-Through: Removing column '12 Months Ended
Dec. 31, 2011'</Log>
  </Logs>
  <InputFiles>
    <File>usbi-20131231.xml</File>
    <File>usbi-20131231.xsd</File>
    <File>usbi-20131231_cal.xml</File>
    <File>usbi-20131231_def.xml</File>
    <File>usbi-20131231_lab.xml</File>
    <File>usbi-20131231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles />
  <BaseTaxonomies />
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
