<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.4.0.3</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>90</ContextCount>
  <ElementCount>217</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>27</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>7</UnitCount>
  <MyReports>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/CondensedConsolidatedBalanceSheets</Role>
      <ShortName>CONDENSED CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1001501 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/CondensedConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1002000 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/CondensedConsolidatedStatementsOfOperationsAndComprehensiveIncome</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1003000 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/CondensedConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Financial Statement Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/FinancialStatementPresentation</Role>
      <ShortName>Financial Statement Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>2103100 - Disclosure - Accounting Standards Updates (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.onassignment.com/role/AccountingStandardsUpdatesNotes</Role>
      <ShortName>Accounting Standards Updates (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2104100 - Disclosure - Acquisitions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/Acquisitions</Role>
      <ShortName>Acquisitions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - Discontinued Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/DiscontinuedOperations</Role>
      <ShortName>Discontinued Operations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2107100 - Disclosure - Goodwill and Identifiable Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/GoodwillAndIdentifiableAssets</Role>
      <ShortName>Goodwill and Identifiable Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2109100 - Disclosure - Long-Term Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/LongTermDebt</Role>
      <ShortName>Long-Term Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2110100 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/CommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2111100 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/FairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2112100 - Disclosure - Accumulated Other Comprehensive Income (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.onassignment.com/role/AccumulatedOtherComprehensiveIncomeNotes</Role>
      <ShortName>Accumulated Other Comprehensive Income (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2114100 - Disclosure - Stock-based compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/StockBasedCompensation</Role>
      <ShortName>Stock-based compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2115100 - Disclosure - Earnings per share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/EarningsPerShare</Role>
      <ShortName>Earnings per share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2116100 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2117100 - Disclosure - Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/SegmentReporting</Role>
      <ShortName>Segment Reporting</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2204201 - Disclosure - Acquisitions Acquisitions (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/AcquisitionsAcquisitionsPolicies</Role>
      <ShortName>Acquisitions Acquisitions (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/AccountingStandardsUpdatesNotes</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2211201 - Disclosure - Fair Value Measurements (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/FairValueMeasurementsPolicies</Role>
      <ShortName>Fair Value Measurements (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/AccountingStandardsUpdatesNotes</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2304302 - Disclosure - Acquisitions Acquisitions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/AcquisitionsAcquisitionsTables</Role>
      <ShortName>Acquisitions Acquisitions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2305301 - Disclosure - Discontinued Operations (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/DiscontinuedOperationsTables</Role>
      <ShortName>Discontinued Operations (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/DiscontinuedOperations</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2307301 - Disclosure - Goodwill and Identifiable Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/GoodwillAndIdentifiableAssetsTables</Role>
      <ShortName>Goodwill and Identifiable Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/GoodwillAndIdentifiableAssets</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2309301 - Disclosure - Long-Term Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/LongTermDebtTables</Role>
      <ShortName>Long-Term Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/LongTermDebt</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2311302 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/FairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/FairValueMeasurements</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2315301 - Disclosure - Earnings per share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings per share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/EarningsPerShare</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2317301 - Disclosure - Segment Reporting (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/SegmentReportingTables</Role>
      <ShortName>Segment Reporting (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/SegmentReporting</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2404403 - Disclosure - Acquisitions Acquisitions Costs, by Acquisition (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/AcquisitionsAcquisitionsCostsByAcquisitionDetails</Role>
      <ShortName>Acquisitions Acquisitions Costs, by Acquisition (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2404404 - Disclosure - Acquisitions Schedule of Purchase Price Allocation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/AcquisitionsScheduleOfPurchasePriceAllocationDetails</Role>
      <ShortName>Acquisitions Schedule of Purchase Price Allocation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2404405 - Disclosure - Acquisitions Schedule of Intangible Assets Acquired as Part of Business Combination (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/AcquisitionsScheduleOfIntangibleAssetsAcquiredAsPartOfBusinessCombinationDetails</Role>
      <ShortName>Acquisitions Schedule of Intangible Assets Acquired as Part of Business Combination (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2404406 - Disclosure - Acquisitions Business Acquisition, Pro Forma Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/AcquisitionsBusinessAcquisitionProFormaInformationDetails</Role>
      <ShortName>Acquisitions Business Acquisition, Pro Forma Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - Discontinued Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/DiscontinuedOperationsDetails</Role>
      <ShortName>Discontinued Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/DiscontinuedOperationsTables</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2407402 - Disclosure - Goodwill and Identifiable Assets (Goodwill) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/GoodwillAndIdentifiableAssetsGoodwillDetails</Role>
      <ShortName>Goodwill and Identifiable Assets (Goodwill) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/GoodwillAndIdentifiableAssetsTables</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2407403 - Disclosure - Goodwill and Identifiable Assets (Acquired Intangible Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/GoodwillAndIdentifiableAssetsAcquiredIntangibleAssetsDetails</Role>
      <ShortName>Goodwill and Identifiable Assets (Acquired Intangible Assets) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/GoodwillAndIdentifiableAssetsTables</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2407404 - Disclosure - Goodwill and Identifiable Assets (Future Amortization Expense) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/GoodwillAndIdentifiableAssetsFutureAmortizationExpenseDetails</Role>
      <ShortName>Goodwill and Identifiable Assets (Future Amortization Expense) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/GoodwillAndIdentifiableAssetsTables</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2409402 - Disclosure - Long-Term Debt Long-Term Debt Schedule (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/LongTermDebtLongTermDebtScheduleDetails</Role>
      <ShortName>Long-Term Debt Long-Term Debt Schedule (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2409403 - Disclosure - Long-Term Debt Long-term Debt text (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/LongTermDebtLongTermDebtTextDetails</Role>
      <ShortName>Long-Term Debt Long-term Debt text (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2410402 - Disclosure - Commitments and Contingencies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/CommitmentsAndContingenciesDetails</Role>
      <ShortName>Commitments and Contingencies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/CommitmentsAndContingencies</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2411403 - Disclosure - Fair Value Measurements (Fair Value Inputs, Liabilities, Quantitative Information) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/FairValueMeasurementsFairValueInputsLiabilitiesQuantitativeInformationDetails</Role>
      <ShortName>Fair Value Measurements (Fair Value Inputs, Liabilities, Quantitative Information) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2411405 - Disclosure - Fair Value Measurements (Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/FairValueMeasurementsLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationDetails</Role>
      <ShortName>Fair Value Measurements (Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2412402 - Disclosure - Accumulated Other Comprehensive Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/AccumulatedOtherComprehensiveIncomeDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/AccumulatedOtherComprehensiveIncomeNotes</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2414401 - Disclosure - Stock-based compensation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/StockBasedCompensationDetails</Role>
      <ShortName>Stock-based compensation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/StockBasedCompensation</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2415402 - Disclosure - Earnings per share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/EarningsPerShareDetails</Role>
      <ShortName>Earnings per share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/EarningsPerShareTables</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2417402 - Disclosure - Segment Reporting (Segment Reporting Information) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/SegmentReportingSegmentReportingInformationDetails</Role>
      <ShortName>Segment Reporting (Segment Reporting Information) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/SegmentReportingTables</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="asgn-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2417403 - Disclosure - Segment Reporting (Revenue by Geographical Areas) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onassignment.com/role/SegmentReportingRevenueByGeographicalAreasDetails</Role>
      <ShortName>Segment Reporting (Revenue by Geographical Areas) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onassignment.com/role/SegmentReportingTables</ParentRole>
      <Position>45</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>asgn-20160331.xml</File>
    <File>asgn-20160331.xsd</File>
    <File>asgn-20160331_cal.xml</File>
    <File>asgn-20160331_def.xml</File>
    <File>asgn-20160331_lab.xml</File>
    <File>asgn-20160331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies/>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
