<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>864</ContextCount>
  <ElementCount>475</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>206</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>11</UnitCount>
  <MyReports>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001001 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001002 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1002003 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1003004 - Statement - Consolidated Statements of (Loss) Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/ConsolidatedStatementsofLossIncome</Role>
      <ShortName>Consolidated Statements of (Loss) Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1004005 - Statement - Consolidated Statements of Comprehensive (Loss) Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/ConsolidatedStatementsofComprehensiveLossIncome</Role>
      <ShortName>Consolidated Statements of Comprehensive (Loss) Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1005006 - Statement - Consolidated Statements of Shareholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/ConsolidatedStatementsofShareholdersEquity</Role>
      <ShortName>Consolidated Statements of Shareholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1006007 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/ConsolidatedStatementsofCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2101101 - Disclosure - General</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/General</Role>
      <ShortName>General</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2102102 - Disclosure - Summary of significant accounting policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Summaryofsignificantaccountingpolicies</Role>
      <ShortName>Summary of significant accounting policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2105103 - Disclosure - Significant transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Significanttransactions</Role>
      <ShortName>Significant transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2108104 - Disclosure - Segment information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Segmentinformation</Role>
      <ShortName>Segment information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2112105 - Disclosure - Reserves for unpaid losses and loss adjustment expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Reservesforunpaidlossesandlossadjustmentexpenses</Role>
      <ShortName>Reserves for unpaid losses and loss adjustment expenses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2116106 - Disclosure - Third party reinsurance</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Thirdpartyreinsurance</Role>
      <ShortName>Third party reinsurance</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2118107 - Disclosure - Allowance for expected credit losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Allowanceforexpectedcreditlosses</Role>
      <ShortName>Allowance for expected credit losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2121108 - Disclosure - Investment securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Investmentsecurities</Role>
      <ShortName>Investment securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2130109 - Disclosure - Fair value measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Fairvaluemeasurements</Role>
      <ShortName>Fair value measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2137110 - Disclosure - Debt and standby letters of credit facilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Debtandstandbylettersofcreditfacilities</Role>
      <ShortName>Debt and standby letters of credit facilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2144111 - Disclosure - Income taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Incometaxes</Role>
      <ShortName>Income taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2147112 - Disclosure - Derivatives</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Derivatives</Role>
      <ShortName>Derivatives</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2152113 - Disclosure - Share-based compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Sharebasedcompensation</Role>
      <ShortName>Share-based compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2157114 - Disclosure - Common shareholder's equity, mezzanine equity, and non-controlling interests</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Commonshareholdersequitymezzanineequityandnoncontrollinginterests</Role>
      <ShortName>Common shareholder's equity, mezzanine equity, and non-controlling interests</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2162115 - Disclosure - Earnings per share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Earningspershare</Role>
      <ShortName>Earnings per share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2165116 - Disclosure - Investments in unconsolidated entities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Investmentsinunconsolidatedentities</Role>
      <ShortName>Investments in unconsolidated entities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2169117 - Disclosure - Variable interest entities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Variableinterestentities</Role>
      <ShortName>Variable interest entities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2173118 - Disclosure - Transactions with related parties</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Transactionswithrelatedparties</Role>
      <ShortName>Transactions with related parties</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2175119 - Disclosure - Commitments and contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/Commitmentsandcontingencies</Role>
      <ShortName>Commitments and contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2203201 - Disclosure - Summary of significant accounting policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/SummaryofsignificantaccountingpoliciesPolicies</Role>
      <ShortName>Summary of significant accounting policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2309301 - Disclosure - Segment information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/SegmentinformationTables</Role>
      <ShortName>Segment information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Segmentinformation</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2313302 - Disclosure - Reserves for unpaid losses and loss adjustment expenses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/ReservesforunpaidlossesandlossadjustmentexpensesTables</Role>
      <ShortName>Reserves for unpaid losses and loss adjustment expenses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Reservesforunpaidlossesandlossadjustmentexpenses</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2319303 - Disclosure - Allowance for expected credit losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/AllowanceforexpectedcreditlossesTables</Role>
      <ShortName>Allowance for expected credit losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Allowanceforexpectedcreditlosses</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2322304 - Disclosure - Investment securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/InvestmentsecuritiesTables</Role>
      <ShortName>Investment securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Investmentsecurities</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2331305 - Disclosure - Fair value measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/FairvaluemeasurementsTables</Role>
      <ShortName>Fair value measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Fairvaluemeasurements</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2338306 - Disclosure - Debt and standby letters of credit facilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/DebtandstandbylettersofcreditfacilitiesTables</Role>
      <ShortName>Debt and standby letters of credit facilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Debtandstandbylettersofcreditfacilities</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2348307 - Disclosure - Derivatives (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/DerivativesTables</Role>
      <ShortName>Derivatives (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Derivatives</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2353308 - Disclosure - Share-based compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/SharebasedcompensationTables</Role>
      <ShortName>Share-based compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Sharebasedcompensation</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2358309 - Disclosure - Common shareholder's equity, mezzanine equity, and non-controlling interests (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/CommonshareholdersequitymezzanineequityandnoncontrollinginterestsTables</Role>
      <ShortName>Common shareholder's equity, mezzanine equity, and non-controlling interests (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Commonshareholdersequitymezzanineequityandnoncontrollinginterests</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2363310 - Disclosure - Earnings per share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/EarningspershareTables</Role>
      <ShortName>Earnings per share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Earningspershare</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2366311 - Disclosure - Investments in unconsolidated entities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/InvestmentsinunconsolidatedentitiesTables</Role>
      <ShortName>Investments in unconsolidated entities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Investmentsinunconsolidatedentities</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2370312 - Disclosure - Variable interest entities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/VariableinterestentitiesTables</Role>
      <ShortName>Variable interest entities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Variableinterestentities</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2376313 - Disclosure - Commitments and contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/CommitmentsandcontingenciesTables</Role>
      <ShortName>Commitments and contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Commitmentsandcontingencies</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2404401 - Disclosure - Summary of significant accounting policies - Recent adopted changes in accounting principle and Recent accounting pronouncements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/SummaryofsignificantaccountingpoliciesRecentadoptedchangesinaccountingprincipleandRecentaccountingpronouncementsDetails</Role>
      <ShortName>Summary of significant accounting policies - Recent adopted changes in accounting principle and Recent accounting pronouncements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2406402 - Disclosure - Significant transactions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/SignificanttransactionsDetails</Role>
      <ShortName>Significant transactions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Significanttransactions</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2407403 - Disclosure - Significant transactions - Loss Portfolio Transfer (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/SignificanttransactionsLossPortfolioTransferDetails</Role>
      <ShortName>Significant transactions - Loss Portfolio Transfer (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2410404 - Disclosure - Segment information - Segment results (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/SegmentinformationSegmentresultsDetails</Role>
      <ShortName>Segment information - Segment results (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2411405 - Disclosure - Segment information - Net premiums written by client location and underwriting location by reportable segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/SegmentinformationNetpremiumswrittenbyclientlocationandunderwritinglocationbyreportablesegmentDetails</Role>
      <ShortName>Segment information - Net premiums written by client location and underwriting location by reportable segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2414406 - Disclosure - Reserves for unpaid losses and loss adjustment expenses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/ReservesforunpaidlossesandlossadjustmentexpensesDetails</Role>
      <ShortName>Reserves for unpaid losses and loss adjustment expenses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/ReservesforunpaidlossesandlossadjustmentexpensesTables</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2415407 - Disclosure - Reserves for unpaid losses and loss adjustment expenses - By segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/ReservesforunpaidlossesandlossadjustmentexpensesBysegmentDetails</Role>
      <ShortName>Reserves for unpaid losses and loss adjustment expenses - By segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/ReservesforunpaidlossesandlossadjustmentexpensesTables</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2417408 - Disclosure - Third party reinsurance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/ThirdpartyreinsuranceDetails</Role>
      <ShortName>Third party reinsurance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Thirdpartyreinsurance</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2420409 - Disclosure - Allowance for expected credit losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/AllowanceforexpectedcreditlossesDetails</Role>
      <ShortName>Allowance for expected credit losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/AllowanceforexpectedcreditlossesTables</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2423410 - Disclosure - Investment securities - Net investment income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/InvestmentsecuritiesNetinvestmentincomeDetails</Role>
      <ShortName>Investment securities - Net investment income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2424411 - Disclosure - Investment securities - Net realized and unrealized investment gains (losses) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/InvestmentsecuritiesNetrealizedandunrealizedinvestmentgainslossesDetails</Role>
      <ShortName>Investment securities - Net realized and unrealized investment gains (losses) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2425412 - Disclosure - Investment securities - Fixed maturity investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/InvestmentsecuritiesFixedmaturityinvestmentsDetails</Role>
      <ShortName>Investment securities - Fixed maturity investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2426413 - Disclosure - Investment securities - Equity securities and Other long-term investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/InvestmentsecuritiesEquitysecuritiesandOtherlongterminvestmentsDetails</Role>
      <ShortName>Investment securities - Equity securities and Other long-term investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2427414 - Disclosure - Investment securities - Hedge funds and Private equity funds (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/InvestmentsecuritiesHedgefundsandPrivateequityfundsDetails</Role>
      <ShortName>Investment securities - Hedge funds and Private equity funds (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2428415 - Disclosure - Investment securities - Hedge Funds redemption frequency and Private Equity Fund lock-up period (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/InvestmentsecuritiesHedgeFundsredemptionfrequencyandPrivateEquityFundlockupperiodDetails</Role>
      <ShortName>Investment securities - Hedge Funds redemption frequency and Private Equity Fund lock-up period (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2429416 - Disclosure - Investment securities - Investments held on deposit or as collateral and Unsettled investment purchases and sales (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/InvestmentsecuritiesInvestmentsheldondepositorascollateralandUnsettledinvestmentpurchasesandsalesDetails</Role>
      <ShortName>Investment securities - Investments held on deposit or as collateral and Unsettled investment purchases and sales (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2432417 - Disclosure - Fair value measurements - Fair value measurements by level (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/FairvaluemeasurementsFairvaluemeasurementsbylevelDetails</Role>
      <ShortName>Fair value measurements - Fair value measurements by level (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2433418 - Disclosure - Fair value measurements - Rollforward of level 3 fair value measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/FairvaluemeasurementsRollforwardoflevel3fairvaluemeasurementsDetails</Role>
      <ShortName>Fair value measurements - Rollforward of level 3 fair value measurements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2434419 - Disclosure - Fair value measurements - Transfers between levels (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/FairvaluemeasurementsTransfersbetweenlevelsDetails</Role>
      <ShortName>Fair value measurements - Transfers between levels (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2435420 - Disclosure - Fair value measurements - Significant unobservable inputs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/FairvaluemeasurementsSignificantunobservableinputsDetails</Role>
      <ShortName>Fair value measurements - Significant unobservable inputs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2436421 - Disclosure - Fair value measurements - Financial instruments disclosed, but not carried at fair value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/FairvaluemeasurementsFinancialinstrumentsdisclosedbutnotcarriedatfairvalueDetails</Role>
      <ShortName>Fair value measurements - Financial instruments disclosed, but not carried at fair value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2439422 - Disclosure - Debt and standby letters of credit facilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/DebtandstandbylettersofcreditfacilitiesDetails</Role>
      <ShortName>Debt and standby letters of credit facilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/DebtandstandbylettersofcreditfacilitiesTables</ParentRole>
      <Position>62</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2440423 - Disclosure - Debt and standby letters of credit facilities - 2017 SEK Subordinated Notes and 2016 SIG Senior Notes (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.siriusgroup.com/role/Debtandstandbylettersofcreditfacilities2017SEKSubordinatedNotesand2016SIGSeniorNotesDetails</Role>
      <ShortName>Debt and standby letters of credit facilities - 2017 SEK Subordinated Notes and 2016 SIG Senior Notes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2441424 - Disclosure - Debt and standby letters of credit facilities - Standby Letter of Credit Facilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/DebtandstandbylettersofcreditfacilitiesStandbyLetterofCreditFacilitiesDetails</Role>
      <ShortName>Debt and standby letters of credit facilities - Standby Letter of Credit Facilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2442425 - Disclosure - Debt and standby letters of credit facilities - Revolving Credit Facility (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/DebtandstandbylettersofcreditfacilitiesRevolvingCreditFacilityDetails</Role>
      <ShortName>Debt and standby letters of credit facilities - Revolving Credit Facility (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2443426 - Disclosure - Debt and standby letters of credit facilities - Interest (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/DebtandstandbylettersofcreditfacilitiesInterestDetails</Role>
      <ShortName>Debt and standby letters of credit facilities - Interest (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/DebtandstandbylettersofcreditfacilitiesTables</ParentRole>
      <Position>66</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2445427 - Disclosure - Income taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/IncometaxesDetails</Role>
      <ShortName>Income taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Incometaxes</ParentRole>
      <Position>67</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2446428 - Disclosure - Income taxes - Deferred tax asset, net of valuation allowance and Uncertain tax positions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/IncometaxesDeferredtaxassetnetofvaluationallowanceandUncertaintaxpositionsDetails</Role>
      <ShortName>Income taxes - Deferred tax asset, net of valuation allowance and Uncertain tax positions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2449429 - Disclosure - Derivatives - Interest rate cap (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/DerivativesInterestratecapDetails</Role>
      <ShortName>Derivatives - Interest rate cap (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2450430 - Disclosure - Derivatives - Classification and fair value of derivatives in consolidated balance sheets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/DerivativesClassificationandfairvalueofderivativesinconsolidatedbalancesheetsDetails</Role>
      <ShortName>Derivatives - Classification and fair value of derivatives in consolidated balance sheets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2451431 - Disclosure - Derivatives - Classification and fair value of derivatives in consolidated of (loss) income statements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/DerivativesClassificationandfairvalueofderivativesinconsolidatedoflossincomestatementsDetails</Role>
      <ShortName>Derivatives - Classification and fair value of derivatives in consolidated of (loss) income statements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2454432 - Disclosure - Share-based compensation - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/SharebasedcompensationNarrativeDetails</Role>
      <ShortName>Share-based compensation - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2455433 - Disclosure - Share-based compensation - Unrecognized Compensation Cost and Period Of Recognition (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/SharebasedcompensationUnrecognizedCompensationCostandPeriodOfRecognitionDetails</Role>
      <ShortName>Share-based compensation - Unrecognized Compensation Cost and Period Of Recognition (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2456434 - Disclosure - Share-based compensation - Unvested Share-based Awards Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/SharebasedcompensationUnvestedSharebasedAwardsActivityDetails</Role>
      <ShortName>Share-based compensation - Unvested Share-based Awards Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2459435 - Disclosure - Common shareholder's equity, mezzanine equity, and non controlling interests - Common shareholder's equity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/CommonshareholdersequitymezzanineequityandnoncontrollinginterestsCommonshareholdersequityDetails</Role>
      <ShortName>Common shareholder's equity, mezzanine equity, and non controlling interests - Common shareholder's equity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2460436 - Disclosure - Common shareholder's equity, mezzanine equity, and non controlling interests - Mezzanine equity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/CommonshareholdersequitymezzanineequityandnoncontrollinginterestsMezzanineequityDetails</Role>
      <ShortName>Common shareholder's equity, mezzanine equity, and non controlling interests - Mezzanine equity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2461437 - Disclosure - Common shareholder's equity, mezzanine equity, and non controlling interests - Non-controlling interests (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/CommonshareholdersequitymezzanineequityandnoncontrollinginterestsNoncontrollinginterestsDetails</Role>
      <ShortName>Common shareholder's equity, mezzanine equity, and non controlling interests - Non-controlling interests (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2464438 - Disclosure - Earnings per share - Basic and Diluted earnings per share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/EarningspershareBasicandDilutedearningspershareDetails</Role>
      <ShortName>Earnings per share - Basic and Diluted earnings per share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2467439 - Disclosure - Investments in unconsolidated entities - Other long-term investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/InvestmentsinunconsolidatedentitiesOtherlongterminvestmentsDetails</Role>
      <ShortName>Investments in unconsolidated entities - Other long-term investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2468440 - Disclosure - Investments in unconsolidated entities - Equity method unconsolidated entities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/InvestmentsinunconsolidatedentitiesEquitymethodunconsolidatedentitiesDetails</Role>
      <ShortName>Investments in unconsolidated entities - Equity method unconsolidated entities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>2471441 - Disclosure - Variable interest entities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/VariableinterestentitiesDetails</Role>
      <ShortName>Variable interest entities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/VariableinterestentitiesTables</ParentRole>
      <Position>81</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>2472442 - Disclosure - Variable interest entities - Unconsolidated VIE (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/VariableinterestentitiesUnconsolidatedVIEDetails</Role>
      <ShortName>Variable interest entities - Unconsolidated VIE (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>2474443 - Disclosure - Transactions with related parties (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/TransactionswithrelatedpartiesDetails</Role>
      <ShortName>Transactions with related parties (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.siriusgroup.com/role/Transactionswithrelatedparties</ParentRole>
      <Position>83</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>2477444 - Disclosure - Commitments and contingencies - Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/CommitmentsandcontingenciesLeasesDetails</Role>
      <ShortName>Commitments and contingencies - Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>2478445 - Disclosure - Commitments and contingencies - Lease balances (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/CommitmentsandcontingenciesLeasebalancesDetails</Role>
      <ShortName>Commitments and contingencies - Lease balances (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>2479446 - Disclosure - Commitments and contingencies - Weighted average remaining lease term and weighted average discount rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/CommitmentsandcontingenciesWeightedaverageremainingleasetermandweightedaveragediscountrateDetails</Role>
      <ShortName>Commitments and contingencies - Weighted average remaining lease term and weighted average discount rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="sg-20200930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>2480447 - Disclosure - Commitments and contingencies - Future annual minimum rental payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.siriusgroup.com/role/CommitmentsandcontingenciesFutureannualminimumrentalpaymentsDetails</Role>
      <ShortName>Commitments and contingencies - Future annual minimum rental payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" original="sg-20200930.htm">sg-20200930.htm</File>
    <File>sg-20200930.xsd</File>
    <File>sg-20200930_cal.xml</File>
    <File>sg-20200930_def.xml</File>
    <File>sg-20200930_lab.xml</File>
    <File>sg-20200930_pre.xml</File>
    <File>sg-20200930xexx1091.htm</File>
    <File>sg-20200930xexx1092.htm</File>
    <File>sg-20200930xexx1093.htm</File>
    <File>sg-20200930xexx1094.htm</File>
    <File>sg-20200930xexx1095.htm</File>
    <File>sg-20200930xexx1096.htm</File>
    <File>sg-20200930xexx1097.htm</File>
    <File>sg-20200930xexx1098.htm</File>
    <File>sg-20200930xexx311.htm</File>
    <File>sg-20200930xexx312.htm</File>
    <File>sg-20200930xexx321.htm</File>
    <File>sg-20200930xexx322.htm</File>
    <File>sg-20200930xexx991.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
