<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.19.3.a.u2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>914</ContextCount>
  <ElementCount>632</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>134</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>11</UnitCount>
  <MyReports>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/Cover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1001501 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1002000 - Statement - Consolidated Statements of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/ConsolidatedStatementsOfOperations</Role>
      <ShortName>Consolidated Statements of Operations</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1003000 - Statement - Consolidated Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/ConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>Consolidated Statements of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1005000 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/ConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1006000 - Statement - Consolidated Statements of Changes in Stockholders' Equity / Member's Interest and Redeemable Noncontrolling Interest</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/ConsolidatedStatementsOfChangesInStockholdersEquityMembersInterestAndRedeemableNoncontrollingInterest</Role>
      <ShortName>Consolidated Statements of Changes in Stockholders' Equity / Member's Interest and Redeemable Noncontrolling Interest</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>1006001 - Statement - Consolidated Statements of Changes in Redeemable Noncontrolling Interest and Stockholders' Equity / Member's Interest and Redeemable Noncontrolling Interest (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/ConsolidatedStatementsOfChangesInRedeemableNoncontrollingInterestAndStockholdersEquityMembersInterestAndRedeemableNoncontrollingInterestParenthetical</Role>
      <ShortName>Consolidated Statements of Changes in Redeemable Noncontrolling Interest and Stockholders' Equity / Member's Interest and Redeemable Noncontrolling Interest (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Summary of Organization and Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SummaryOfOrganizationAndSignificantAccountingPolicies</Role>
      <ShortName>Summary of Organization and Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2102100 - Disclosure - Acquisitions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/Acquisitions</Role>
      <ShortName>Acquisitions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2103100 - Disclosure - Goodwill</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/Goodwill</Role>
      <ShortName>Goodwill</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2104100 - Disclosure - Revenue Recognition</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/RevenueRecognition</Role>
      <ShortName>Revenue Recognition</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - Inventories</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/Inventories</Role>
      <ShortName>Inventories</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2106100 - Disclosure - Property, Plant and Equipment, net and Intangibles, net</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/PropertyPlantAndEquipmentNetAndIntangiblesNet</Role>
      <ShortName>Property, Plant and Equipment, net and Intangibles, net</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2107100 - Disclosure - Accrued Expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/AccruedExpenses</Role>
      <ShortName>Accrued Expenses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/Debt</Role>
      <ShortName>Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2109100 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2110100 - Disclosure - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EarningsPerShare</Role>
      <ShortName>Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2111100 - Disclosure - Stockholder's Equity/Members' Interest</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/StockholdersEquityMembersInterest</Role>
      <ShortName>Stockholder's Equity/Members' Interest</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2112100 - Disclosure - Supplemental Cash Flow Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SupplementalCashFlowInformation</Role>
      <ShortName>Supplemental Cash Flow Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2113100 - Disclosure - Stock-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/StockBasedCompensation</Role>
      <ShortName>Stock-Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2114100 - Disclosure - Employee Benefit Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EmployeeBenefitPlans</Role>
      <ShortName>Employee Benefit Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2115100 - Disclosure - Accrued Mining and Landfill Reclamation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/AccruedMiningAndLandfillReclamation</Role>
      <ShortName>Accrued Mining and Landfill Reclamation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2116100 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/CommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2117100 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/Leases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2118100 - Disclosure - Fair Value of Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/FairValueOfFinancialInstruments</Role>
      <ShortName>Fair Value of Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2119100 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2121100 - Disclosure - Senior Notes' Guarantor and Non-Guarantor Financial Information</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.summitmaterials.com/role/SeniorNotesGuarantorAndNonGuarantorFinancialInformation</Role>
      <ShortName>Senior Notes' Guarantor and Non-Guarantor Financial Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2129100 - Disclosure - Supplementary Data (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SupplementaryDataUnaudited</Role>
      <ShortName>Supplementary Data (Unaudited)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2201201 - Disclosure - Summary of Organization and Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SummaryOfOrganizationAndSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Organization and Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/SummaryOfOrganizationAndSignificantAccountingPolicies</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2302301 - Disclosure - Acquisitions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/AcquisitionsTables</Role>
      <ShortName>Acquisitions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/Acquisitions</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2303301 - Disclosure - Goodwill (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/GoodwillTables</Role>
      <ShortName>Goodwill (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/Goodwill</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2304301 - Disclosure - Revenue Recognition (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/RevenueRecognitionTables</Role>
      <ShortName>Revenue Recognition (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/RevenueRecognition</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2305301 - Disclosure - Inventories (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/InventoriesTables</Role>
      <ShortName>Inventories (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/Inventories</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2306301 - Disclosure - Property, Plant and Equipment, net and Intangibles, net (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/PropertyPlantAndEquipmentNetAndIntangiblesNetTables</Role>
      <ShortName>Property, Plant and Equipment, net and Intangibles, net (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/PropertyPlantAndEquipmentNetAndIntangiblesNet</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2307301 - Disclosure - Accrued Expenses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/AccruedExpensesTables</Role>
      <ShortName>Accrued Expenses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/AccruedExpenses</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2308301 - Disclosure - Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/DebtTables</Role>
      <ShortName>Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/Debt</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2309301 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/IncomeTaxes</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2310301 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/EarningsPerShare</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2311301 - Disclosure - Stockholder's Equity/Members' Interest (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/StockholdersEquityMembersInterestTables</Role>
      <ShortName>Stockholder's Equity/Members' Interest (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/StockholdersEquityMembersInterest</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2312301 - Disclosure - Supplemental Cash Flow Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SupplementalCashFlowInformationTables</Role>
      <ShortName>Supplemental Cash Flow Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/SupplementalCashFlowInformation</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2313301 - Disclosure - Stock-Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/StockBasedCompensationTables</Role>
      <ShortName>Stock-Based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/StockBasedCompensation</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2314301 - Disclosure - Employee Benefit Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EmployeeBenefitPlansTables</Role>
      <ShortName>Employee Benefit Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/EmployeeBenefitPlans</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2315301 - Disclosure - Accrued Mining and Landfill Reclamation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/AccruedMiningAndLandfillReclamationTables</Role>
      <ShortName>Accrued Mining and Landfill Reclamation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/AccruedMiningAndLandfillReclamation</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2317301 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/LeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/Leases</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2318301 - Disclosure - Fair Value (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/FairValueTables</Role>
      <ShortName>Fair Value (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/FairValueOfFinancialInstruments</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2319301 - Disclosure - Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SegmentInformationTables</Role>
      <ShortName>Segment Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/SegmentInformation</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2321301 - Disclosure - Senior Notes' Guarantor and Non-Guarantor Financial Information (Tables)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.summitmaterials.com/role/SeniorNotesGuarantorAndNonGuarantorFinancialInformationTables</Role>
      <ShortName>Senior Notes' Guarantor and Non-Guarantor Financial Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/SeniorNotesGuarantorAndNonGuarantorFinancialInformation</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2329301 - Disclosure - Supplementary Data (Unaudited) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SupplementaryDataUnauditedTables</Role>
      <ShortName>Supplementary Data (Unaudited) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/SupplementaryDataUnaudited</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2401402 - Disclosure - Summary of Organization and Significant Accounting Policies - General Information and Equity Offerings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SummaryOfOrganizationAndSignificantAccountingPoliciesGeneralInformationAndEquityOfferingsDetails</Role>
      <ShortName>Summary of Organization and Significant Accounting Policies - General Information and Equity Offerings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2401403 - Disclosure - Summary of Organization and Significant Accounting Policies - New Accounting Standards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SummaryOfOrganizationAndSignificantAccountingPoliciesNewAccountingStandardsDetails</Role>
      <ShortName>Summary of Organization and Significant Accounting Policies - New Accounting Standards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2402402 - Disclosure - Acquisitions - Acquisitions by Region (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/AcquisitionsAcquisitionsByRegionDetails</Role>
      <ShortName>Acquisitions - Acquisitions by Region (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2402403 - Disclosure - Acquisitions - Summary of Assets Acquired and Liabilities Assumed (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/AcquisitionsSummaryOfAssetsAcquiredAndLiabilitiesAssumedDetail</Role>
      <ShortName>Acquisitions - Summary of Assets Acquired and Liabilities Assumed (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2402404 - Disclosure - Acquisitions - Contractual Obligations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/AcquisitionsContractualObligationsDetails</Role>
      <ShortName>Acquisitions - Contractual Obligations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2403402 - Disclosure - Goodwill - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/GoodwillNarrativeDetails</Role>
      <ShortName>Goodwill - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2403403 - Disclosure - Goodwill - Summary of Goodwill by Reportable Segments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/GoodwillSummaryOfGoodwillByReportableSegmentsDetails</Role>
      <ShortName>Goodwill - Summary of Goodwill by Reportable Segments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2404402 - Disclosure - Revenue Recognition - By Product (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/RevenueRecognitionByProductDetails</Role>
      <ShortName>Revenue Recognition - By Product (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2404403 - Disclosure - Revenue Recognition - Contract Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/RevenueRecognitionContractAssetsAndLiabilitiesDetails</Role>
      <ShortName>Revenue Recognition - Contract Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2404404 - Disclosure - Revenue Recognition - Summary of Accounts Receivable, Net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/RevenueRecognitionSummaryOfAccountsReceivableNetDetails</Role>
      <ShortName>Revenue Recognition - Summary of Accounts Receivable, Net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - Inventories - Components of Inventories (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/InventoriesComponentsOfInventoriesDetails</Role>
      <ShortName>Inventories - Components of Inventories (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2406402 - Disclosure - Property, Plant and Equipment, net and Intangibles, net - Components of Property, Plant and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/PropertyPlantAndEquipmentNetAndIntangiblesNetComponentsOfPropertyPlantAndEquipmentDetails</Role>
      <ShortName>Property, Plant and Equipment, net and Intangibles, net - Components of Property, Plant and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2406403 - Disclosure - Property, Plant and Equipment, net and Intangibles, net - Capital and Finance Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/PropertyPlantAndEquipmentNetAndIntangiblesNetCapitalAndFinanceLeasesDetails</Role>
      <ShortName>Property, Plant and Equipment, net and Intangibles, net - Capital and Finance Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2406404 - Disclosure - Property, Plant and Equipment, net and Intangibles, net - Intangible Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/PropertyPlantAndEquipmentNetAndIntangiblesNetIntangibleAssetsDetails</Role>
      <ShortName>Property, Plant and Equipment, net and Intangibles, net - Intangible Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2407402 - Disclosure - Accrued Expenses - Components of Accrued Expenses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/AccruedExpensesComponentsOfAccruedExpensesDetails</Role>
      <ShortName>Accrued Expenses - Components of Accrued Expenses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2408402 - Disclosure - Debt - Schedule of Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/DebtScheduleOfDebtDetails</Role>
      <ShortName>Debt - Schedule of Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2408403 - Disclosure - Debt - Schedule of Contractual Payments of Long-Term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/DebtScheduleOfContractualPaymentsOfLongTermDebtDetails</Role>
      <ShortName>Debt - Schedule of Contractual Payments of Long-Term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2408404 - Disclosure - Debt - Senior Notes (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.summitmaterials.com/role/DebtSeniorNotesDetails</Role>
      <ShortName>Debt - Senior Notes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2408405 - Disclosure - Debt - Senior Secured Credit Facilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/DebtSeniorSecuredCreditFacilitiesDetails</Role>
      <ShortName>Debt - Senior Secured Credit Facilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2408406 - Disclosure - Debt - Summary of Activity for Deferred Financing Fees (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/DebtSummaryOfActivityForDeferredFinancingFeesDetails</Role>
      <ShortName>Debt - Summary of Activity for Deferred Financing Fees (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2408407 - Disclosure - Debt - Other Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/DebtOtherNarrativeDetails</Role>
      <ShortName>Debt - Other Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2409402 - Disclosure - Income Taxes - Components of Income Tax Benefit (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/IncomeTaxesComponentsOfIncomeTaxBenefitDetail</Role>
      <ShortName>Income Taxes - Components of Income Tax Benefit (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2409403 - Disclosure - Income Taxes - Schedule of Income Tax Benefit (Expense) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/IncomeTaxesScheduleOfIncomeTaxBenefitExpenseDetails</Role>
      <ShortName>Income Taxes - Schedule of Income Tax Benefit (Expense) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2409404 - Disclosure - Income Taxes - Components of Net Deferred Income Tax Assets and Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/IncomeTaxesComponentsOfNetDeferredIncomeTaxAssetsAndLiabilitiesDetail</Role>
      <ShortName>Income Taxes - Components of Net Deferred Income Tax Assets and Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2409405 - Disclosure - Income Taxes - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/IncomeTaxesNarrativeDetails</Role>
      <ShortName>Income Taxes - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2409406 - Disclosure - Income Taxes - Unrecognized Tax Benefits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/IncomeTaxesUnrecognizedTaxBenefitsDetails</Role>
      <ShortName>Income Taxes - Unrecognized Tax Benefits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2409407 - Disclosure - Incomes Taxes - Deferred Tax Asset Valuation Allowance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/IncomesTaxesDeferredTaxAssetValuationAllowanceDetails</Role>
      <ShortName>Incomes Taxes - Deferred Tax Asset Valuation Allowance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2410402 - Disclosure - Earnings Per Share - Schedule of Basic to Diluted Income Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EarningsPerShareScheduleOfBasicToDilutedIncomePerShareDetails</Role>
      <ShortName>Earnings Per Share - Schedule of Basic to Diluted Income Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2411402 - Disclosure - Stockholders' Equity/Members' Interest - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/StockholdersEquityMembersInterestNarrativeDetails</Role>
      <ShortName>Stockholders' Equity/Members' Interest - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2411403 - Disclosure - Stockholders' Equity/Members' Interest - Change in Ownership (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/StockholdersEquityMembersInterestChangeInOwnershipDetails</Role>
      <ShortName>Stockholders' Equity/Members' Interest - Change in Ownership (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2411404 - Disclosure - Stockholders' Equity/Members' Interest - Accumulated Other Comprehensive Income (Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/StockholdersEquityMembersInterestAccumulatedOtherComprehensiveIncomeLossDetails</Role>
      <ShortName>Stockholders' Equity/Members' Interest - Accumulated Other Comprehensive Income (Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>2412402 - Disclosure - Supplemental Cash Flow Information - Schedule of Supplemental Cash Flow Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SupplementalCashFlowInformationScheduleOfSupplementalCashFlowInformationDetails</Role>
      <ShortName>Supplemental Cash Flow Information - Schedule of Supplemental Cash Flow Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>2413402 - Disclosure - Stock-Based Compensation - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/StockBasedCompensationNarrativeDetails</Role>
      <ShortName>Stock-Based Compensation - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>2413403 - Disclosure - Stock-Based Compensation - Summary of Award Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/StockBasedCompensationSummaryOfAwardActivityDetails</Role>
      <ShortName>Stock-Based Compensation - Summary of Award Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>2413404 - Disclosure - Stock-Based Compensation - Weighted Average Assumptions Used to Estimate the Fair Value of Grants (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/StockBasedCompensationWeightedAverageAssumptionsUsedToEstimateFairValueOfGrantsDetails</Role>
      <ShortName>Stock-Based Compensation - Weighted Average Assumptions Used to Estimate the Fair Value of Grants (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>2414402 - Disclosure - Employee Benefit Plans - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EmployeeBenefitPlansNarrativeDetails</Role>
      <ShortName>Employee Benefit Plans - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>2414403 - Disclosure - Employee Benefit Plans - Obligations and Funded Status (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EmployeeBenefitPlansObligationsAndFundedStatusDetail</Role>
      <ShortName>Employee Benefit Plans - Obligations and Funded Status (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>2414404 - Disclosure - Employee Benefit Plans - Amounts Recognized in Other Comprehensive (Gain) Loss (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EmployeeBenefitPlansAmountsRecognizedInOtherComprehensiveGainLossDetail</Role>
      <ShortName>Employee Benefit Plans - Amounts Recognized in Other Comprehensive (Gain) Loss (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>2414405 - Disclosure - Employee Benefit Plans - Components of Net Periodic Benefit Cost (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EmployeeBenefitPlansComponentsOfNetPeriodicBenefitCostDetail</Role>
      <ShortName>Employee Benefit Plans - Components of Net Periodic Benefit Cost (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>2414406 - Disclosure - Employee Benefit Plans - Weighted-Average Assumptions Used to Determine Benefit Obligations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EmployeeBenefitPlansWeightedAverageAssumptionsUsedToDetermineBenefitObligationsDetail</Role>
      <ShortName>Employee Benefit Plans - Weighted-Average Assumptions Used to Determine Benefit Obligations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>2414407 - Disclosure - Employee Benefit Plans - Weighted-Average Assumptions Used to Determine Net Periodic Benefit Cost (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EmployeeBenefitPlansWeightedAverageAssumptionsUsedToDetermineNetPeriodicBenefitCostDetail</Role>
      <ShortName>Employee Benefit Plans - Weighted-Average Assumptions Used to Determine Net Periodic Benefit Cost (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>2414408 - Disclosure - Employee Benefit Plans - Effects of One Percentage-Point Change in Assumed Health Care Cost Trend Rates (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EmployeeBenefitPlansEffectsOfOnePercentagePointChangeInAssumedHealthCareCostTrendRatesDetail</Role>
      <ShortName>Employee Benefit Plans - Effects of One Percentage-Point Change in Assumed Health Care Cost Trend Rates (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>2414409 - Disclosure - Employee Benefit Plans - Fair Value of Company's Pension Plans' Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EmployeeBenefitPlansFairValueOfCompanysPensionPlansAssetsDetail</Role>
      <ShortName>Employee Benefit Plans - Fair Value of Company's Pension Plans' Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>2414410 - Disclosure - Employee Benefit Plans - Estimated Benefit Payments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EmployeeBenefitPlansEstimatedBenefitPaymentsDetail</Role>
      <ShortName>Employee Benefit Plans - Estimated Benefit Payments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>2414411 - Disclosure - Employee Benefit Plans - Multiemployer Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/EmployeeBenefitPlansMultiemployerPlansDetails</Role>
      <ShortName>Employee Benefit Plans - Multiemployer Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>2415402 - Disclosure - Accrued Mining and Landfill Reclamation - Narrative (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/AccruedMiningAndLandfillReclamationNarrativeDetail</Role>
      <ShortName>Accrued Mining and Landfill Reclamation - Narrative (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>2415403 - Disclosure - Accrued Mining and Landfill Reclamation - Activity for Asset Retirement Obligations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/AccruedMiningAndLandfillReclamationActivityForAssetRetirementObligationsDetails</Role>
      <ShortName>Accrued Mining and Landfill Reclamation - Activity for Asset Retirement Obligations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>2416401 - Disclosure - Commitments and Contingencies - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/CommitmentsAndContingenciesNarrativeDetails</Role>
      <ShortName>Commitments and Contingencies - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>2417402 - Disclosure - Leases - Operating and Finance Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/LeasesOperatingAndFinanceLeasesDetails</Role>
      <ShortName>Leases - Operating and Finance Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>2417403 - Disclosure - Leases - Narrative (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/LeasesNarrativeDetail</Role>
      <ShortName>Leases - Narrative (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>2417404 - Disclosure - Leases - Schedule of Minimum Contractual Agreements Under Royalty Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/LeasesScheduleOfMinimumContractualAgreementsUnderRoyaltyAgreementsDetails</Role>
      <ShortName>Leases - Schedule of Minimum Contractual Agreements Under Royalty Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>2418402 - Disclosure - Fair Value of Financial Instruments - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/FairValueOfFinancialInstrumentsNarrativeDetails</Role>
      <ShortName>Fair Value of Financial Instruments - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>2418403 - Disclosure - Fair Value of Financial Instruments - Fair Value Measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/FairValueOfFinancialInstrumentsFairValueMeasurementsDetails</Role>
      <ShortName>Fair Value of Financial Instruments - Fair Value Measurements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>2418404 - Disclosure - Fair Value of Financial Instruments - Carrying Value and Fair Value of Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/FairValueOfFinancialInstrumentsCarryingValueAndFairValueOfFinancialInstrumentsDetails</Role>
      <ShortName>Fair Value of Financial Instruments - Carrying Value and Fair Value of Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>2419402 - Disclosure - Segment Information - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SegmentInformationNarrativeDetails</Role>
      <ShortName>Segment Information - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>2419403 - Disclosure - Segment Information - Financial Data (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SegmentInformationFinancialDataDetails</Role>
      <ShortName>Segment Information - Financial Data (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>105</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>2421402 - Disclosure - Senior Notes' Guarantor and Non-Guarantor Financial Information - Schedule of Condensed Consolidating Balance Sheets (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.summitmaterials.com/role/SeniorNotesGuarantorAndNonGuarantorFinancialInformationScheduleOfCondensedConsolidatingBalanceSheetsDetails</Role>
      <ShortName>Senior Notes' Guarantor and Non-Guarantor Financial Information - Schedule of Condensed Consolidating Balance Sheets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>106</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>2421403 - Disclosure - Senior Notes' Guarantor and Non-Guarantor Financial Information - Schedule of Condensed Consolidating Statements of Operations and Comprehensive Income (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.summitmaterials.com/role/SeniorNotesGuarantorAndNonGuarantorFinancialInformationScheduleOfCondensedConsolidatingStatementsOfOperationsAndComprehensiveIncomeDetails</Role>
      <ShortName>Senior Notes' Guarantor and Non-Guarantor Financial Information - Schedule of Condensed Consolidating Statements of Operations and Comprehensive Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>107</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>2421404 - Disclosure - Senior Notes' Guarantor and Non-Guarantor Financial Information - Schedule of Condensed Consolidating Statements of Cash Flows (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.summitmaterials.com/role/SeniorNotesGuarantorAndNonGuarantorFinancialInformationScheduleOfCondensedConsolidatingStatementsOfCashFlowsDetails</Role>
      <ShortName>Senior Notes' Guarantor and Non-Guarantor Financial Information - Schedule of Condensed Consolidating Statements of Cash Flows (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>108</Position>
    </Report>
    <Report instance="a10k2019.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>2429402 - Disclosure - Supplementary Data (Unaudited) - Supplemental Financial Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.summitmaterials.com/role/SupplementaryDataUnauditedSupplementalFinancialInformationDetail</Role>
      <ShortName>Supplementary Data (Unaudited) - Supplemental Financial Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.summitmaterials.com/role/SupplementaryDataUnauditedTables</ParentRole>
      <Position>109</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-K" original="a10k2019.htm">a10k2019.htm</File>
    <File doctype="10-K" original="exhibit99110k2019.htm">exhibit99110k2019.htm</File>
    <File>ex2110k2019.htm</File>
    <File>ex23110k2019.htm</File>
    <File>ex31110k2019.htm</File>
    <File>ex31210k2019.htm</File>
    <File>ex31310k2019.htm</File>
    <File>ex31410k2019.htm</File>
    <File>ex32110k2019.htm</File>
    <File>ex32210k2019.htm</File>
    <File>ex32310k2019.htm</File>
    <File>ex32410k2019.htm</File>
    <File>ex4110k2019.htm</File>
    <File>exhibit95110k2019.htm</File>
    <File>sum-20191228.xsd</File>
    <File>sum-20191228_cal.xml</File>
    <File>sum-20191228_def.xml</File>
    <File>sum-20191228_lab.xml</File>
    <File>sum-20191228_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>corpassetmap2018a02.jpg</File>
    <File>corpstructurea15.jpg</File>
    <File>productionvaluechaina01.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/srt/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2019-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
