<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.10.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>199</ContextCount>
  <ElementCount>316</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>69</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>9</UnitCount>
  <MyReports>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>100000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>100010 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/StatementConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>100020 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/StatementConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>100030 - Statement - Consolidated Statements of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/StatementConsolidatedStatementsOfOperations</Role>
      <ShortName>Consolidated Statements of Operations</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>100040 - Statement - Consolidated Statements of Comprehensive Loss</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/StatementConsolidatedStatementsOfComprehensiveLoss</Role>
      <ShortName>Consolidated Statements of Comprehensive Loss</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>100050 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/StatementConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>100060 - Disclosure - Nature of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureNatureOfOperations</Role>
      <ShortName>Nature of Operations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>100070 - Disclosure - Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureSignificantAccountingPolicies</Role>
      <ShortName>Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>100080 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureFairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>100090 - Disclosure - Balance Sheet Detail</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureBalanceSheetDetail</Role>
      <ShortName>Balance Sheet Detail</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>100100 - Disclosure - Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureIntangibleAssets</Role>
      <ShortName>Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>100110 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>100120 - Disclosure - Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureDebt</Role>
      <ShortName>Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>100130 - Disclosure - Convertible Preferred Stock and Shareholders' Deficit</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureConvertiblePreferredStockAndShareholdersDeficit</Role>
      <ShortName>Convertible Preferred Stock and Shareholders' Deficit</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>100140 - Disclosure - Equity Incentive Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureEquityIncentivePlans</Role>
      <ShortName>Equity Incentive Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>100150 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureIncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>100160 - Disclosure - Net Loss Per Share Attributable to Common Shareholders</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureNetLossPerShareAttributableToCommonShareholders</Role>
      <ShortName>Net Loss Per Share Attributable to Common Shareholders</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>100170 - Disclosure - Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.avalara.com/20180930/taxonomy/role/DisclosureSignificantAccountingPolicies</ParentRole>
      <Position>18</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>100180 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureFairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.avalara.com/20180930/taxonomy/role/DisclosureFairValueMeasurements</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>100190 - Disclosure - Balance Sheet Detail (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureBalanceSheetDetailTables</Role>
      <ShortName>Balance Sheet Detail (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.avalara.com/20180930/taxonomy/role/DisclosureBalanceSheetDetail</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>100200 - Disclosure - Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureIntangibleAssetsTables</Role>
      <ShortName>Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.avalara.com/20180930/taxonomy/role/DisclosureIntangibleAssets</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>100210 - Disclosure - Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureDebtTables</Role>
      <ShortName>Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.avalara.com/20180930/taxonomy/role/DisclosureDebt</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>100220 - Disclosure - Convertible Preferred Stock and Shareholders' Deficit (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureConvertiblePreferredStockAndShareholdersDeficitTables</Role>
      <ShortName>Convertible Preferred Stock and Shareholders' Deficit (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.avalara.com/20180930/taxonomy/role/DisclosureConvertiblePreferredStockAndShareholdersDeficit</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>100230 - Disclosure - Equity Incentive Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureEquityIncentivePlansTables</Role>
      <ShortName>Equity Incentive Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.avalara.com/20180930/taxonomy/role/DisclosureEquityIncentivePlans</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>100240 - Disclosure - Net Loss Per Share Attributable to Common Shareholders (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureNetLossPerShareAttributableToCommonShareholdersTables</Role>
      <ShortName>Net Loss Per Share Attributable to Common Shareholders (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.avalara.com/20180930/taxonomy/role/DisclosureNetLossPerShareAttributableToCommonShareholders</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>100250 - Disclosure - Significant Accounting Policies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureSignificantAccountingPoliciesAdditionalInformationDetails</Role>
      <ShortName>Significant Accounting Policies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>100260 - Disclosure - Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureFairValueMeasurementsScheduleOfAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>100270 - Disclosure - Fair Value Measurements - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureFairValueMeasurementsAdditionalInformationDetails</Role>
      <ShortName>Fair Value Measurements - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>100280 - Disclosure - Fair Value Measurements - Summary of Reconciliation of Beginning and Ending Balances of Recurring Fair Value Measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureFairValueMeasurementsSummaryOfReconciliationOfBeginningAndEndingBalancesOfRecurringFairValueMeasurementsDetails</Role>
      <ShortName>Fair Value Measurements - Summary of Reconciliation of Beginning and Ending Balances of Recurring Fair Value Measurements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>100290 - Disclosure - Fair Value Measurements - Summary of Reconciliation of Beginning and Ending Balances of Recurring Fair Value Measurements (Parenthetical) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureFairValueMeasurementsSummaryOfReconciliationOfBeginningAndEndingBalancesOfRecurringFairValueMeasurementsParentheticalDetails</Role>
      <ShortName>Fair Value Measurements - Summary of Reconciliation of Beginning and Ending Balances of Recurring Fair Value Measurements (Parenthetical) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>100300 - Disclosure - Balance Sheet Detail - Property and Equipment, Net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureBalanceSheetDetailPropertyAndEquipmentNetDetails</Role>
      <ShortName>Balance Sheet Detail - Property and Equipment, Net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>100310 - Disclosure - Balance Sheet Detail - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureBalanceSheetDetailAdditionalInformationDetails</Role>
      <ShortName>Balance Sheet Detail - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>100320 - Disclosure - Balance Sheet Detail - Prepaid Expenses and Other Current Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureBalanceSheetDetailPrepaidExpensesAndOtherCurrentAssetsDetails</Role>
      <ShortName>Balance Sheet Detail - Prepaid Expenses and Other Current Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>100330 - Disclosure - Balance Sheet Detail - Accrued Expenses and Trade Payables (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureBalanceSheetDetailAccruedExpensesAndTradePayablesDetails</Role>
      <ShortName>Balance Sheet Detail - Accrued Expenses and Trade Payables (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>100340 - Disclosure - Intangible Assets - Schedule of Finite-lived Intangible Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureIntangibleAssetsScheduleOfFiniteLivedIntangibleAssetsDetails</Role>
      <ShortName>Intangible Assets - Schedule of Finite-lived Intangible Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>100350 - Disclosure - Intangible Assets - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureIntangibleAssetsAdditionalInformationDetails</Role>
      <ShortName>Intangible Assets - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>100360 - Disclosure - Intangible Assets - Schedule of Changes in Carrying Amount of Goodwill (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureIntangibleAssetsScheduleOfChangesInCarryingAmountOfGoodwillDetails</Role>
      <ShortName>Intangible Assets - Schedule of Changes in Carrying Amount of Goodwill (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>100370 - Disclosure - Commitments and Contingencies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetails</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>100380 - Disclosure - Debt - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureDebtAdditionalInformationDetails</Role>
      <ShortName>Debt - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>100390 - Disclosure - Debt - Schedule of Outstanding Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureDebtScheduleOfOutstandingBorrowingsDetails</Role>
      <ShortName>Debt - Schedule of Outstanding Borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>100400 - Disclosure - Convertible Preferred Stock and Shareholders' Equity (Deficit) - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureConvertiblePreferredStockAndShareholdersEquityDeficitAdditionalInformationDetails</Role>
      <ShortName>Convertible Preferred Stock and Shareholders' Equity (Deficit) - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.avalara.com/20180930/taxonomy/role/DisclosureConvertiblePreferredStockAndShareholdersDeficitTables</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>100410 - Disclosure - Convertible Preferred Stock and Shareholders' Equity (Deficit) - Summary of Changes of Convertible Preferred Stock and Shareholders' Deficit (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureConvertiblePreferredStockAndShareholdersEquityDeficitSummaryOfChangesOfConvertiblePreferredStockAndShareholdersDeficitDetails</Role>
      <ShortName>Convertible Preferred Stock and Shareholders' Equity (Deficit) - Summary of Changes of Convertible Preferred Stock and Shareholders' Deficit (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.avalara.com/20180930/taxonomy/role/DisclosureConvertiblePreferredStockAndShareholdersDeficitTables</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>100420 - Disclosure - Equity Incentive Plans - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureEquityIncentivePlansAdditionalInformationDetails</Role>
      <ShortName>Equity Incentive Plans - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>100430 - Disclosure - Equity Incentive Plans - Summary of Stock-Based Compensation Expense Related to Equity Incentive Rewards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureEquityIncentivePlansSummaryOfStockBasedCompensationExpenseRelatedToEquityIncentiveRewardsDetails</Role>
      <ShortName>Equity Incentive Plans - Summary of Stock-Based Compensation Expense Related to Equity Incentive Rewards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>100440 - Disclosure - Equity Incentive Plans - Summary of Stock Option Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureEquityIncentivePlansSummaryOfStockOptionActivityDetails</Role>
      <ShortName>Equity Incentive Plans - Summary of Stock Option Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>100450 - Disclosure - Equity Incentive Plans - Summary of Options Outstanding and Vested (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureEquityIncentivePlansSummaryOfOptionsOutstandingAndVestedDetails</Role>
      <ShortName>Equity Incentive Plans - Summary of Options Outstanding and Vested (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>100460 - Disclosure - Equity Incentive Plans - Summary of Fair Value Estimated Using Black-Scholes Option Pricing Model Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureEquityIncentivePlansSummaryOfFairValueEstimatedUsingBlackScholesOptionPricingModelAssumptionsDetails</Role>
      <ShortName>Equity Incentive Plans - Summary of Fair Value Estimated Using Black-Scholes Option Pricing Model Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>100470 - Disclosure - Equity Incentive Plans - Summary of Fair Value of ESPP Estimated Using Black-Scholes Option Pricing Model Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureEquityIncentivePlansSummaryOfFairValueOfESPPEstimatedUsingBlackScholesOptionPricingModelAssumptionsDetails</Role>
      <ShortName>Equity Incentive Plans - Summary of Fair Value of ESPP Estimated Using Black-Scholes Option Pricing Model Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>100480 - Disclosure - Income Taxes - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureIncomeTaxesAdditionalInformationDetails</Role>
      <ShortName>Income Taxes - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>100490 - Disclosure - Net Loss Per Share Attributable to Common Shareholders - Schedule of Computation of Basic and Diluted Net Loss Per Common Share (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureNetLossPerShareAttributableToCommonShareholdersScheduleOfComputationOfBasicAndDilutedNetLossPerCommonShareDetail</Role>
      <ShortName>Net Loss Per Share Attributable to Common Shareholders - Schedule of Computation of Basic and Diluted Net Loss Per Common Share (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="avlr-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>100500 - Disclosure - Net Loss Per Share Attributable to Common Shareholders - Schedule of Weighted-Average Outstanding Shares of Common Stock Equivalents Were Excluded from Computation of Diluted Net Loss Per Share (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.avalara.com/20180930/taxonomy/role/DisclosureNetLossPerShareAttributableToCommonShareholdersScheduleOfWeightedAverageOutstandingSharesOfCommonStockEquivalentsWereExcludedFromComputationOfDilutedNetLossPerShareDetail</Role>
      <ShortName>Net Loss Per Share Attributable to Common Shareholders - Schedule of Weighted-Average Outstanding Shares of Common Stock Equivalents Were Excluded from Computation of Diluted Net Loss Per Share (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>avlr-20180930.xml</File>
    <File>avlr-20180930.xsd</File>
    <File>avlr-20180930_cal.xml</File>
    <File>avlr-20180930_def.xml</File>
    <File>avlr-20180930_lab.xml</File>
    <File>avlr-20180930_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/srt/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2018-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
