<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.22.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>188</ContextCount>
  <ElementCount>297</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>46</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>8</UnitCount>
  <MyReports>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>00090 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>00100 - Statement - Condensed Consolidated Balance Sheets (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/StatementCondensedConsolidatedBalanceSheetsUnaudited</Role>
      <ShortName>Condensed Consolidated Balance Sheets (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>00105 - Statement - Condensed Consolidated Balance Sheets (Parenthetical) (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/StatementCondensedConsolidatedBalanceSheetsParentheticalUnaudited</Role>
      <ShortName>Condensed Consolidated Balance Sheets (Parenthetical) (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>00200 - Statement - Condensed Consolidated Statements of Operations and Comprehensive Loss (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/StatementCondensedConsolidatedStatementsOfOperationsAndComprehensiveLossUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Operations and Comprehensive Loss (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>00300 - Statement - Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/StatementCondensedConsolidatedStatementsOfConvertiblePreferredStockAndStockholdersEquityDeficitUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>00400 - Statement - Condensed Consolidated Statements of Cash Flows (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/StatementCondensedConsolidatedStatementsOfCashFlowsUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Cash Flows (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>10101 - Disclosure - Organization and Nature of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureOrganizationAndNatureOfOperations</Role>
      <ShortName>Organization and Nature of Operations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>10201 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>10301 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureFairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>10401 - Disclosure - Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureInvestments</Role>
      <ShortName>Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>10501 - Disclosure - Condensed Consolidated Balance Sheet Details</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureCondensedConsolidatedBalanceSheetDetails</Role>
      <ShortName>Condensed Consolidated Balance Sheet Details</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>10601 - Disclosure - Convertible Preferred Stock and Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureConvertiblePreferredStockAndStockholdersEquity</Role>
      <ShortName>Convertible Preferred Stock and Stockholders' Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>10701 - Disclosure - License Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureLicenseAgreements</Role>
      <ShortName>License Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>10801 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>10901 - Disclosure - Employee Benefits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureEmployeeBenefits</Role>
      <ShortName>Employee Benefits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>11001 - Disclosure - Net Loss Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureNetLossPerShare</Role>
      <ShortName>Net Loss Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>11101 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureSubsequentEvents</Role>
      <ShortName>Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>20202 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>18</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>30303 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureFairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureFairValueMeasurements</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>30403 - Disclosure - Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureInvestmentsTables</Role>
      <ShortName>Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureInvestments</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>30503 - Disclosure - Condensed Consolidated Balance Sheet Details (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureCondensedConsolidatedBalanceSheetDetailsTables</Role>
      <ShortName>Condensed Consolidated Balance Sheet Details (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureCondensedConsolidatedBalanceSheetDetails</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>30603 - Disclosure - Convertible Preferred Stock and Stockholders' Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureConvertiblePreferredStockAndStockholdersEquityTables</Role>
      <ShortName>Convertible Preferred Stock and Stockholders' Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureConvertiblePreferredStockAndStockholdersEquity</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>30803 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureCommitmentsAndContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureCommitmentsAndContingencies</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>31003 - Disclosure - Net Loss Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureNetLossPerShareTables</Role>
      <ShortName>Net Loss Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureNetLossPerShare</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>40101 - Disclosure - Organization and Nature of Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureOrganizationAndNatureOfOperationsDetails</Role>
      <ShortName>Organization and Nature of Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureOrganizationAndNatureOfOperations</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>40201 - Disclosure - Summary of Significant Accounting Policies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureSummaryOfSignificantAccountingPoliciesDetails</Role>
      <ShortName>Summary of Significant Accounting Policies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>40301 - Disclosure - Fair Value Measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureFairValueMeasurementsDetails</Role>
      <ShortName>Fair Value Measurements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureFairValueMeasurementsTables</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>40302 - Disclosure - Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureFairValueMeasurementsSummaryOfFinancialAssetsMeasuredAtFairValueOnRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>40303 - Disclosure - Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureFairValueMeasurementsSummaryOfFinancialAssetsMeasuredAtFairValueOnRecurringBasisAdditionalInformationDetails</Role>
      <ShortName>Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>40401 - Disclosure - Investments - Summary of Cash Equivalents and Investments Measured at Fair Value on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureInvestmentsSummaryOfCashEquivalentsAndInvestmentsMeasuredAtFairValueOnRecurringBasisDetails</Role>
      <ShortName>Investments - Summary of Cash Equivalents and Investments Measured at Fair Value on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>40402 - Disclosure - Investments - Schedule of Contractual Maturities of AFS Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureInvestmentsScheduleOfContractualMaturitiesOfAfsSecuritiesDetails</Role>
      <ShortName>Investments - Schedule of Contractual Maturities of AFS Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>40403 - Disclosure - Investments - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureInvestmentsNarrativeDetails</Role>
      <ShortName>Investments - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>40501 - Disclosure - Condensed Consolidated Balance Sheet Details - Schedule of Prepaid and Other Current Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureCondensedConsolidatedBalanceSheetDetailsScheduleOfPrepaidAndOtherCurrentAssetsDetails</Role>
      <ShortName>Condensed Consolidated Balance Sheet Details - Schedule of Prepaid and Other Current Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>40502 - Disclosure - Condensed Consolidated Balance Sheet Details - Schedule of Property and Equipment, Net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureCondensedConsolidatedBalanceSheetDetailsScheduleOfPropertyAndEquipmentNetDetails</Role>
      <ShortName>Condensed Consolidated Balance Sheet Details - Schedule of Property and Equipment, Net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>40503 - Disclosure - Condensed Consolidated Balance Sheet Details - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureCondensedConsolidatedBalanceSheetDetailsNarrativeDetails</Role>
      <ShortName>Condensed Consolidated Balance Sheet Details - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>40504 - Disclosure - Condensed Consolidated Balance Sheet Details - Schedule of Other Non-Current Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureCondensedConsolidatedBalanceSheetDetailsScheduleOfOtherNonCurrentAssetsDetails</Role>
      <ShortName>Condensed Consolidated Balance Sheet Details - Schedule of Other Non-Current Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>40601 - Disclosure - Convertible Preferred Stock and Stockholders' Equity - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureConvertiblePreferredStockAndStockholdersEquityNarrativeDetails</Role>
      <ShortName>Convertible Preferred Stock and Stockholders' Equity - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureConvertiblePreferredStockAndStockholdersEquityTables</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>40602 - Disclosure - Convertible Preferred Stock and Stockholders' Equity - Summary of Stock Option Activities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureConvertiblePreferredStockAndStockholdersEquitySummaryOfStockOptionActivitiesDetails</Role>
      <ShortName>Convertible Preferred Stock and Stockholders' Equity - Summary of Stock Option Activities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>40603 - Disclosure - Convertible Preferred Stock and Stockholders' Equity - Summary of Stock-Based Compensation Expense Recognized (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureConvertiblePreferredStockAndStockholdersEquitySummaryOfStockBasedCompensationExpenseRecognizedDetails</Role>
      <ShortName>Convertible Preferred Stock and Stockholders' Equity - Summary of Stock-Based Compensation Expense Recognized (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>40604 - Disclosure - Convertible Preferred Stock and Stockholders' Equity - Weighted-Average Assumptions Used in Black-Scholes Option Pricing Model to Determine Fair Value of Stock Option Grants (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureConvertiblePreferredStockAndStockholdersEquityWeightedAverageAssumptionsUsedInBlackScholesOptionPricingModelToDetermineFairValueOfStockOptionGrantsDetails</Role>
      <ShortName>Convertible Preferred Stock and Stockholders' Equity - Weighted-Average Assumptions Used in Black-Scholes Option Pricing Model to Determine Fair Value of Stock Option Grants (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>40605 - Disclosure - Convertible Preferred Stock and Stockholders' Equity - Schedule of Common Stock Reserved for Future Issuance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureConvertiblePreferredStockAndStockholdersEquityScheduleOfCommonStockReservedForFutureIssuanceDetails</Role>
      <ShortName>Convertible Preferred Stock and Stockholders' Equity - Schedule of Common Stock Reserved for Future Issuance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>40701 - Disclosure - License Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureLicenseAgreementsDetails</Role>
      <ShortName>License Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureLicenseAgreements</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>40801 - Disclosure - Commitments and Contingencies - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureCommitmentsAndContingenciesNarrativeDetails</Role>
      <ShortName>Commitments and Contingencies - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>40802 - Disclosure - Commitments and Contingencies - Schedule of Future Minimum Lease Payments Required under Operating Lease (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureCommitmentsAndContingenciesScheduleOfFutureMinimumLeasePaymentsRequiredUnderOperatingLeaseDetails</Role>
      <ShortName>Commitments and Contingencies - Schedule of Future Minimum Lease Payments Required under Operating Lease (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>40803 - Disclosure - Commitments and Contingencies - Summary of Lease Costs and Cash Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureCommitmentsAndContingenciesSummaryOfLeaseCostsAndCashPaymentsDetails</Role>
      <ShortName>Commitments and Contingencies - Summary of Lease Costs and Cash Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>40901 - Disclosure - Employee Benefits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureEmployeeBenefitsDetails</Role>
      <ShortName>Employee Benefits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://rainthera.com/role/DisclosureEmployeeBenefits</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>41001 - Disclosure - Net Loss Per Share - Schedule of Computation of Basic and Diluted Net Loss per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureNetLossPerShareScheduleOfComputationOfBasicAndDilutedNetLossPerShareDetails</Role>
      <ShortName>Net Loss Per Share - Schedule of Computation of Basic and Diluted Net Loss per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="rain-20220630x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>41002 - Disclosure - Net Loss Per Share - Schedule of Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://rainthera.com/role/DisclosureNetLossPerShareScheduleOfPotentiallyDilutiveSecuritiesExcludedFromCalculationOfDilutedNetLossPerShareDetails</Role>
      <ShortName>Net Loss Per Share - Schedule of Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" original="rain-20220630x10q.htm">rain-20220630x10q.htm</File>
    <File>rain-20220630.xsd</File>
    <File>rain-20220630_cal.xml</File>
    <File>rain-20220630_def.xml</File>
    <File>rain-20220630_lab.xml</File>
    <File>rain-20220630_pre.xml</File>
    <File>rain-20220630xex10d2.htm</File>
    <File>rain-20220630xex10d3.htm</File>
    <File>rain-20220630xex10d4.htm</File>
    <File>rain-20220630xex31d1.htm</File>
    <File>rain-20220630xex31d2.htm</File>
    <File>rain-20220630xex32d1.htm</File>
  </InputFiles>
  <SupplementalFiles>
    <File>rain-20220630x10q007.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="511">http://fasb.org/us-gaap/2022</BaseTaxonomy>
    <BaseTaxonomy items="33">http://xbrl.sec.gov/dei/2022</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
