<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.22.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>222</ContextCount>
  <ElementCount>430</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>75</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>7</UnitCount>
  <MyReports>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001001 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0002002 - Document - Audit Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/AuditInformation</Role>
      <ShortName>Audit Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1001003 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1002004 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1003005 - Statement - Consolidated Statements of Operations and Comprehensive (Loss) Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/ConsolidatedStatementsofOperationsandComprehensiveLossIncome</Role>
      <ShortName>Consolidated Statements of Operations and Comprehensive (Loss) Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1004006 - Statement - Consolidated Statements of Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/ConsolidatedStatementsofEquity</Role>
      <ShortName>Consolidated Statements of Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1005007 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/ConsolidatedStatementsofCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2101101 - Disclosure - The Company and Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/TheCompanyandSummaryofSignificantAccountingPolicies</Role>
      <ShortName>The Company and Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2106102 - Disclosure - Fair Value Disclosures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/FairValueDisclosures</Role>
      <ShortName>Fair Value Disclosures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2109103 - Disclosure - Investments, Available-for-Sale</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/InvestmentsAvailableforSale</Role>
      <ShortName>Investments, Available-for-Sale</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2112104 - Disclosure - Balance Sheet Details</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/BalanceSheetDetails</Role>
      <ShortName>Balance Sheet Details</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2116105 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/Leases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2121106 - Disclosure - Equity Method Investment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/EquityMethodInvestment</Role>
      <ShortName>Equity Method Investment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2126107 - Disclosure - Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/StockholdersEquity</Role>
      <ShortName>Stockholders' Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2134108 - Disclosure - Collaborations and License Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/CollaborationsandLicenseAgreements</Role>
      <ShortName>Collaborations and License Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2138109 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2145110 - Disclosure - Legal Proceedings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/LegalProceedings</Role>
      <ShortName>Legal Proceedings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2146111 - Disclosure - Subsequent Event</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/SubsequentEvent</Role>
      <ShortName>Subsequent Event</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2202201 - Disclosure - The Company and Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesPolicies</Role>
      <ShortName>The Company and Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.arenapharm.com/role/TheCompanyandSummaryofSignificantAccountingPolicies</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2303301 - Disclosure - The Company and Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesTables</Role>
      <ShortName>The Company and Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.arenapharm.com/role/TheCompanyandSummaryofSignificantAccountingPolicies</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2307302 - Disclosure - Fair Value Disclosures (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/FairValueDisclosuresTables</Role>
      <ShortName>Fair Value Disclosures (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.arenapharm.com/role/FairValueDisclosures</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2310303 - Disclosure - Investments, Available-for-Sale (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/InvestmentsAvailableforSaleTables</Role>
      <ShortName>Investments, Available-for-Sale (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.arenapharm.com/role/InvestmentsAvailableforSale</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2313304 - Disclosure - Balance Sheet Details (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/BalanceSheetDetailsTables</Role>
      <ShortName>Balance Sheet Details (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.arenapharm.com/role/BalanceSheetDetails</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2317305 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/LeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.arenapharm.com/role/Leases</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2322306 - Disclosure - Equity Method Investment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/EquityMethodInvestmentTables</Role>
      <ShortName>Equity Method Investment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.arenapharm.com/role/EquityMethodInvestment</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2327307 - Disclosure - Stockholders' Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/StockholdersEquityTables</Role>
      <ShortName>Stockholders' Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.arenapharm.com/role/StockholdersEquity</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2335308 - Disclosure - Collaborations and License Agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/CollaborationsandLicenseAgreementsTables</Role>
      <ShortName>Collaborations and License Agreements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.arenapharm.com/role/CollaborationsandLicenseAgreements</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2339309 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.arenapharm.com/role/IncomeTaxes</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2404401 - Disclosure - The Company and Summary of Significant Accounting Policies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesAdditionalInformationDetail</Role>
      <ShortName>The Company and Summary of Significant Accounting Policies - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - The Company and Summary of Significant Accounting Policies - Reconciliation of Components of Cash, Cash Equivalents and Restricted Cash (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesReconciliationofComponentsofCashCashEquivalentsandRestrictedCashDetail</Role>
      <ShortName>The Company and Summary of Significant Accounting Policies - Reconciliation of Components of Cash, Cash Equivalents and Restricted Cash (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2408403 - Disclosure - Fair Value Disclosures - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/FairValueDisclosuresFinancialAssetsandLiabilitiesMeasuredatFairValueonRecurringBasisDetail</Role>
      <ShortName>Fair Value Disclosures - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2411404 - Disclosure - Investments, Available-for-Sale (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/InvestmentsAvailableforSaleDetail</Role>
      <ShortName>Investments, Available-for-Sale (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.arenapharm.com/role/InvestmentsAvailableforSaleTables</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2414405 - Disclosure - Balance Sheet Details - Land, Property and Equipment, Net (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/BalanceSheetDetailsLandPropertyandEquipmentNetDetail</Role>
      <ShortName>Balance Sheet Details - Land, Property and Equipment, Net (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2415406 - Disclosure - Balance Sheet Details - Accounts Payable and Other Accrued Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/BalanceSheetDetailsAccountsPayableandOtherAccruedLiabilitiesDetail</Role>
      <ShortName>Balance Sheet Details - Accounts Payable and Other Accrued Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2418407 - Disclosure - Leases - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/LeasesAdditionalInformationDetail</Role>
      <ShortName>Leases - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2419408 - Disclosure - Leases - Future Minimum Lease Payments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/LeasesFutureMinimumLeasePaymentsDetail</Role>
      <ShortName>Leases - Future Minimum Lease Payments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2420409 - Disclosure - Leases - Expected Minimum Rental Payments to be Received under Sublease (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/LeasesExpectedMinimumRentalPaymentstobeReceivedunderSubleaseDetail</Role>
      <ShortName>Leases - Expected Minimum Rental Payments to be Received under Sublease (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2423410 - Disclosure - Equity Method Investment - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/EquityMethodInvestmentNarrativeDetails</Role>
      <ShortName>Equity Method Investment - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2424411 - Disclosure - Equity Method Investment - Carrying Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/EquityMethodInvestmentCarryingValueDetails</Role>
      <ShortName>Equity Method Investment - Carrying Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2425412 - Disclosure - Equity Method Investment - Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/EquityMethodInvestmentActivityDetails</Role>
      <ShortName>Equity Method Investment - Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2428413 - Disclosure - Stockholders' Equity - Narrative (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/StockholdersEquityNarrativeDetail</Role>
      <ShortName>Stockholders' Equity - Narrative (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2429414 - Disclosure - Stockholders Equity - Schedule of Share Based Compensation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/StockholdersEquityScheduleofShareBasedCompensationDetails</Role>
      <ShortName>Stockholders Equity - Schedule of Share Based Compensation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2430415 - Disclosure - Stockholders' Equity - Schedule of Share Based Compensation Expense by Grant Type (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/StockholdersEquityScheduleofShareBasedCompensationExpensebyGrantTypeDetails</Role>
      <ShortName>Stockholders' Equity - Schedule of Share Based Compensation Expense by Grant Type (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2431416 - Disclosure - Stockholders' Equity - Weighted-Average Assumptions and Estimated Fair Value of Stock Options (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/StockholdersEquityWeightedAverageAssumptionsandEstimatedFairValueofStockOptionsDetail</Role>
      <ShortName>Stockholders' Equity - Weighted-Average Assumptions and Estimated Fair Value of Stock Options (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2432417 - Disclosure - Stockholders' Equity - Summary of Stock Option Activity (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/StockholdersEquitySummaryofStockOptionActivityDetail</Role>
      <ShortName>Stockholders' Equity - Summary of Stock Option Activity (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2433418 - Disclosure - Stockholders' Equity - Schedule of Restricted Stock and Performance Stock (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/StockholdersEquityScheduleofRestrictedStockandPerformanceStockDetail</Role>
      <ShortName>Stockholders' Equity - Schedule of Restricted Stock and Performance Stock (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2436419 - Disclosure - Collaborations and License Agreements - Summary of Revenue Disaggregated by Major Customers, Timing of Revenue Recognition and Revenue Classification (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/CollaborationsandLicenseAgreementsSummaryofRevenueDisaggregatedbyMajorCustomersTimingofRevenueRecognitionandRevenueClassificationDetail</Role>
      <ShortName>Collaborations and License Agreements - Summary of Revenue Disaggregated by Major Customers, Timing of Revenue Recognition and Revenue Classification (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2437420 - Disclosure - Collaborations and License Agreements - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/CollaborationsandLicenseAgreementsAdditionalInformationDetail</Role>
      <ShortName>Collaborations and License Agreements - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2440421 - Disclosure - Income Taxes - Summary of (Loss) Income Before Provision (Benefit) for Income Taxes by Region (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/IncomeTaxesSummaryofLossIncomeBeforeProvisionBenefitforIncomeTaxesbyRegionDetail</Role>
      <ShortName>Income Taxes - Summary of (Loss) Income Before Provision (Benefit) for Income Taxes by Region (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2441422 - Disclosure - Income Taxes - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/IncomeTaxesAdditionalInformationDetail</Role>
      <ShortName>Income Taxes - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2442423 - Disclosure - Income Taxes - Reconciliation of Provision (Benefit) for Income Taxes at Statutory Federal Rate to Provision (Benefit) for Income Taxes (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/IncomeTaxesReconciliationofProvisionBenefitforIncomeTaxesatStatutoryFederalRatetoProvisionBenefitforIncomeTaxesDetail</Role>
      <ShortName>Income Taxes - Reconciliation of Provision (Benefit) for Income Taxes at Statutory Federal Rate to Provision (Benefit) for Income Taxes (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2443424 - Disclosure - Income Taxes - Components of Net Deferred Tax Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/IncomeTaxesComponentsofNetDeferredTaxAssetsDetail</Role>
      <ShortName>Income Taxes - Components of Net Deferred Tax Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2444425 - Disclosure - Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/IncomeTaxesReconciliationofUnrecognizedTaxBenefitsDetail</Role>
      <ShortName>Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="arna-20211231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2447426 - Disclosure - Subsequent Event (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.arenapharm.com/role/SubsequentEventDetail</Role>
      <ShortName>Subsequent Event (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.arenapharm.com/role/SubsequentEvent</ParentRole>
      <Position>54</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="arna-20211231.htm">arna-20211231.htm</File>
    <File>arna-20211231.xsd</File>
    <File>arna-20211231_cal.xml</File>
    <File>arna-20211231_def.xml</File>
    <File>arna-20211231_lab.xml</File>
    <File>arna-20211231_pre.xml</File>
    <File>arna-20211231xexx1029.htm</File>
    <File>arna-20211231xexx1030.htm</File>
    <File>arna-20211231xexx105.htm</File>
    <File>arna-20211231xexx1053.htm</File>
    <File>arna-20211231xexx1059.htm</File>
    <File>arna-20211231xexx211.htm</File>
    <File>arna-20211231xexx231.htm</File>
    <File>arna-20211231xexx311.htm</File>
    <File>arna-20211231xexx312.htm</File>
    <File>arna-20211231xexx321.htm</File>
  </InputFiles>
  <SupplementalFiles>
    <File>arna-20211231_g1.gif</File>
    <File>arna-20211231_g2.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="720">http://fasb.org/us-gaap/2021-01-31</BaseTaxonomy>
    <BaseTaxonomy items="38">http://xbrl.sec.gov/dei/2021q4</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
