<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.2.0.727</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>264</ContextCount>
  <ElementCount>311</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>88</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>8</UnitCount>
  <MyReports>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - Condensed Consolidated Statements of Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/CondensedConsolidatedStatementsOfIncomeUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1002000 - Statement - Condensed Consolidated Statements of Comprehensive Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/CondensedConsolidatedStatementsOfComprehensiveIncomeUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Comprehensive Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1002501 - Statement - Condensed Consolidated Statements of Comprehensive Income (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/CondensedConsolidatedStatementsOfComprehensiveIncomeUnauditedParenthetical</Role>
      <ShortName>Condensed Consolidated Statements of Comprehensive Income (Unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1003000 - Statement - Condensed Consolidated Balance Sheets (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/CondensedConsolidatedBalanceSheetsUnaudited</Role>
      <ShortName>Condensed Consolidated Balance Sheets (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1003501 - Statement - Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/CondensedConsolidatedBalanceSheetsUnauditedParenthetical</Role>
      <ShortName>Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1004000 - Statement - Condensed Consolidated Statements of Cash Flows (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/CondensedConsolidatedStatementsOfCashFlowsUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Cash Flows (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/SummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2104100 - Disclosure - Acquisitions Acquisitions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/AcquisitionsAcquisitions</Role>
      <ShortName>Acquisitions Acquisitions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2106100 - Disclosure - Reserves for Discounts and Allowances</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/ReservesForDiscountsAndAllowances</Role>
      <ShortName>Reserves for Discounts and Allowances</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2107100 - Disclosure - Inventory</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/Inventory</Role>
      <ShortName>Inventory</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - Intangible Assets and Goodwill</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/IntangibleAssetsAndGoodwill</Role>
      <ShortName>Intangible Assets and Goodwill</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2109100 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2110100 - Disclosure - Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FinancialInstruments</Role>
      <ShortName>Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2112100 - Disclosure - Derivative Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/DerivativeInstruments</Role>
      <ShortName>Derivative Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2114100 - Disclosure - Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/Equity</Role>
      <ShortName>Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2115100 - Disclosure - Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLoss</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2116100 - Disclosure - Earnings per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/EarningsPerShare</Role>
      <ShortName>Earnings per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2117100 - Disclosure - Share-based Payments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/ShareBasedPayments</Role>
      <ShortName>Share-based Payments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2118100 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2119100 - Disclosure - Other Consolidated Financial Statement Detail</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetail</Role>
      <ShortName>Other Consolidated Financial Statement Detail</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2120100 - Disclosure - Investments in Variable Interest Entities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/InvestmentsInVariableInterestEntities</Role>
      <ShortName>Investments in Variable Interest Entities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2121100 - Disclosure - Collaborative and Other Relationships</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/CollaborativeAndOtherRelationships</Role>
      <ShortName>Collaborative and Other Relationships</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2122100 - Disclosure - Litigation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/Litigation</Role>
      <ShortName>Litigation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2123100 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/CommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2124100 - Disclosure - Subsequent Events Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/SubsequentEventsSubsequentEvents</Role>
      <ShortName>Subsequent Events Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2201201 - Disclosure - Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/SummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>27</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2304301 - Disclosure - Acquisitions Acquisitions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/AcquisitionsAcquisitionsTables</Role>
      <ShortName>Acquisitions Acquisitions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/AcquisitionsAcquisitions</ParentRole>
      <Position>28</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2306301 - Disclosure - Reserves for Discounts and Allowances (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/ReservesForDiscountsAndAllowancesTables</Role>
      <ShortName>Reserves for Discounts and Allowances (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/ReservesForDiscountsAndAllowances</ParentRole>
      <Position>29</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2307301 - Disclosure - Inventory (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/InventoryTables</Role>
      <ShortName>Inventory (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/Inventory</ParentRole>
      <Position>30</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2308301 - Disclosure - Intangible Assets and Goodwill (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillTables</Role>
      <ShortName>Intangible Assets and Goodwill (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/IntangibleAssetsAndGoodwill</ParentRole>
      <Position>31</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2309301 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/FairValueMeasurements</ParentRole>
      <Position>32</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2310301 - Disclosure - Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FinancialInstrumentsTables</Role>
      <ShortName>Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/FinancialInstruments</ParentRole>
      <Position>33</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2312301 - Disclosure - Derivative Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/DerivativeInstrumentsTables</Role>
      <ShortName>Derivative Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/DerivativeInstruments</ParentRole>
      <Position>34</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2314301 - Disclosure - Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/EquityTables</Role>
      <ShortName>Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/Equity</ParentRole>
      <Position>35</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2315301 - Disclosure - Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossTables</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLoss</ParentRole>
      <Position>36</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2316301 - Disclosure - Earnings per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/EarningsPerShare</ParentRole>
      <Position>37</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2317301 - Disclosure - Share-Based Payments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/ShareBasedPaymentsTables</Role>
      <ShortName>Share-Based Payments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2318301 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/IncomeTaxes</ParentRole>
      <Position>39</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2319301 - Disclosure - Other Consolidated Financial Statement Detail (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailTables</Role>
      <ShortName>Other Consolidated Financial Statement Detail (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetail</ParentRole>
      <Position>40</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2401402 - Disclosure - Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesDetails</Role>
      <ShortName>Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesPolicies</ParentRole>
      <Position>41</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2404402 - Disclosure - Acquisitions  Acquisitions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/AcquisitionsAcquisitionsDetails</Role>
      <ShortName>Acquisitions Acquisitions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/AcquisitionsAcquisitionsTables</ParentRole>
      <Position>42</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2404403 - Disclosure - Acquisitions Acquisitions (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/AcquisitionsAcquisitionsDetails1</Role>
      <ShortName>Acquisitions Acquisitions (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/AcquisitionsAcquisitionsTables</ParentRole>
      <Position>43</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2404404 - Disclosure - Acquisitions Acquisitions (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/AcquisitionsAcquisitionsDetailsTextual</Role>
      <ShortName>Acquisitions Acquisitions (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/AcquisitionsAcquisitionsTables</ParentRole>
      <Position>44</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2406402 - Disclosure - Reserves for Discounts and Allowances (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/ReservesForDiscountsAndAllowancesDetails</Role>
      <ShortName>Reserves for Discounts and Allowances (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/ReservesForDiscountsAndAllowancesTables</ParentRole>
      <Position>45</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2406403 - Disclosure - Reserves for Discounts and Allowances (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/ReservesForDiscountsAndAllowancesDetails1</Role>
      <ShortName>Reserves for Discounts and Allowances (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/ReservesForDiscountsAndAllowancesTables</ParentRole>
      <Position>46</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2407402 - Disclosure - Inventory (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/InventoryDetails</Role>
      <ShortName>Inventory (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/InventoryTables</ParentRole>
      <Position>47</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2407403 - Disclosure - Inventory Inventory (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/InventoryInventoryDetailsTextual</Role>
      <ShortName>Inventory Inventory (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2408402 - Disclosure - Intangible Assets and Goodwill (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillDetails</Role>
      <ShortName>Intangible Assets and Goodwill (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillTables</ParentRole>
      <Position>49</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2408403 - Disclosure - Intangible Assets and Goodwill (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillDetails1</Role>
      <ShortName>Intangible Assets and Goodwill (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillTables</ParentRole>
      <Position>50</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2409402 - Disclosure - Fair Value Measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FairValueMeasurementsDetails</Role>
      <ShortName>Fair Value Measurements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2409403 - Disclosure - Fair Value Measurements (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FairValueMeasurementsDetails1</Role>
      <ShortName>Fair Value Measurements (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>52</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2409404 - Disclosure - Fair Value Measurements (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FairValueMeasurementsDetails2</Role>
      <ShortName>Fair Value Measurements (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>53</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2409405 - Disclosure - Fair Value Measurements (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FairValueMeasurementsDetailsTextual</Role>
      <ShortName>Fair Value Measurements (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>54</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2410402 - Disclosure - Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FinancialInstrumentsDetails</Role>
      <ShortName>Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/FinancialInstrumentsTables</ParentRole>
      <Position>55</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2410403 - Disclosure - Financial Instruments (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FinancialInstrumentsDetails1</Role>
      <ShortName>Financial Instruments (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/FinancialInstrumentsTables</ParentRole>
      <Position>56</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2410404 - Disclosure - Financial Instruments (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FinancialInstrumentsDetails2</Role>
      <ShortName>Financial Instruments (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/FinancialInstrumentsTables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2410405 - Disclosure - Financial Instruments (Details 3)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FinancialInstrumentsDetails3</Role>
      <ShortName>Financial Instruments (Details 3)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/FinancialInstrumentsTables</ParentRole>
      <Position>58</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2410406 - Disclosure - Financial Instruments (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FinancialInstrumentsDetailsTextual</Role>
      <ShortName>Financial Instruments (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/FinancialInstrumentsTables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2410407 - Disclosure - Financial Instruments Financial Instruments (Details Textual 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/FinancialInstrumentsFinancialInstrumentsDetailsTextual2</Role>
      <ShortName>Financial Instruments Financial Instruments (Details Textual 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2412402 - Disclosure - Derivative Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/DerivativeInstrumentsDetails</Role>
      <ShortName>Derivative Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/DerivativeInstrumentsTables</ParentRole>
      <Position>61</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2414402 - Disclosure - Equity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/EquityDetails</Role>
      <ShortName>Equity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/EquityTables</ParentRole>
      <Position>62</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2414403 - Disclosure - Equity (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/EquityDetailsTextual</Role>
      <ShortName>Equity (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/EquityTables</ParentRole>
      <Position>63</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2415402 - Disclosure - Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossTables</ParentRole>
      <Position>64</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2415403 - Disclosure - Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossDetails1</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossTables</ParentRole>
      <Position>65</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2416402 - Disclosure - Earnings per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/EarningsPerShareDetails</Role>
      <ShortName>Earnings per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/EarningsPerShareTables</ParentRole>
      <Position>66</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2417402 - Disclosure - Share-Based Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/ShareBasedPaymentsDetails</Role>
      <ShortName>Share-Based Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/ShareBasedPaymentsTables</ParentRole>
      <Position>67</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2417403 - Disclosure - Share-Based Payments (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/ShareBasedPaymentsDetails1</Role>
      <ShortName>Share-Based Payments (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/ShareBasedPaymentsTables</ParentRole>
      <Position>68</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2417404 - Disclosure - Share-Based Payments (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/ShareBasedPaymentsDetails2</Role>
      <ShortName>Share-Based Payments (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/ShareBasedPaymentsTables</ParentRole>
      <Position>69</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2417405 - Disclosure - Share-Based Payments (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/ShareBasedPaymentsDetailsTextual</Role>
      <ShortName>Share-Based Payments (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/ShareBasedPaymentsTables</ParentRole>
      <Position>70</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2418402 - Disclosure - Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/IncomeTaxesDetails</Role>
      <ShortName>Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/IncomeTaxesTables</ParentRole>
      <Position>71</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2418403 - Disclosure - Income Taxes (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/IncomeTaxesDetailsTextual</Role>
      <ShortName>Income Taxes (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/IncomeTaxesTables</ParentRole>
      <Position>72</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2419402 - Disclosure - Other Consolidated Financial Statement Detail (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailDetails</Role>
      <ShortName>Other Consolidated Financial Statement Detail (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailTables</ParentRole>
      <Position>73</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2419403 - Disclosure - Other Consolidated Financial Statement Detail Other Consolidated Financial Statement (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailOtherConsolidatedFinancialStatementDetailsTextual</Role>
      <ShortName>Other Consolidated Financial Statement Detail Other Consolidated Financial Statement (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2420401 - Disclosure - Investments in Variable Interest Entities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/InvestmentsInVariableInterestEntitiesDetails</Role>
      <ShortName>Investments in Variable Interest Entities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/InvestmentsInVariableInterestEntities</ParentRole>
      <Position>75</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2421401 - Disclosure - Collaborative and Other Relationships Collaborative and Other Relationships - Equity Method Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsEquityMethodInvestmentsDetails</Role>
      <ShortName>Collaborative and Other Relationships Collaborative and Other Relationships - Equity Method Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2422401 - Disclosure - Litigation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/LitigationDetails</Role>
      <ShortName>Litigation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/Litigation</ParentRole>
      <Position>77</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2423401 - Disclosure - Commitments and Contingences (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/CommitmentsAndContingencesDetails</Role>
      <ShortName>Commitments and Contingences (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/CommitmentsAndContingencies</ParentRole>
      <Position>78</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2423402 - Disclosure - Commitments and Contingencies Commitments and Contingencies (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/CommitmentsAndContingenciesCommitmentsAndContingenciesDetails1</Role>
      <ShortName>Commitments and Contingencies Commitments and Contingencies (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2424401 - Disclosure - Subsequent Events Subsequent Events (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.biogenidec.com/role/SubsequentEventsSubsequentEventsDetails</Role>
      <ShortName>Subsequent Events Subsequent Events (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.biogenidec.com/role/SubsequentEventsSubsequentEvents</ParentRole>
      <Position>80</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Info">In ''Condensed Consolidated Balance Sheets (Unaudited)'', column(s) 2, 4, 5, 6 are contained in other reports, so were removed by flow through suppression.</Log>
    <Log type="Info">In ''Condensed Consolidated Statements of Cash Flows (Unaudited)'', column(s) 1, 2 are contained in other reports, so were removed by flow through suppression.</Log>
  </Logs>
  <InputFiles>
    <File>biib-20150630.xml</File>
    <File>biib-20150630_cal.xml</File>
    <File>biib-20150630_def.xml</File>
    <File>biib-20150630_lab.xml</File>
    <File>biib-20150630_pre.xml</File>
    <File>biib-20150630.xsd</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies/>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>

