XML 91 R60.htm IDEA: XBRL DOCUMENT
v3.21.1

Financial Instruments Financial Instruments (Details Textual 2) - USD ($) $ / shares in Units, shares in Millions, number in Millions, $ in Millions | 1 Months Ended | 3 Months Ended |
--- | --- | --- | ---
Nov. 30, 2020 | Apr. 30, 2020 | Feb. 29, 2020 | Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Business Acquisition [Line Items] |  |  |  |  |  |
Proceeds from sales of strategic investments |  |  |  | $ 91.2 | $ 0.5 |
Sage Therapeutics Inc. |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Global Licensing Collaboration Agreement, Amount, Shares Purchased | $ 650.0 |  |  |  |  |
Global Licensing Collaboration Agreement, Shares, Purchased | 6.2 |  |  |  |  |
Global Licensing Collaboration Agreement, Purchase Price Per Share | $ 104.14 |  |  |  |  |
Dividend yield percentage |  |  |  | 0.00% |  |
Sangamo Therapeutics, Inc. Agreement |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Global Licensing Collaboration Agreement, Amount, Shares Purchased |  |  | $ 225.0 |  |  |
Global Licensing Collaboration Agreement, Shares, Purchased |  |  | 24.0 |  |  |
Strategic Investments |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Strategic investment portfolio |  |  |  | $ 1,493.1 |  |
Other noncurrent assets | Strategic Investments |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Strategic investment portfolio |  |  |  |  |  | $ 2,024.6
Denali Therapeutics |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Investment in common stock, shares purchased |  |  |  | 13.0 |  |
Dividend yield percentage |  |  |  | 0.00% |  |
Sangamo |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Dividend yield percentage |  | 0.00% |  |  |  |

X
- Definition Dividend Yield Percentage + References No definition available. + Details
Name: | biib_DividendYieldPercentage
Namespace Prefix: | biib_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Global Licensing Collaboration Agreement, Amount, Shares Purchased + References No definition available. + Details
Name: | biib_GlobalLicensingCollaborationAgreementAmountSharesPurchased
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Global Licensing Collaboration Agreement, Purchase Price Per Share + References No definition available. + Details
Name: | biib_GlobalLicensingCollaborationAgreementPurchasePricePerShare
Namespace Prefix: | biib_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- Definition Global Licensing Collaboration Agreement, Shares, Purchased + References No definition available. + Details
Name: | biib_GlobalLicensingCollaborationAgreementSharesPurchased
Namespace Prefix: | biib_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Investment in common stock, shares purchased + References No definition available. + Details
Name: | biib_Investmentincommonstocksharespurchased
Namespace Prefix: | biib_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Strategic investment portfolio. + References No definition available. + Details
Name: | biib_StrategicInvestmentPortfolio
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_BusinessAcquisitionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The cash inflow associated with the sale of equity securities classified as available-for-sale securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26853-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3179-108585 + Details
Name: | us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesEquity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_TypeOfArrangementAxis=biib_SageTherapeuticsIncMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_TypeOfArrangementAxis=biib_SangamoTherapeuticsInc.AgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=biib_StrategicInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=biib_DenaliTherapeuticsIncMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=biib_SangamoCommonStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |