XML 74 R42.htm IDEA: XBRL DOCUMENT
v3.19.1

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2019 USD ($) segment | Jan. 01, 2019 USD ($) | Dec. 31, 2018 USD ($)
Operating lease assets | $ 447.8 |  | $ 0.0
Operating lease liabilities | $ 508.6 |  |
Number of reportable segments | segment | 1 |  |
Interest in subsidiary (less than given percentage) | 100.00% |  |
Accounting Standards Update 2016-02 |  |  |
Operating lease assets |  | $ 463.0 |
Operating lease liabilities |  | $ 526.0 |

X
- Definition Interest in subsidiary. + References No definition available. + Details
Name: | biib_InterestInSubsidiary
Namespace Prefix: | biib_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Present value of lessee's discounted obligation for lease payments from operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977 + Details
Name: | us-gaap_OperatingLeaseLiability
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of lessee's right to use underlying asset under operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977 + Details
Name: | us-gaap_OperatingLeaseRightOfUseAsset
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_ProspectiveAdoptionOfNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201602Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |