XML 124 R70.htm IDEA: XBRL DOCUMENT
v3.20.4

Fair Value Measurements - Narrative (Details) - USD ($) | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Feb. 28, 2015 | Dec. 31, 2010 | Dec. 31, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Jun. 07, 2019 | Mar. 31, 2012
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |  |  |
Technological and regulatory success, probability |  |  |  |  |  | 39.90% |  |  |  |
Reduction in contingent consideration |  |  |  |  |  | $ 51,000,000.0 | $ 61,200,000 |  |  |
Maximum contingent consideration in the form of development and approval milestones |  |  | $ 400,000,000.0 |  |  | 400,000,000.0 |  |  |  |
In-process research and development |  |  |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |  |  |
Impairment |  |  |  | $ 215,900,000 | $ 189,300,000 |  | 215,900,000 | $ 189,300,000 |  |
Intangible asset, fair value |  |  | 0 | $ 0 |  | 0 |  |  |  |
Convergence Pharmaceuticals |  |  |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |  |  |
Additions | $ 274,500,000 |  |  |  |  |  |  |  |  |
Fair Value |  |  | 259,800,000 |  |  | 259,800,000 | 244,600,000 |  |  |
Maximum contingent consideration in the form of development and approval milestones |  |  | 400,000,000.0 |  |  | 400,000,000.0 |  |  |  |
Intangible assets | $ 424,600,000 |  |  |  |  |  |  |  |  |
Biogen Idec International Neuroscience GmbH |  |  |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |  |  |
Additions |  | $ 81,200,000 |  |  |  | 0 |  |  |  |
Fair Value |  |  | 101,500,000 |  |  | 101,500,000 |  |  |  |
Biogen Idec International Neuroscience GmbH | In-process research and development |  |  |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |  |  |
Acquired intangible asset |  |  |  |  |  | 110,900,000 |  |  |  |
Stromedix, Inc. |  |  |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  | $ 219,200,000
Nightstar |  |  |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  | $ 585,000,000.0 |
Nightstar | BIIB054 |  |  |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |  |  |
Impairment |  |  | $ 75,400,000 |  |  | $ 75,400,000 |  |  |  |
Discount rate | Convergence Pharmaceuticals |  |  |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |  |  |
Fair value measurement and measurement inputs, recurring and nonrecurring |  |  | 0.60% |  |  | 0.60% |  |  |  |
Other long-term liabilities |  |  |  |  |  |  |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |  |  |  |  |  |  |
Contingent consideration obligations |  |  | $ 110,300,000 |  |  | $ 110,300,000 | $ 197,700,000 |  |  |

X
- Definition Fair Value Measurement Inputs + References No definition available. + Details
Name: | biib_FairValueMeasurementInputs
Namespace Prefix: | biib_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Technological and Regulatory Success, Probability + References No definition available. + Details
Name: | biib_TechnologicalAndRegulatorySuccessProbability
Namespace Prefix: | biib_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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_BusinessAcquisitionContingentConsiderationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of increase (decrease) in the value of right to a contingent consideration asset. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 4 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e7008-128479 + Details
Name: | us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationAsset1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479 + Details
Name: | us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of liability recognized arising from contingent consideration in a business combination. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 35 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116859824&loc=d3e6819-128478 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 25 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476 + Details
Name: | us-gaap_BusinessCombinationContingentConsiderationLiability
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of liability recognized arising from contingent consideration in a business combination, expected to be settled beyond one year or the normal operating cycle, if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 25 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 35 -Paragraph 1 -Subparagraph b -URI http://asc.fasb.org/extlink&oid=116859824&loc=d3e6819-128478 + Details
Name: | us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of assets, excluding financial assets and goodwill, that lack physical substance, having a projected indefinite period of benefit, acquired at the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121647850&loc=d3e4845-128472 + Details
Name: | us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Fair value portion of assets, excluding financial assets, that lack physical substance, having a limited useful life. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
Name: | us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275 + Details
Name: | us-gaap_FinitelivedIntangibleAssetsAcquired1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of impairment loss resulting from write-down of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit to fair value. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16373-109275 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 + Details
Name: | us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_InProcessResearchAndDevelopmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=biib_ConvergencePharmaceuticalsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=biib_BiogenIdecInternationalNeuroscienceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=biib_StromedixMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=biib_NightstarMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | biib_ResearchanddevelopmentassetAxis=biib_BIIB054Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContingentConsiderationByTypeAxis=us-gaap_OtherNoncurrentLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |