Fair Value Measurement Inputs

No definition available.

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- Definition

Technological and Regulatory Success, Probability

+ References

No definition available.

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- 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.

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- 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)
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e7008-128479

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- 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
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-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

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- 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
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- 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
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- 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
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- 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
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-SubTopic 10
-Section 50
-Paragraph 2
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- 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

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- 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
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- 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
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Fair Value Measurements - Narrative (Details) - USD ($)	1 Months Ended		3 Months Ended			12 Months Ended
Fair Value Measurements - Narrative (Details) - USD ($)	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