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Summary of Significant Accounting Policies (Details)	3 Months Ended
	Mar. 31, 2021 product segment
Accounting Policies [Abstract]
Number of potential products, exclusive rights | product	2
Number of reportable segments | segment	1
Interest in subsidiary (less than given percentage)	100.00%

X

- Definition

Interest in subsidiary.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

X

- Definition

Number of Ophthalmolgy Biosimilar Products, Exclusive Rights

+ References

No definition available.

+ Details

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Namespace Prefix:	biib_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8672-108599

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