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Collaborative and Other Relationships - ADUHELM Collaboration Agreement (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Collaborative arrangements and non-collaborative arrangement transactions
Excess and obsolescence charges related to inventory	$ 44.4	$ 36.8
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Amounts receivable	10.3		$ 1.4
Amounts payable	$ 171.8		$ 118.4

X

- Definition

Accounts Payable, To Collaborator

+ References

No definition available.

+ Details

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Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Accounts Receivable, From Collaborator

+ References

No definition available.

+ Details

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Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 808
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479402/808-10-50-1

+ Details

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X

- Definition

Amount of loss from reductions in inventory due to subsequent measurement adjustments, including, but not limited to, physical deterioration, obsolescence, or changes in price levels.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 330
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483080/330-10-50-2

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- Details

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