Collaborative and Other Relationships - Profit Sharing (Details) $ in Millions	3 Months Ended
Mar. 31, 2026 USD ($)
RITUXAN
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of co promotion operating profits first fifty million	30.00%
After Second GAZYVA Threshold Date	35.00%
Until Second GAZYVA Threshold Date	37.50%
LUNSUMIO
Collaborative arrangements and non-collaborative arrangement transactions
Limit of gross sale of GAZYVA to be achieved in preceding 12 months under option one	$ 500.0
Sales trigger gross sales threshold	$ 350.0
After Second GAZYVA Threshold Date	35.00%
GAZYVA
Collaborative arrangements and non-collaborative arrangement transactions
After Second GAZYVA Threshold Date	35.00%

Percentage of co promotion operating profits first fifty million.

No definition available.

Name:	biib_PercentageOfCoPromotionOperatingProfitsFirstFiftyMillion
Namespace Prefix:	biib_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of co promotion operating profits greater than first fifty million option two sub option three.

+ References

No definition available.

+ Details

Name:	biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionThree
Namespace Prefix:	biib_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of co promotion operating profits greater than first fifty million option two sub option two.

+ References

No definition available.

+ Details

Name:	biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionTwo
Namespace Prefix:	biib_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Sales trigger gross sales threshold.

+ References

No definition available.

+ Details

Name:	biib_SalesTriggerGrossSalesThreshold
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Threshold of gross sales of to be achieved in preceding twelve consecutive months under option one.

+ References

No definition available.

+ Details

Name:	biib_ThresholdOfGrossSalesOfToBeAchievedInPrecedingTwelveConsecutiveMonthsUnderOptionOne
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_ProductOrServiceAxis=biib_RituxanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=biib_LUNSUMIOMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=biib_GAZYVAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: