Long Term Debt, Exchanged, Amount

No definition available.

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Carrying value as of the balance sheet date of notes payable (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermNotesPayable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Sum of the carrying values as of the balance sheet date of the portions of long-term notes payable due within one year or the operating cycle if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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- Details

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- Details

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- Details

Name:	us-gaap_DebtInstrumentAxis=biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember
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Balance Type:
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- Details

Name:	us-gaap_DebtInstrumentAxis=biib_A315SeniorNotesDueMay12050Member
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- Details

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Indebtedness - Summary of Indebtedness (Details) - USD ($) $ in Millions	1 Months Ended
	Feb. 28, 2021	Dec. 31, 2021	Dec. 31, 2020	Apr. 30, 2020
Non-current portion:
Non-current portion of notes payable		$ 7,567.3
Senior Notes
Current portion:
Current portion of notes payable		999.1	$ 0.0
Non-current portion:
Non-current portion of notes payable		6,274.0	7,426.2
3.625% Senior Notes due September 15, 2022 \| Senior Notes
Current portion:
Current portion of notes payable		999.1	0.0
Non-current portion:
Non-current portion of notes payable		$ 0.0	997.9
Interest rate on senior notes		3.625%
4.050% Senior Notes due September 15, 2025 \| Senior Notes
Non-current portion:
Non-current portion of notes payable		$ 1,742.9	1,741.2
Interest rate on senior notes		4.05%
2.250% Senior Notes due May 1, 2030 \| Senior Notes
Non-current portion:
Non-current portion of notes payable		$ 1,492.0	1,491.1
Interest rate on senior notes		2.25%		2.25%
5.200% Senior Notes due September 15, 2045 \| Senior Notes
Non-current portion:
Non-current portion of notes payable		$ 1,099.9	1,723.4
Interest rate on senior notes		5.20%
Long term debt, exchanged amount	$ 624.6
3.150% Senior Notes due May 1, 2050 \| Senior Notes
Non-current portion:
Non-current portion of notes payable		$ 1,473.2	1,472.6
Interest rate on senior notes		3.15%		3.15%
3.250% Senior Notes due February 15, 2051 \| Senior Notes
Non-current portion:
Non-current portion of notes payable		$ 466.0	$ 0.0
Interest rate on senior notes		3.25%
Long term debt, exchanged amount	$ 700.7