Financial Instruments (Details 1) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Marketable debt securities
Amortized Cost	$ 2,381.5	$ 2,049.1
Gross unrealized gains	1.1	2.3
Gross unrealized losses	(1.0)	(0.4)
Fair value	2,381.6	2,051.0
Marketable equity securities
Amortized Cost	1,168.9	1,239.5
Gross Unrealized Gains	257.9	749.7
Gross Unrealized Losses	(260.6)	(14.9)
Fair Value	1,166.2	1,974.3
Corporate debt securities Current
Marketable debt securities
Amortized Cost	1,135.8	897.8
Gross unrealized gains	0.3	0.4
Gross unrealized losses	(0.4)	(0.2)
Fair value	1,135.7	898.0
Corporate debt securities Non-current
Marketable debt securities
Amortized Cost	350.7	402.5
Gross unrealized gains	0.5	1.1
Gross unrealized losses	(0.1)	(0.1)
Fair value	351.1	403.5
Government securities Current
Marketable debt securities
Amortized Cost	277.4	380.6
Gross unrealized gains	0.1	0.1
Gross unrealized losses	0.0	0.0
Fair value	277.5	380.7
Government securities Non-current
Marketable debt securities
Amortized Cost	435.5	245.9
Gross unrealized gains	0.1	0.5
Gross unrealized losses	(0.2)	0.0
Fair value	435.4	246.4
Mortgage and other asset backed securities Current
Marketable debt securities
Amortized Cost	0.1	0.2
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Fair value	0.1	0.2
Mortgage and other asset backed securities Non-current
Marketable debt securities
Amortized Cost	182.0	122.1
Gross unrealized gains	0.1	0.2
Gross unrealized losses	(0.3)	(0.1)
Fair value	181.8	122.2
Equity Securities, Current
Marketable equity securities
Amortized Cost	35.5	70.6
Gross Unrealized Gains	19.5	15.9
Gross Unrealized Losses	0.0	0.0
Fair Value	55.0	86.5
Equity Securities, Non-Current
Marketable equity securities
Amortized Cost	1,133.4	1,168.9
Gross Unrealized Gains	238.4	733.8
Gross Unrealized Losses	(260.6)	(14.9)
Fair Value	$ 1,111.2	$ 1,887.8

Equity Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax

No definition available.

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

- Definition

Equity Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, Before Tax

+ References

No definition available.

+ Details

Name:	biib_EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

Name:	us-gaap_EquitySecuritiesFvNi
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

+ References

No definition available.

+ Details

Name:	us-gaap_EquitySecuritiesFvNiCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=biib_CorporateDebtSecuritiesCurrentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=biib_CorporateDebtSecuritiesNonCurrentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=biib_GovernmentSecuritiesCurrentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=biib_GovernmentSecuritiesNonCurrentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=biib_EquitySecuritiesCurrentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=biib_EquitySecuritiesNonCurrentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: