Fair values as of the balance sheet date of all assets of deferred compensation plans.

No definition available.

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

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

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

- Details

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

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

+ Details

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

- Definition

Amount of liability recognized arising from contingent consideration in a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 25

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 35

-Paragraph 1

-Subparagraph b

-URI http://asc.fasb.org/extlink&oid=6911338&loc=d3e6819-128478

+ Details

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

- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash

-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

-URI http://asc.fasb.org/extlink&oid=6507016

+ Details

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

- Definition

Fair value portion of asset contracts related to the exchange of different currencies, including, but not limited to, foreign currency options, forward contracts, and swaps.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of liability contracts related to the exchange of different currencies, including, but not limited to, foreign currency options, forward (delivery or nondelivery) contracts, and swaps entered into.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph a, f, g

-Article 7

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560

+ Details

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

- Definition

Fair value of financial and nonfinancial obligations.

+ References

+ Details

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

- Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Assets:
Marketable debt securities	$ 2,051.2us-gaap_AvailableForSaleSecurities	$ 2,111.1us-gaap_AvailableForSaleSecurities
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	752.5	716.3
Derivative contracts	168.2	72.7
Plan assets for deferred compensation	44.1	36.9
Total	3,034.3	2,958.4
Liabilities:
Derivative contracts	12.6	5.4
Contingent consideration obligations	461.8	215.5	275.1	280.9
Total	474.4	220.9
Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	991.0	1,063.0
Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	850.6	849.8
Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	209.6	198.3
Fair Value, Measurements, Recurring \| Marketable equity securities
Assets:
Investments	6.2	6.9
Fair Value, Measurements, Recurring \| Venture capital investments
Assets:
Investments	12.1	14.5	23.6	21.9
Quoted Prices in Active Markets, Level 1 \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0	0
Derivative contracts	0	0
Plan assets for deferred compensation	0	0
Total	6.2	6.9
Liabilities:
Derivative contracts	0	0
Contingent consideration obligations	0	0
Total	0	0
Quoted Prices in Active Markets, Level 1 \| Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	0	0
Quoted Prices in Active Markets, Level 1 \| Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	0	0
Quoted Prices in Active Markets, Level 1 \| Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	0	0
Quoted Prices in Active Markets, Level 1 \| Fair Value, Measurements, Recurring \| Marketable equity securities
Assets:
Investments	6.2	6.9
Quoted Prices in Active Markets, Level 1 \| Fair Value, Measurements, Recurring \| Venture capital investments
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	752.5	716.3
Derivative contracts	168.2	72.7
Plan assets for deferred compensation	44.1	36.9
Total	3,016.0	2,937.0
Liabilities:
Derivative contracts	12.6	5.4
Contingent consideration obligations	0	0
Total	12.6	5.4
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	991.0	1,063.0
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	850.6	849.8
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	209.6	198.3
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Marketable equity securities
Assets:
Investments	0	0
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Venture capital investments
Assets:
Investments	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0	0
Derivative contracts	0	0
Plan assets for deferred compensation	0	0
Total	12.1	14.5
Liabilities:
Derivative contracts	0	0
Contingent consideration obligations	461.8	215.5
Total	461.8	215.5
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Marketable debt securities	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Government securities
Assets:
Marketable debt securities	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Mortgage and other asset backed securities
Assets:
Marketable debt securities	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Marketable equity securities
Assets:
Investments	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Venture capital investments
Assets:
Investments	$ 12.1	$ 14.5