XML 91 R72.htm IDEA: XBRL DOCUMENT
v3.24.1.u1

Financial Instruments (Details 1) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Marketable equity securities |  |
Amortized Cost | $ 928.4 | $ 979.9
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (598.9) | (563.1)
Fair Value | 329.5 | 416.8
Equity Securities, Current |  |
Marketable equity securities |  |
Amortized Cost | 11.7 | 31.6
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (11.0) | (21.0)
Fair Value | 0.7 | 10.6
Equity Securities, Non-Current |  |
Marketable equity securities |  |
Amortized Cost | 916.7 | 948.3
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (587.9) | (542.1)
Fair Value | $ 328.8 | $ 406.2

X
- Definition Equity Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax + References No definition available. + Details
Name: | biib_EquitySecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- 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

X
- References No definition available. + Details
Name: | us-gaap_DebtSecuritiesTradingAndEquitySecuritiesFvNiCostAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(2)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483467/210-10-45-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1A -Publisher FASB -URI https://asc.fasb.org//1943274/2147482736/825-10-45-1A + Details
Name: | us-gaap_EquitySecuritiesFvNi
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1 + Details
Name: | us-gaap_EquitySecuritiesFvNiCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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