XML 89 R58.htm IDEA: XBRL DOCUMENT
v3.21.2

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

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
- 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

X
- 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

X
- 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

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

X
- 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

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). + 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

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 No definition available. + 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_CorporateDebtSecuritiesCurrentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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

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

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: |