XML 95 R63.htm IDEA: XBRL DOCUMENT
v3.21.2

Derivative Instruments (Details) $ in Millions, ₩ in Billions | 1 Months Ended | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Nov. 30, 2018 USD ($) | Nov. 30, 2018 KRW (₩) | Jun. 30, 2021 USD ($) | Jun. 30, 2020 USD ($) | Dec. 31, 2019 USD ($) | Jun. 30, 2021 USD ($) | Jun. 30, 2020 USD ($) | Sep. 30, 2020 | Dec. 31, 2020 USD ($)
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Gain/Loss on fair value of foreign currency forward contracts |  |  | $ 53.7 |  |  | $ 53.7 |  |  | $ (212.5)
Gain on interest rate swap |  |  |  |  |  | 0.0 | $ 3.3 |  |
Net gains (losses) of other income (expense) related to foreign currency forward contracts |  |  | (4.8) | $ (8.3) |  | (12.6) | 5.9 |  |
Unrealized Gross Losses on Foreign Currency Derivatives, Net, before Tax |  |  | 67.0 |  |  | 67.0 |  |  |
Unrealized Gross Gains on Foreign Currency Derivatives, Net, before Tax |  |  | 13.3 |  |  | 13.3 |  |  |
Gains (losses) on net investment hedges |  |  | (2.3) | (6.2) |  | 20.1 | 16.8 |  |
Ionis, Sangamo, Denali and Sage |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Net losses recognized on the decrease in fair value of equity securities |  |  | 105.8 |  |  | $ 284.8 |  |  |
Minimum |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Range of durations of foreign currency forward contracts |  |  |  |  |  | 1 month |  | 1 month |
Maximum |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Range of durations of foreign currency forward contracts |  |  |  |  |  | 18 months |  | 24 months |
Not designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  |  | 1,205.1 |  |  | $ 1,205.1 |  |  | 1,158.0
Other current assets | Designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative asset, fair value |  |  | 9.7 |  |  | 9.7 |  |  | 0.0
Other current assets | Not designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative asset, fair value |  |  | 3.3 |  |  | 3.3 |  |  | 20.5
Other noncurrent assets | Designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative asset, fair value |  |  | 7.4 |  |  | 7.4 |  |  | 0.0
Accrued expenses and other | Designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative liability, fair value |  |  | 55.2 |  |  | 55.2 |  |  | 157.1
Accrued expenses and other | Not designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative liability, fair value |  |  | 17.2 |  |  | 17.2 |  |  | 4.7
Foreign exchange contract | Designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  |  | 2,623.5 |  |  | 2,623.5 |  |  | 3,229.7
Interest rate swap | Designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  |  | 675.0 |  |  | 675.0 |  |  |
Cash Flow Hedging [Member] | Foreign exchange contract | Other long-term liabilities |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative liability, fair value |  |  | 0.0 |  |  | $ 0.0 |  |  | 35.7
Net Investment Hedging |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Remaining duration of Net Investment Hedges |  |  |  |  |  | 4 months |  |  |
Unrealized gain (loss) on net investment hedges in AOCI |  |  |  |  |  | $ 0.1 |  |  | 21.2
Derivative qualifying as net investment hedge, excluded component |  |  | 0.1 |  |  | 0.1 |  |  | 0.2
Gains (losses) on net investment hedges |  |  | (2.5) | (8.8) |  | 21.3 | 15.4 |  |
Gains (losses) on net investment hedge, excluded component |  |  | 0.3 | 3.5 |  | (1.1) | 3.2 |  |
Net Investment Hedging | Foreign exchange contract | Other current assets |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative asset, fair value |  |  | 0.4 |  |  | 0.4 |  |  | 0.0
Net Investment Hedging | Foreign exchange contract | Accrued expenses and other |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative liability, fair value |  |  | 0.0 |  |  | 0.0 |  |  | 19.7
Interest Expense | Interest rate swap |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Gain on interest rate swap |  |  |  |  |  | 3.3 |  |  |
Revenue | Foreign exchange contract |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax |  |  | (30.7) | 23.7 |  | (53.8) | 50.7 |  |
Revenue | Cash flows, revenue | Foreign exchange contract |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Gain on interest rate swap |  |  | (0.8) | (1.6) |  | (3.8) | 7.7 |  |
Operating expense | Foreign exchange contract |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax |  |  | 0.4 | 0.0 |  | 0.0 | (0.1) |  |
Operating expense | Cash flows, operating expenses | Foreign exchange contract |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Gain on interest rate swap |  |  | (0.3) | (0.2) |  | (0.4) | (1.1) |  |
Other income (expense) | Net Investment Hedging |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Gain (loss) recognized in net income, excluded component |  |  | 0.0 | $ 0.8 |  | 0.1 | $ 1.7 |  |
Euro | Foreign exchange contract | Designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  |  | 2,269.2 |  |  | 2,269.2 |  |  | 2,979.1
British pound | Foreign exchange contract | Designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  |  | 62.5 |  |  | 62.5 |  |  | 0.0
Swiss franc | Foreign exchange contract | Designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  |  | 128.6 |  |  | 128.6 |  |  | 250.6
Japan, Yen | Foreign exchange contract | Designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  |  | 53.3 |  |  | 53.3 |  |  | 0.0
Canadian dollar | Foreign exchange contract | Designated as hedging instrument |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  |  | $ 109.9 |  |  | $ 109.9 |  |  | $ 0.0
2.900% Senior Notes due 2020 |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Senior notes interest rate |  |  | 2.90% |  |  | 2.90% |  |  |
Samsung Bioepis |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Percentage of stake in entity |  |  |  | 5.00% |  |  | 5.00% |  |
Equity Method Investment, Ownership Percentage |  |  | 49.90% |  |  | 49.90% |  |  |
Payments to acquire additional investment in equity method investment | $ 676.6 | ₩ 759.5 |  |  | $ 676.6 |  |  |  |
Short-term derivative |  |  |  |  |  |  |  |  |
Derivatives [Line Items] |  |  |  |  |  |  |  |  |
Gain/Loss on fair value of foreign currency forward contracts |  |  | $ 58.2 |  |  | $ 58.2 |  |  |
Range of durations of foreign currency forward contracts |  |  |  |  |  | 12 months |  |  |

X
- Definition Derivatives used in net investment hedge, net of tax, excluded component increase (decrease) + References No definition available. + Details
Name: | biib_Derivativesusedinnetinvestmenthedgenetoftaxexcludedcomponentincreasedecrease
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Equity Method Investment, Ownership Percentage Before Additional Purchase Transaction + References No definition available. + Details
Name: | biib_EquityMethodInvestmentOwnershipPercentageBeforeAdditionalPurchaseTransaction
Namespace Prefix: | biib_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of net unrealized gain (loss) related to the change in fair value of foreign currency exchange rate derivatives designated as cash flow hedging instruments. Recorded in accumulated other comprehensive income to the extent that the cash flow hedge is determined to be effective. + References No definition available. + Details
Name: | biib_UnrealizedGainLossesOnForeignCurrencyDerivativesNetBeforeTax
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Unrealized Gross Gains on Foreign Currency Derivatives, Net, before Tax + References No definition available. + Details
Name: | biib_UnrealizedGrossGainsOnForeignCurrencyDerivativesNetBeforeTax
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Unrealized Gross Losses on Foreign Currency Derivatives, Net, before Tax + References No definition available. + Details
Name: | biib_UnrealizedGrossLossesOnForeignCurrencyDerivativesNetBeforeTax
Namespace Prefix: | biib_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount, after tax, of accumulated gain (loss) from increase (decrease) in value of excluded component of derivative hedge. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121549185&loc=d3e80784-113994 + Details
Name: | us-gaap_AociDerivativeQualifyingAsHedgeExcludedComponentAfterTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Contractual interest rate for funds borrowed, under the debt agreement. + 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(a)(1)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4D -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=121570589&loc=SL51823488-111719 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624163-113959 + Details
Name: | us-gaap_DerivativeFairValueOfDerivativeAsset
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4D -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=121570589&loc=SL51823488-111719 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624163-113959 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313 + Details
Name: | us-gaap_DerivativeFairValueOfDerivativeLiability
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 25 -Section 50 -Paragraph 1 -Subparagraph (a)(1),(2) -URI http://asc.fasb.org/extlink&oid=121577467&loc=d3e76258-113986 + Details
Name: | us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Nominal or face amount used to calculate payment on derivative. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959 + Details
Name: | us-gaap_DerivativeNotionalAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

X
- Definition Period remaining until the derivative contract matures, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_DerivativeRemainingMaturity1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_DerivativeTermOfContract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DerivativesFairValueLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 + Details
Name: | us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of unrealized loss on 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 321 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=120269885&loc=SL75117546-209714 + Details
Name: | us-gaap_EquitySecuritiesFvNiUnrealizedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5618551-113959 + Details
Name: | us-gaap_GainLossOnDerivativeInstrumentsNetPretax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of gain (loss) recognized in earnings in the period from the increase (decrease) in fair value of foreign currency derivatives not designated as hedging instruments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5618551-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959 + Details
Name: | us-gaap_GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount, before tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e689-108580 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e637-108580 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959 + Details
Name: | us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3213-108585 + Details
Name: | us-gaap_PaymentsToAcquireEquityMethodInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of increase (decrease) in the cumulative translation adjustment from gain (loss), after tax, on foreign currency derivatives, that are designated as, and are effective as, economic hedges of a net investment in a foreign entity. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 30 -Section 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=118261656&loc=d3e32211-110900 + Details
Name: | us-gaap_TranslationAdjustmentForNetInvestmentHedgeNetOfTaxPeriodIncreaseDecrease
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of net unrealized gain (loss) related to the change in fair value of foreign currency exchange rate derivatives designated as cash flow hedging instruments. Recorded in accumulated other comprehensive income to the extent that the cash flow hedge is determined to be effective. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959 + Details
Name: | us-gaap_UnrealizedGainLossOnForeignCurrencyDerivativesNetBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=biib_IonisSangamoDenaliAndSageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_OtherCurrentAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_AccruedLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_NetInvestmentHedgingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=us-gaap_InterestExpenseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=us-gaap_SalesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=biib_CashflowsrevenueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=us-gaap_OperatingExpenseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=biib_CashflowsoperatingexpensesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=us-gaap_OtherNonoperatingIncomeExpenseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CurrencyAxis=currency_EUR
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CurrencyAxis=currency_GBP
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CurrencyAxis=currency_CHF
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CurrencyAxis=currency_JPY
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CurrencyAxis=currency_CAD
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=biib_SamsungBiosimilarAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | biib_DerivativeMaturityAxis=biib_ShorttermderivativeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |