Litigation	6 Months Ended
Jun. 30, 2021
Loss Contingency, Information about Litigation Matters [Abstract]
Litigation	We are currently involved in various claims and legal proceedings, including the matters described below. For information as to our accounting policies relating to claims and legal proceedings, including use of estimates and contingencies, please read Note 1, Summary of Significant Accounting Policies, to our consolidated financial statements included in our 2020 Form 10-K. With respect to some loss contingencies, an estimate of the possible loss or range of loss cannot be made until management has further information, including, for example, (i) which claims, if any, will survive dispositive motion practice; (ii) information to be obtained through discovery; (iii) information as to the parties' damages claims and supporting evidence; (iv) the parties’ legal theories; and (v) the parties' settlement positions. The claims and legal proceedings in which we are involved also include challenges to the scope, validity or enforceability of the patents relating to our products, pipeline or processes and challenges to the scope, validity or enforceability of the patents held by others. These include claims by third parties that we infringe their patents. An adverse outcome in any of these proceedings could result in one or more of the following and have a material impact on our business or consolidated results of operations and financial position: (i) loss of patent protection; (ii) inability to continue to engage in certain activities; and (iii) payment of significant damages, royalties, penalties and/or license fees to third parties. Loss Contingencies ADUHELM Securities Litigation We and certain current and former officers are named as defendants in an action filed by a shareholder on November 13, 2020, in the U.S. District Court for the Central District of California and transferred to the U.S. District Court for the District of Massachusetts in March 2021. The action alleges violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and seeks a declaration of the action as a class action and an award of damages, interest and attorneys' fees. An estimate of the possible loss or range of loss cannot be made at this time. No trial date has been set. We have filed a motion to dismiss the action, which is pending. IMRALDI Patent Litigation In September 2018 Fresenius Kabi Deutschland GmbH (Fresenius Kabi) commenced proceedings for damages and injunctive relief against Biogen France SAS in the Tribunal de Grande Instance de Paris, alleging that IMRALDI, the adalimumab biosimilar product of Samsung Bioepis UK Limited that Biogen has commercialized in Europe, infringes the French counterpart of European Patent No. 3 148 510 (the '510 Patent), which was issued in June 2018 and expires in May 2035. No hearing has been scheduled. In October 2018 Fresenius Kabi commenced preliminary injunction proceedings against Biogen (Denmark) Manufacturing ApS and Biogen Denmark A/S in Denmark's Maritime and Commercial High Court alleging infringement of Danish Utility Models. The request for preliminary injunction was denied in June 2019 and the decision was affirmed on appeal in February 2021. In July 2020 the Danish Patent Board of Appeal revoked the Danish Utility Models that were the subject of Fresenius Kabi’s October 2018 request for a preliminary injunction and Fresenius Kabi has appealed those revocations to Denmark’s Maritime and Commercial High Court. No hearing has been scheduled in that appeal. In June 2020 Fresenius Kabi commenced preliminary injunction proceedings against Biogen (Denmark) Manufacturing ApS and Biogen (Denmark) A/S in Denmark’s Maritime and Commercial High Court alleging infringement of another Danish Utility Model. A hearing was held in May and June 2021. In November 2018 Fresenius Kabi commenced infringement proceedings for damages and injunctive relief against Biogen GmbH in the Düsseldorf Regional Court relating to the German counterpart of the ‘510 Patent. The case has been stayed pending proceedings at the European Patent Office (EPO). In July 2019 Gedeon Richter PLC commenced proceedings against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of the German counterpart of European Patent No. 3 212 667, which was issued in September 2018 and expires in October 2035, and seeking damages and injunctive relief. A hearing has been set for November 2021. An estimate of the possible loss or range of loss in the IMRALDI patent litigation described above cannot be made at this time. Qui Tam Litigation In July 2015 a qui tam action filed by Michael Bawduniak on behalf of the U.S. and certain states was unsealed by the U.S. District Court for the District of Massachusetts. The action alleges sales and promotional activities in violation of the federal False Claims Act and state law counterparts and seeks single and treble damages, civil penalties, interest, attorneys’ fees and costs. No trial date has been set. The U.S. has not made an intervention decision. An estimate of the possible loss or range of loss cannot be made at this time. Dispute with Former Convergence Shareholders In November and December 2019 Shareholder Representative Services LLC, on behalf of the former shareholders of Convergence, sent us correspondence asserting claims of $200.0 million for alleged breach of the contract under which we acquired Convergence. We dispute the claims. Samsung BioLogics Arbitration In December 2020 we requested arbitration in the International Chamber of Commerce Court of International Arbitration against Samsung BioLogics seeking interpretation of certain provisions in the Joint Venture Agreement executed on December 6, 2011, as amended, by and between Biogen and Samsung BioLogics (the Joint Venture Agreement). Samsung BioLogics has asserted counterclaims, including breach of the Joint Venture Agreement, and seeks declaratory relief and unspecified damages. An estimate of the possible loss or range of loss cannot be made at this time. We expect a hearing in the fourth quarter of 2021. Other Matters Petition for Inter Partes Review In July 2018 Mylan Pharmaceuticals, Inc. (Mylan) filed a petition that was granted by the U.S. Patent Trial and Appeal Board (PTAB) for inter partes review of our U.S. Patent No. 8,399,514 (the '514 Patent). The '514 Patent includes claims covering treatment of MS with 480 mg of dimethyl fumarate per day as provided for in our TECFIDERA label. In February 2020 the PTAB issued a final written decision upholding the patentability of the ‘514 Patent and in April 2020 Mylan filed an appeal in the U.S. Court of Appeals for the Federal Circuit (the Federal Circuit), which is pending. Hatch-Waxman Act Litigation relating to TECFIDERA Orange-Book Listed Patents In 2017 to 2020, we filed patent infringement proceedings relating to TECFIDERA Orange-Book listed patents pursuant to the Drug Price Competition and Patent Term Restoration Act of 1984, commonly known as the Hatch-Waxman Act (the Delaware Actions), against Accord Healthcare Inc., Alkem Laboratories Ltd., Amneal Pharmaceuticals LLC, Cipla Limited, Graviti Pharmaceuticals Pvt. Ltd., Hetero USA, Inc., Lupin Atlantis Holdings SA, Macleods Pharmaceuticals, Ltd., MSN Laboratories Pvt. Ltd., Pharmathen S.A., Prinston Pharmaceutical Inc., Sandoz Inc., Shilpa Medicare Limited, Slayback Pharma LLC, Sun Pharmaceutical Industries, Ltd., Sun Pharmaceutical Industries, Inc., Sun Pharma Global FZE, Torrent Pharmaceuticals Ltd., TWi Pharmaceuticals, Inc., Windlas Healthcare Pvt. Ltd. and Zydus Pharmaceuticals (USA) Inc. (collectively, the Delaware Defendants) in the U.S. District Court for the District of Delaware (the Delaware Court) and against Mylan in the U.S. District Court for the Northern District of West Virginia (the West Virginia Court). On June 22, 2020, the West Virginia Court entered judgment for Mylan that the asserted claims of the ‘514 Patent are invalid for lack of written description. We appealed the judgment to the Federal Circuit and the appeal is pending. The Delaware Court entered judgment for the Delaware Defendants on the grounds that the judgment of the West Virginia Court applies to the Delaware Actions under principles of collateral estoppel. We have appealed the judgments and the appeal is pending. Hatch-Waxman Act Litigation relating to VUMERITY Orange-Book Listed Patents In March 2021 Biogen and Alkermes Pharma Ireland Limited filed patent infringement proceedings relating to VUMERITY Orange-Book listed patents (U.S. Patent Nos. 8,699,281, 9,090,558 and 10,080,733) pursuant to the Hatch-Waxman Act in the Delaware Court against Teva Pharmaceuticals Development, Inc. A trial date has not yet been set. European Patent Office Oppositions In 2016 the EPO revoked our European Patent No. 2 137 537, which covers the treatment of MS with 480 mg of dimethyl fumarate as provided for in our TECFIDERA label. We have appealed to the Technical Boards of Appeal of the EPO and a hearing date has been set for January 2022. In March 2018 the EPO revoked Forward Pharma A/S' (Forward Pharma) European Patent No. 2 801 355, which expires in October 2025. Forward Pharma has filed an appeal to the Technical Boards of Appeal of the EPO and a hearing has been set for September 2021. TYSABRI Patent Revocation Matters In November 2017 Bioeq GMBH, affiliated with the Polpharma Group, brought an action in the Polish Patent Office seeking to revoke Polish Patent No. 215263 (the Polish '263 Patent), which corresponds to our European Patent No. 1 485 127 (the E.U. '127 Patent) and covers administration of natalizumab (TYSABRI) to treat MS. The Polish '263 Patent expires in February 2023. The Polish Patent Office dismissed the action in February 2021. In August 2020 a related entity, Polpharma Biologics S.A., also brought an action seeking to revoke the Polish ‘263 Patent in the Polish Patent Office. The action was suspended by the Polish Patent Office in April 2021. Swiss Pharma International AG, also affiliated with the Polpharma Group, filed actions in the District Court of the Hague, Netherlands (January 2016), the German Patents Court (March 2016) and the Commercial Court of Rome (November 2017) seeking to invalidate the Dutch, German and Italian counterparts, respectively, of the E.U. '127 Patent, which also cover administration of natalizumab (TYSABRI) to treat MS and expire in February 2023. The Dutch and German counterparts were ruled invalid. The decision in the Dutch action was affirmed on appeal and the German appeal has been withdrawn. A hearing in the Italian action will be held in June 2022. Annulment Proceedings in General Court of the European Union relating to TECFIDERA Pharmaceutical Works Polpharma SA (Polpharma) and Mylan Ireland Ltd. (Mylan Ireland) each filed applications in the General Court of the European Union (Polpharma in October 2018 and Mylan Ireland in November 2020) seeking to annul decisions of the European Medicines Agency (EMA) refusing to validate Polpharma’s and Mylan Ireland’s respective applications to market a generic version of TECFIDERA. The EMA’s refusals were on the grounds that TECFIDERA benefits from regulatory data protection. Biogen and the European Commission were granted leave to intervene in support of the EMA in the case brought by Polpharma. On May 5, 2021, the European General Court annulled the EMA's non-validation decision with respect to a generic application related to TECFIDERA in the E.U. This generic application is now before the EMA, which is reassessing TECFIDERA's regulatory data protection by performing a scientific assessment pursuant to the European General Court's decision. The result of the scientific assessment of the EMA is expected in the fourth quarter of 2021. We have appealed the European General Court's decision to the European Court of Justice and the appeal is pending. The case brought by Mylan Ireland has been stayed. Product Liability and Other Legal Proceedings We are also involved in product liability claims and other legal proceedings generally incidental to our normal business activities. While the outcome of any of these proceedings cannot be accurately predicted, we do not believe the ultimate resolution of any of these existing matters would have a material adverse effect on our business or financial condition.

We are currently involved in various claims and legal proceedings, including the matters described below. For information as to our accounting policies relating to claims and legal proceedings, including use of estimates and contingencies, please read Note 1, Summary of Significant Accounting Policies, to our consolidated financial statements included in our 2020 Form 10-K.

With respect to some loss contingencies, an estimate of the possible loss or range of loss cannot be made until management has further information, including, for example, (i) which claims, if any, will survive dispositive motion practice; (ii) information to be obtained through discovery; (iii) information as to the parties' damages claims and supporting evidence; (iv) the parties’ legal theories; and (v) the parties' settlement positions.

The claims and legal proceedings in which we are involved also include challenges to the scope, validity or enforceability of the patents relating to our products, pipeline or processes and challenges to the scope, validity or enforceability of the patents held by others. These include claims by third parties that we infringe their patents. An adverse outcome in any of these proceedings could result in one or more of the following and have a material impact on our business or consolidated results of operations and financial position: (i) loss of patent protection; (ii) inability to continue to engage in certain activities; and (iii) payment of significant damages, royalties, penalties and/or license fees to third parties.

We and certain current and former officers are named as defendants in an action filed by a shareholder on November 13, 2020, in the U.S. District Court for the Central District of California and transferred to the U.S. District Court for the District of Massachusetts in March 2021. The action alleges violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and seeks a declaration of the action as a class action and an award of damages, interest and attorneys' fees. An estimate of the possible loss or range of loss cannot be made at this time. No trial date has been set. We have filed a motion to dismiss the action, which is pending.

In September 2018 Fresenius Kabi Deutschland GmbH (Fresenius Kabi) commenced proceedings for damages and injunctive relief against Biogen France SAS in the Tribunal de Grande Instance de Paris, alleging that IMRALDI, the adalimumab biosimilar product of Samsung Bioepis UK Limited that Biogen has commercialized in Europe, infringes the French counterpart of European Patent No. 3 148 510 (the '510 Patent), which was issued in June 2018 and expires in May 2035. No hearing has been scheduled.

In October 2018 Fresenius Kabi commenced preliminary injunction proceedings against Biogen (Denmark) Manufacturing ApS and Biogen Denmark A/S in Denmark's Maritime and Commercial High Court alleging infringement of Danish Utility Models. The request for preliminary injunction was denied in June 2019 and the decision was affirmed on appeal in February 2021. In July 2020 the Danish Patent Board of Appeal revoked the Danish Utility Models that were the subject of Fresenius Kabi’s October 2018 request for a preliminary injunction and Fresenius Kabi has appealed those revocations to Denmark’s Maritime and Commercial High Court. No hearing has been scheduled in that appeal.

In June 2020 Fresenius Kabi commenced preliminary injunction proceedings against Biogen (Denmark) Manufacturing ApS and Biogen (Denmark) A/S in Denmark’s Maritime and Commercial High Court alleging infringement of another Danish Utility Model. A hearing was held in May and June 2021.

In November 2018 Fresenius Kabi commenced infringement proceedings for damages and injunctive relief against Biogen GmbH in the Düsseldorf Regional Court relating to the German counterpart of the ‘510 Patent. The case has been stayed pending proceedings at the European Patent Office (EPO).

In July 2019 Gedeon Richter PLC commenced proceedings against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of the German counterpart of European Patent No. 3 212 667, which was issued in September 2018 and expires in October 2035, and seeking damages and injunctive relief. A hearing has been set for November 2021.

In July 2015 a qui tam action filed by Michael Bawduniak on behalf of the U.S. and certain states was unsealed by the U.S. District Court for the District of Massachusetts. The action alleges sales and promotional activities in violation of the federal False Claims Act and state law counterparts and seeks single and treble damages, civil penalties, interest, attorneys’ fees and costs. No trial date has been set. The U.S. has not made an intervention decision. An estimate of the possible loss or range of loss cannot be made at this time.

In November and December 2019 Shareholder Representative Services LLC, on behalf of the former shareholders of Convergence, sent us correspondence asserting claims of $200.0 million for alleged breach of the contract under which we acquired Convergence. We dispute the claims.

In December 2020 we requested arbitration in the International Chamber of Commerce Court of International Arbitration against Samsung BioLogics seeking interpretation of certain provisions in the Joint Venture Agreement executed on December 6, 2011, as amended, by and between Biogen and Samsung BioLogics (the Joint Venture Agreement). Samsung BioLogics has asserted counterclaims, including breach of the Joint Venture Agreement, and seeks declaratory relief and unspecified damages. An estimate of the possible loss or range of loss cannot be made at this time. We expect a hearing in the fourth quarter of 2021.

In July 2018 Mylan Pharmaceuticals, Inc. (Mylan) filed a petition that was granted by the U.S. Patent Trial and Appeal Board (PTAB) for inter partes review of our U.S. Patent No. 8,399,514 (the '514 Patent). The '514 Patent includes claims covering treatment of MS with 480 mg of dimethyl fumarate per day as provided for in our TECFIDERA label. In February 2020 the PTAB issued a final written decision upholding the patentability of the ‘514 Patent and in April 2020 Mylan filed an appeal in the U.S. Court of Appeals for the Federal Circuit (the Federal Circuit), which is pending.

In 2017 to 2020, we filed patent infringement proceedings relating to TECFIDERA Orange-Book listed patents pursuant to the Drug Price Competition and Patent Term Restoration Act of 1984, commonly known as the Hatch-Waxman Act (the Delaware Actions), against Accord Healthcare Inc., Alkem Laboratories Ltd., Amneal Pharmaceuticals LLC, Cipla Limited, Graviti Pharmaceuticals Pvt. Ltd., Hetero USA, Inc., Lupin Atlantis Holdings SA, Macleods Pharmaceuticals, Ltd., MSN Laboratories Pvt. Ltd., Pharmathen S.A., Prinston Pharmaceutical Inc., Sandoz Inc., Shilpa Medicare Limited, Slayback Pharma LLC, Sun Pharmaceutical Industries, Ltd., Sun Pharmaceutical Industries, Inc., Sun Pharma Global FZE, Torrent Pharmaceuticals Ltd., TWi Pharmaceuticals, Inc., Windlas Healthcare Pvt. Ltd. and Zydus Pharmaceuticals (USA) Inc. (collectively, the Delaware Defendants) in the U.S. District Court for the District of Delaware (the Delaware Court) and against Mylan in the U.S. District Court for the Northern District of West Virginia (the West Virginia Court).

The Delaware Court entered judgment for the Delaware Defendants on the grounds that the judgment of the West Virginia Court applies to the Delaware Actions under principles of collateral estoppel. We have appealed the judgments and the appeal is pending.

In March 2021 Biogen and Alkermes Pharma Ireland Limited filed patent infringement proceedings relating to VUMERITY Orange-Book listed patents (U.S. Patent Nos. 8,699,281, 9,090,558 and 10,080,733) pursuant to the Hatch-Waxman Act in the Delaware Court against Teva Pharmaceuticals Development, Inc. A trial date has not yet been set.

In 2016 the EPO revoked our European Patent No. 2 137 537, which covers the treatment of MS with 480 mg of dimethyl fumarate as provided for in our TECFIDERA label. We have appealed to the Technical Boards of Appeal of the EPO and a hearing date has been set for January 2022.

In March 2018 the EPO revoked Forward Pharma A/S' (Forward Pharma) European Patent No. 2 801 355, which expires in October 2025. Forward Pharma has filed an appeal to the Technical Boards of Appeal of the EPO and a hearing has been set for September 2021.

In November 2017 Bioeq GMBH, affiliated with the Polpharma Group, brought an action in the Polish Patent Office seeking to revoke Polish Patent No. 215263 (the Polish '263 Patent), which corresponds to our European Patent No. 1 485 127 (the E.U. '127 Patent) and covers administration of natalizumab (TYSABRI) to treat MS. The Polish '263 Patent expires in February 2023. The Polish Patent Office dismissed the action in February 2021. In August 2020 a related entity, Polpharma Biologics S.A., also brought an action seeking to revoke the Polish ‘263 Patent in the Polish Patent Office. The action was suspended by the Polish Patent Office in April 2021.

Swiss Pharma International AG, also affiliated with the Polpharma Group, filed actions in the District Court of the Hague, Netherlands (January 2016), the German Patents Court (March 2016) and the Commercial Court of Rome (November 2017) seeking to invalidate the Dutch, German and Italian counterparts, respectively, of the E.U. '127 Patent, which also cover administration of natalizumab (TYSABRI) to treat MS and expire in February 2023. The Dutch and German counterparts were ruled invalid. The decision in the Dutch action was affirmed on appeal and the German appeal has been withdrawn. A hearing in the Italian action will be held in June 2022.

Pharmaceutical Works Polpharma SA (Polpharma) and Mylan Ireland Ltd. (Mylan Ireland) each filed applications in the General Court of the European Union (Polpharma in October 2018 and Mylan Ireland in November 2020) seeking to annul decisions of the European Medicines Agency (EMA) refusing to validate Polpharma’s and Mylan Ireland’s respective applications to market a generic version of TECFIDERA. The EMA’s refusals were on the grounds that TECFIDERA benefits from regulatory data protection. Biogen and the European Commission were granted leave to intervene in support of the EMA in the case brought by Polpharma. On May 5, 2021, the European General Court annulled the EMA's non-validation decision with respect to a generic application related to TECFIDERA in the E.U. This generic application is now before the EMA, which is reassessing TECFIDERA's regulatory data protection by performing a scientific assessment pursuant to the European General Court's decision. The result of the scientific assessment of the EMA is expected in the fourth quarter of 2021. We have appealed the European General Court's decision to the European Court of Justice and the appeal is pending. The case brought by Mylan Ireland has been stayed.

We are also involved in product liability claims and other legal proceedings generally incidental to our normal business activities. While the outcome of any of these proceedings cannot be accurately predicted, we do not believe the ultimate resolution of any of these existing matters would have a material adverse effect on our business or financial condition.