Litigation	6 Months Ended
Jun. 30, 2022
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 2021 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 actions filed by shareholders on November 13, 2020 (the November 2020 Securities Action) and February 7, 2022 (the February 2022 Securities Action), and pending in the U.S. District Court for the District of Massachusetts. The actions allege violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and seek declarations of the actions as class actions and monetary relief. We have filed a motion to dismiss the November 2020 Securities Action, which is pending. An estimate of the possible loss or range of loss in these actions cannot be made at this time. Derivative Action We and members of the Board of Directors are named as defendants in an action filed by a shareholder on February 9, 2022, in the U.S. District Court for the District of Massachusetts. The action alleges violations of federal securities laws under 15 U.S.C. §78n(a) and 17 C.F.R. §240 14.a-9, breaches of fiduciary duties and waste of corporate assets, and seeks declaratory and injunctive relief, monetary relief to Biogen, and attorneys’ fees and costs to the plaintiff. The court has stayed the case pending the resolution of the February 2022 Securities Action. An estimate of the possible loss or range of loss cannot be made at this time. 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 (the French proceeding) and in November 2018 against Biogen GmbH in the Düsseldorf Regional Court (the German proceeding), alleging that IMRALDI, the adalimumab biosimilar product of Samsung Bioepis that Biogen has commercialized in Europe, infringes national counterparts of European Patent No. 3 148 510 (the EP '510 Patent, expiring in May 2035). Fresenius Kabi later added European Patent 3 145 488 (the EP ‘488 Patent, expiring in May 2035) to both actions and no hearing has been set in either. In June 2022 the Technical Boards of Appeal (TBA) of the European Patent Office (EPO) affirmed the revocation of the EP ‘510 Patent. The EPO has scheduled a hearing on the validity of the EP ‘488 Patent for October 2022. In June 2020 Fresenius Kabi commenced proceedings in Denmark's Maritime and Commercial High Court alleging that IMRALDI infringes the Danish counterpart of the EP '488 Patent and a Danish utility model. In September 2021 the Court ruled that the patent and utility model are invalid and not infringed. Fresenius Kabi has appealed to the High Court of Eastern Denmark and the appeal is pending. In July 2020 the Danish Patent Board of Appeal revoked the Danish utility models that Fresenius Kabi had asserted against Biogen and Fresenius Kabi has appealed to the Danish Maritime and Commercial High Court. No hearing has been scheduled. In July 2019 Gedeon Richter Nyrt commenced proceedings for damages and injunctive relief against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of the German counterpart of European Patent No. 3 212 667, which expires in October 2035. The case has been stayed pending proceedings in the EPO seeking to invalidate the patent. In November 2020 Gedeon Richter Nyrt commenced additional proceedings against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of a German utility model. In October 2021 Biogen filed cancellation proceedings in respect of the German utility model and the infringement proceedings have been stayed pending the outcome of the cancellation proceedings. 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 Biogen has reached an agreement in principle to resolve previously disclosed litigation pending in the U.S. District Court for the District of Massachusetts alleging violations of the federal False Claims Act and state law counterparts. The litigation was filed by Michael Bawduniak on behalf of the U.S. and certain states and unsealed in 2015. The U.S. has not intervened in the case. The agreement in principle contemplates Biogen making a payment of $900.0 million. The agreement in principle does not include any admission of liability and is subject to the negotiation of final settlement documents and agreements with the named government entities. 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. ERISA Class Action Litigation In September 2020 the U.S. District Court for the District of Massachusetts consolidated two cases filed against us in July and August 2020 by participants in the Biogen 401(k) Savings Plan alleging breach of fiduciary duty under ERISA. Plaintiffs seek a declaration of the action as a class action and monetary and other relief. An estimate of the possible loss or range of loss cannot be made at this time. Humana Patient Assistance Litigation In September 2021 Humana Inc. (Humana) filed suit against us in the U.S. District Court for the District of Massachusetts alleging damages related to our providing MS patients with free medications and making charitable contributions to non-profit organizations that assist MS patients. Humana alleges violation of the federal RICO Act and state laws and seeks statutory treble damages, attorneys' fees and costs. We filed a motion to dismiss, which is pending. An estimate of the possible loss cannot be made at this time. Other Matters Government Investigations The U.S. House of Representatives Committees on Oversight and Reform and Energy and Commerce and the Office of Inspector General of the U.S. Department of Health and Human Services have announced investigations relating to ADUHELM. In addition, the Company has received a civil investigative demand from the Federal Trade Commission and a subpoena from the Securities and Exchange Commission seeking information relating to ADUHELM, including healthcare sites, ADUHELM’s approval and ADUHELM’s marketing. TECFIDERA Patent Matters 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 (the West Virginia Action) in the U.S. District Court for the Northern District of West Virginia (the West Virginia Court). In November 2021 the Federal Circuit affirmed the West Virginia Court judgment that the asserted claims of our U.S. Patent No. 8,399,514 (the '514 Patent) are invalid for lack of written description and in June 2022 we filed a petition for a writ of certiorari to the United States Supreme Court seeking review of the Federal Circuit's decision. 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. The appeals in the Delaware Actions are stayed pending any final action by the United States Supreme Court with respect to the judgment in the West Virginia Action. In July 2018 Mylan Pharmaceuticals, Inc. (Mylan) filed a petition with the U.S. Patent Trial and Appeal Board (PTAB) for inter partes review of the '514 Patent. In November 2021 the Federal Circuit ruled that the PTAB decision upholding the patentability of the ‘514 patent was moot, but in April 2022 the Federal Circuit vacated that ruling and stayed the appeal pending any final action by the United States Supreme Court with respect to the judgment in the West Virginia Action. TYSABRI Patent Revocation Matters In November 2017 Swiss Pharma International AG, affiliated with the Polpharma Group, filed an action in the Commercial Court of Rome to invalidate the Italian counterpart of the European Patent No. 1 485 127 (the EP '127 Patent) which covers administration of natalizumab (TYSABRI) to treat MS and expires in February 2023. A hearing has been set for November 2022. In August 2020 Polpharma Biologics S.A., also affiliated with the Polpharma Group, brought an action in the Polish Patent Office to revoke our Polish Patent No. 215263, which corresponds to the EP '127 Patent and expires in February 2023. The action was suspended by the Polish Patent Office in April 2021 pending examination of our amended patent claims. In June 2021 Polpharma Biologics S.A., Sandoz B.V. and Sandoz AG filed an action in the District Court of the Hague, Netherlands to invalidate the Dutch counterpart of our European Patent 2 676 967 (the EP '967 Patent), which expires in 2027 and covers methods of treatment using natalizumab (TYSABRI) and pre-treatment testing of patients. A hearing has been set for September 2022. In July 2021 the EPO revoked the EP ‘967 Patent. A hearing on our appeal to the TBA of the EPO is set for December 2022. In September 2021 Polpharma Biologics S.A., Sandoz AG, Sandoz Limited and Sandoz GmbH filed an action in the English High Court to revoke the U.K. counterpart of the EP ‘967 Patent and seeking a declaration that the patent would not be infringed by the marketing of Polpharma’s proposed natalizumab biosimilar. A hearing has been set for February 2023. Annulment Proceedings in the General Court of the European Union relating to TECFIDERA Pharmaceutical Works Polpharma SA (Polpharma) and Mylan Ireland Ltd. (Mylan Ireland) each filed actions in the General Court of the European Union (Polpharma in October 2018 and Mylan Ireland in November 2020) to annul the European Medicines Agency's (EMA) decision not to validate their applications to market generic versions of TECFIDERA on the grounds that TECFIDERA benefits from regulatory data protection. On May 5, 2021, the European General Court annulled the EMA's non-validation decision with respect to Polpharma. We have appealed the 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 2021 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 actions filed by shareholders on November 13, 2020 (the November 2020 Securities Action) and February 7, 2022 (the February 2022 Securities Action), and pending in the U.S. District Court for the District of Massachusetts. The actions allege violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and seek declarations of the actions as class actions and monetary relief. We have filed a motion to dismiss the November 2020 Securities Action, which is pending. An estimate of the possible loss or range of loss in these actions cannot be made at this time.

We and members of the Board of Directors are named as defendants in an action filed by a shareholder on February 9, 2022, in the U.S. District Court for the District of Massachusetts. The action alleges violations of federal securities laws under 15 U.S.C. §78n(a) and 17 C.F.R. §240 14.a-9, breaches of fiduciary duties and waste of corporate assets, and seeks declaratory and injunctive relief, monetary relief to Biogen, and attorneys’ fees and costs to the plaintiff. The court has stayed the case pending the resolution of the February 2022 Securities Action. An estimate of the possible loss or range of loss cannot be made at this time.

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 (the French proceeding) and in November 2018 against Biogen GmbH in the Düsseldorf Regional Court (the German proceeding), alleging that IMRALDI, the adalimumab biosimilar product of Samsung Bioepis that Biogen has commercialized in Europe, infringes national counterparts of European Patent No. 3 148 510 (the EP '510 Patent, expiring in May 2035). Fresenius Kabi later added European Patent 3 145 488 (the EP ‘488 Patent, expiring in May 2035) to both actions and no hearing has been set in either. In June 2022 the Technical Boards of Appeal (TBA) of the European Patent Office (EPO) affirmed the revocation of the EP ‘510 Patent. The EPO has scheduled a hearing on the validity of the EP ‘488 Patent for October 2022.

In June 2020 Fresenius Kabi commenced proceedings in Denmark's Maritime and Commercial High Court alleging that IMRALDI infringes the Danish counterpart of the EP '488 Patent and a Danish utility model. In September 2021 the Court ruled that the patent and utility model are invalid and not infringed. Fresenius Kabi has appealed to the High Court of Eastern Denmark and the appeal is pending.

In July 2020 the Danish Patent Board of Appeal revoked the Danish utility models that Fresenius Kabi had asserted against Biogen and Fresenius Kabi has appealed to the Danish Maritime and Commercial High Court. No hearing has been scheduled.

In July 2019 Gedeon Richter Nyrt commenced proceedings for damages and injunctive relief against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of the German counterpart of European Patent No. 3 212 667, which expires in October 2035. The case has been stayed pending proceedings in the EPO seeking to invalidate the patent. In November 2020 Gedeon Richter Nyrt commenced additional proceedings against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of a German utility model. In October 2021 Biogen filed cancellation proceedings in respect of the German utility model and the infringement proceedings have been stayed pending the outcome of the cancellation proceedings.

Biogen has reached an agreement in principle to resolve previously disclosed litigation pending in the U.S. District Court for the District of Massachusetts alleging violations of the federal False Claims Act and state law counterparts. The litigation was filed by Michael Bawduniak on behalf of the U.S. and certain states and unsealed in 2015. The U.S. has not intervened in the case. The agreement in principle contemplates Biogen making a payment of $900.0 million. The agreement in principle does not include any admission of liability and is subject to the negotiation of final settlement documents and agreements with the named government entities.

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 September 2020 the U.S. District Court for the District of Massachusetts consolidated two cases filed against us in July and August 2020 by participants in the Biogen 401(k) Savings Plan alleging breach of fiduciary duty under ERISA. Plaintiffs seek a declaration of the action as a class action and monetary and other relief. An estimate of the possible loss or range of loss cannot be made at this time.

In September 2021 Humana Inc. (Humana) filed suit against us in the U.S. District Court for the District of Massachusetts alleging damages related to our providing MS patients with free medications and making charitable contributions to non-profit organizations that assist MS patients. Humana alleges violation of the federal RICO Act and state laws and seeks statutory treble damages, attorneys' fees and costs. We filed a motion to dismiss, which is pending. An estimate of the possible loss cannot be made at this time.

The U.S. House of Representatives Committees on Oversight and Reform and Energy and Commerce and the Office of Inspector General of the U.S. Department of Health and Human Services have announced investigations relating to ADUHELM. In addition, the Company has received a civil investigative demand from the Federal Trade Commission and a subpoena from the Securities and Exchange Commission seeking information relating to ADUHELM, including healthcare sites, ADUHELM’s approval and ADUHELM’s marketing.

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 (the West Virginia Action) in the U.S. District Court for the Northern District of West Virginia (the West Virginia Court).

In November 2021 the Federal Circuit affirmed the West Virginia Court judgment that the asserted claims of our U.S. Patent No. 8,399,514 (the '514 Patent) are invalid for lack of written description and in June 2022 we filed a petition for a writ of certiorari to the United States Supreme Court seeking review of the Federal Circuit's decision.

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. The appeals in the Delaware Actions are stayed pending any final action by the United States Supreme Court with respect to the judgment in the West Virginia Action.

In July 2018 Mylan Pharmaceuticals, Inc. (Mylan) filed a petition with the U.S. Patent Trial and Appeal Board (PTAB) for inter partes review of the '514 Patent. In November 2021 the Federal Circuit ruled that the PTAB decision upholding the patentability of the ‘514 patent was moot, but in April 2022 the Federal Circuit vacated that ruling and stayed the appeal pending any final action by the United States Supreme Court with respect to the judgment in the West Virginia Action.

In November 2017 Swiss Pharma International AG, affiliated with the Polpharma Group, filed an action in the Commercial Court of Rome to invalidate the Italian counterpart of the European Patent No. 1 485 127 (the EP '127 Patent) which covers administration of natalizumab (TYSABRI) to treat MS and expires in February 2023. A hearing has been set for November 2022.

In August 2020 Polpharma Biologics S.A., also affiliated with the Polpharma Group, brought an action in the Polish Patent Office to revoke our Polish Patent No. 215263, which corresponds to the EP '127 Patent and expires in February 2023. The action was suspended by the Polish Patent Office in April 2021 pending examination of our amended patent claims.

In June 2021 Polpharma Biologics S.A., Sandoz B.V. and Sandoz AG filed an action in the District Court of the Hague, Netherlands to invalidate the Dutch counterpart of our European Patent 2 676 967 (the EP '967 Patent), which expires in 2027 and covers methods of treatment using natalizumab (TYSABRI) and pre-treatment testing of patients. A hearing has been set for September 2022.

In September 2021 Polpharma Biologics S.A., Sandoz AG, Sandoz Limited and Sandoz GmbH filed an action in the English High Court to revoke the U.K. counterpart of the EP ‘967 Patent and seeking a declaration that the patent would not be infringed by the marketing of Polpharma’s proposed natalizumab biosimilar. A hearing has been set for February 2023.

Pharmaceutical Works Polpharma SA (Polpharma) and Mylan Ireland Ltd. (Mylan Ireland) each filed actions in the General Court of the European Union (Polpharma in October 2018 and Mylan Ireland in November 2020) to annul the European Medicines Agency's (EMA) decision not to validate their applications to market generic versions of TECFIDERA on the grounds that TECFIDERA benefits from regulatory data protection. On May 5, 2021, the European General Court annulled the EMA's non-validation decision with respect to Polpharma. We have appealed the 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.