Litigation	6 Months Ended
Jun. 30, 2023
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 2022 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. If an estimate of the possible loss or range of loss can be made at this time, it is included in the potential loss contingency description below. 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 In March 2023 the United States District Court for the District of Massachusetts (the District Court) dismissed the previously disclosed shareholder action related to ADUHELM that had been filed in February 2022 against us and certain current and former officers. In May 2023 the plaintiff filed a motion to alter the judgment and amend the complaint, which is pending. The second previously disclosed shareholder action related to ADUHELM, filed in November 2022 and dismissed by the District Court in September 2021, remains on appeal to the U.S. Court of Appeals for the First Circuit. Both actions alleged violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5. Derivative Action We and members of the Board of Directors are named as defendants in derivative actions filed by shareholders in February and July 2022, in the U.S. District Court for the District of Massachusetts. The actions allege violations of federal securities laws under 15 U.S.C. §78n(a) and 17 C.F.R. §240 14.a-9, and breaches of fiduciary duties and waste of corporate assets, and seek declaratory and injunctive relief, monetary relief payable to Biogen, and attorneys’ fees and costs payable to the plaintiffs. The District Court has stayed both cases. IMRALDI Patent Litigation In June 2022 Fresenius Kabi Deutschland GmbH (Fresenius Kabi) filed a claim 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 that Biogen commercializes in Europe, infringes the French counterpart of European Patent 3 145 488 (the EP ‘488 Patent), which expires in May 2035. In August 2022 Fresenius Kabi filed a claim for damages and injunctive relief against Biogen GmbH in the Düsseldorf Regional Court, alleging infringement of the German counterpart of the EP '488 Patent. A hearing in the Düsseldorf Regional Court has been set for December 2023. No hearing has been set in the French action. In July 2019 Gedeon Richter Nyrt (Gedeon Richter) filed a claim 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 (the EP '667 Patent, expiring in October 2035), and in November 2020 Gedeon Richter filed a claim for damages and injunctive relief against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of a German utility model corresponding to the EP '667 Patent (expiring in October 2025). In July 2023 we and Gedeon Richter entered an agreement resolving the claims. Litigation with Former Convergence Shareholders In 2015 Biogen acquired Convergence, a U.K. company. In 2019 Shareholder Representative Services LLC, on behalf of the former shareholders of Convergence, asserted claims of $200.0 million for alleged breaches of the contract pursuant to which we acquired Convergence. In June 2023 Shareholder Representative Services LLC and 24 former shareholders filed suit against us in the High Court of Justice of England and Wales on one of the previously asserted claims, seeking payment of $49.9 million, interest and costs. No trial date has been set. 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 and seeking a declaration of the action as a class action and monetary and other relief. In June 2023 the parties reached a settlement in principle, subject to approval of the settlement by the court. Humana Patient Assistance Litigation In March 2023 the District Court for the District of Massachusetts dismissed the previously disclosed action filed against us by Humana in September 2020. Humana had alleged damages related to our providing MS patients with free medications and making charitable contributions to non-profit organizations that assist MS patients and had alleged violations of the federal RICO Act and state laws. In April 2023 Humana filed a motion to alter the judgment and amend the complaint, which is pending. Distributor Matter In December 2022 we terminated our distribution agreement with the distributor of products for Biogen in various countries in the Middle East and northern Africa. The former distributor has asserted breach of contract. No suit has been filed. Genentech Litigation In February 2023 Genentech, Inc. filed suit against us in the U.S. District Court for the Northern District of California, alleging that it is owed royalties on sales of TYSABRI that occurred after the expiration of a patent licensed by Genentech to Biogen, together with interest and costs. No trial date has been set. Other Matters Government Investigation The Company has received subpoenas from the Securities and Exchange Commission seeking information relating to ADUHELM, including healthcare sites and ADUHELM’s approval. TYSABRI Biosimilar Patent Matter In September 2022, following Sandoz Inc.'s announcement that the FDA had accepted its biologics license application for a proposed biosimilar referring to TYSABRI, we filed an action in the U.S. District Court for the District of Delaware against Sandoz Inc., other Sandoz entities and Polpharma Biologics S.A. under the Biologics Price Competition and Innovation Act, 42 U.S.C. §262, seeking a declaratory judgment of patent infringement. No trial has been set. Annulment Proceedings in the General Court of the European Union relating to TECFIDERA In November 2020 Mylan Ireland filed an action in the General Court of the European Union (the General Court) to annul the EMA's decision not to validate its applications to market generic versions of TECFIDERA on the grounds that TECFIDERA benefits from regulatory data protection. No trial date has been set. Hatch-Waxman Act Litigation relating to VUMERITY Orange-Book Listed Patents In July 2023 Biogen and Alkermes Pharma Ireland Limited filed patent infringement proceedings relating to VUMERITY Orange-Book listed patents (U.S. Patent Nos. 8,669,281, 9,090,558 and 10,080,733) pursuant to the Drug Price Competition and Patent Term Restoration Act of 1984 (the Hatch-Waxman Act) in the U.S. District Court for the District of Delaware against Zydus Worldwide DMCC. No trial date has been set. 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 2022 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. If an estimate of the possible loss or range of loss can be made at this time, it is included in the potential loss contingency description below.

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.

In March 2023 the United States District Court for the District of Massachusetts (the District Court) dismissed the previously disclosed shareholder action related to ADUHELM that had been filed in February 2022 against us and certain current and former officers. In May 2023 the plaintiff filed a motion to alter the judgment and amend the complaint, which is pending. The second previously disclosed shareholder action related to ADUHELM, filed in November 2022 and dismissed by the District Court in September 2021, remains on appeal to the U.S. Court of Appeals for the First Circuit. Both actions alleged violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5.

We and members of the Board of Directors are named as defendants in derivative actions filed by shareholders in February and July 2022, in the U.S. District Court for the District of Massachusetts. The actions allege violations of federal securities laws under 15 U.S.C. §78n(a) and 17 C.F.R. §240 14.a-9, and breaches of fiduciary duties and waste of corporate assets, and seek declaratory and injunctive relief, monetary relief payable to Biogen, and attorneys’ fees and costs payable to the plaintiffs. The District Court has stayed both cases.

In June 2022 Fresenius Kabi Deutschland GmbH (Fresenius Kabi) filed a claim 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 that Biogen commercializes in Europe, infringes the French counterpart of European Patent 3 145 488 (the EP ‘488 Patent), which expires in May 2035. In August 2022 Fresenius Kabi filed a claim for damages and injunctive relief against Biogen GmbH in the Düsseldorf Regional Court, alleging infringement of the German counterpart of the EP '488 Patent. A hearing in the Düsseldorf Regional Court has been set for December 2023. No hearing has been set in the French action.

In July 2019 Gedeon Richter Nyrt (Gedeon Richter) filed a claim 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 (the EP '667 Patent, expiring in October 2035), and in November 2020 Gedeon Richter filed a claim for damages and injunctive relief against Biogen GmbH in the Düsseldorf Regional Court alleging infringement of a German utility model corresponding to the EP '667 Patent (expiring in October 2025). In July 2023 we and Gedeon Richter entered an agreement resolving the claims.

In 2015 Biogen acquired Convergence, a U.K. company. In 2019 Shareholder Representative Services LLC, on behalf of the former shareholders of Convergence, asserted claims of $200.0 million for alleged breaches of the contract pursuant to which we acquired Convergence. In June 2023 Shareholder Representative Services LLC and 24 former shareholders filed suit against us in the High Court of Justice of England and Wales on one of the previously asserted claims, seeking payment of $49.9 million, interest and costs. No trial date has been set.

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 and seeking a declaration of the action as a class action and monetary and other relief. In June 2023 the parties reached a settlement in principle, subject to approval of the settlement by the court.

In March 2023 the District Court for the District of Massachusetts dismissed the previously disclosed action filed against us by Humana in September 2020. Humana had alleged damages related to our providing MS patients with free medications and making charitable contributions to non-profit organizations that assist MS patients and had alleged violations of the federal RICO Act and state laws. In April 2023 Humana filed a motion to alter the judgment and amend the complaint, which is pending.

In December 2022 we terminated our distribution agreement with the distributor of products for Biogen in various countries in the Middle East and northern Africa. The former distributor has asserted breach of contract. No suit has been filed.

In February 2023 Genentech, Inc. filed suit against us in the U.S. District Court for the Northern District of California, alleging that it is owed royalties on sales of TYSABRI that occurred after the expiration of a patent licensed by Genentech to Biogen, together with interest and costs. No trial date has been set.

The Company has received subpoenas from the Securities and Exchange Commission seeking information relating to ADUHELM, including healthcare sites and ADUHELM’s approval.

In September 2022, following Sandoz Inc.'s announcement that the FDA had accepted its biologics license application for a proposed biosimilar referring to TYSABRI, we filed an action in the U.S. District Court for the District of Delaware against Sandoz Inc., other Sandoz entities and Polpharma Biologics S.A. under the Biologics Price Competition and Innovation Act, 42 U.S.C. §262, seeking a declaratory judgment of patent infringement. No trial has been set.

In November 2020 Mylan Ireland filed an action in the General Court of the European Union (the General Court) to annul the EMA's decision not to validate its applications to market generic versions of TECFIDERA on the grounds that TECFIDERA benefits from regulatory data protection. No trial date has been set.

In July 2023 Biogen and Alkermes Pharma Ireland Limited filed patent infringement proceedings relating to VUMERITY Orange-Book listed patents (U.S. Patent Nos. 8,669,281, 9,090,558 and 10,080,733) pursuant to the Drug Price Competition and Patent Term Restoration Act of 1984 (the Hatch-Waxman Act) in the U.S. District Court for the District of Delaware against Zydus Worldwide DMCC. No trial date has been set.

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.