Litigation	12 Months Ended
Dec. 31, 2017
Loss Contingency, Information about Litigation Matters [Abstract]
Litigation	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 these consolidated financial statements. 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 Qui Tam Litigation On July 6, 2015, a qui tam action filed on behalf of the United States 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. Our motion to dismiss is pending. The United States has not made an intervention decision. An estimate of the possible loss or range of loss cannot be made at this time. Securities Litigation We and certain current and former officers are defendants in an action filed by a shareholder on October 20, 2016 in the U.S. District Court for the District of Massachusetts alleging violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and seeking 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. Other Matters Abbreviated New Drug Application (ANDA) Litigation relating to TECFIDERA In June, July, and September 2017 and January 2018, we initiated patent infringement proceedings pursuant to the Hatch-Waxman act in the U.S. District Court for the District of Delaware against Amneal Pharmaceuticals LLC, Aurobindo Pharma U.S.A., Inc., Caribe Holdings (Cayman) Co. Ltd. DBA Puracap Caribe, Puracap International LLC, Graviti Pharmaceuticals Pvt. Ltd., Hetero USA, Inc., Impax Laboratories, Inc., Prinston Pharmaceutical Inc., Slayback Pharma LLC, Teva Pharmaceuticals USA, Inc., Alkem Laboratories Ltd., Cipla Limited, Glenmark Pharmaceuticals Ltd., Lupin Atlantis Holdings SA, Macleods Pharmaceuticals, Ltd., MSN Laboratories Pvt. Ltd., Pharmathen S.A., Shipla Medicare Limited, Sun Pharma Global FZE, Torrent Pharmaceuticals Ltd., TWi Pharmaceuticals, Inc., Windlas Healthcare Pvt. Ltd., Accord Healthcare Inc., Par Pharmaceutical Inc., Sandoz Inc., Sawai USA, Inc. and Zydus Pharmaceuticals (USA) Inc. In addition, we initiated patent infringement proceedings pursuant to the Hatch-Waxman act against Stason Pharmaceuticals, Inc. in the U.S. District Court for the Central District of California, Zydus Pharmaceuticals (USA) Inc. in the U.S. District Court for the District of New Jersey, Accord Healthcare Inc. in the U.S. District Court for the Middle District of North Carolina, Par Pharmaceutical Inc. in the U.S. District Court for the Southern District of New York, Sandoz Inc. in the U.S. District Court for the District of Colorado and Mylan Pharmaceuticals Inc. in the U.S. District Court for the Northern District of West Virginia. The cases against Accord Healthcare Inc., Zydus Pharmaceuticals (USA) Inc. and Sandoz Inc. have been dismissed in the North Carolina, New Jersey and Colorado courts but will continue against those parties in Delaware. The cases against Par Pharmaceutical Inc. in both New York and Delaware have been dismissed because Par Pharmaceutical Inc.’s ANDA application has been withdrawn. The case against Stason Pharmaceuticals, Inc. in California has been dismissed, but the case against its partner Sawai USA, Inc. will proceed in Delaware. We expect a trial in the Delaware actions in December 2019, and a trial has been set in the West Virginia action in February 2020. Interference Proceeding with Forward Pharma In April 2015 the U.S. Patent and Trademark Office (USPTO) declared an interference between Forward Pharma’s pending U.S. Patent Application No. 11/576,871 and our U.S. Patent No. 8,399,514 (the '514 patent). The '514 patent includes claims covering the treatment of MS with 480 mg of dimethyl fumarate as provided for in our TECFIDERA label. In March 2017 the USPTO ruled against Forward Pharma. Forward Pharma has appealed to the U.S. Court of Appeals for the Federal Circuit and the appeal is pending. For additional information regarding this matter, please read Note 7, Intangibles Assets and Goodwill, to these consolidated financial statements. European Patent Office Oppositions In 2016 the EPO decided to revoke our European patent number 2 137 537 (the '537 patent), which we have appealed. The '537 patent includes claims covering the treatment of MS with 480 mg of dimethyl fumarate as provided for in our TECFIDERA label. In January 2018 the EPO announced its decision revoking Forward Pharma’s European Patent No. 2 801 355, which was issued in May 2015 and expires in October 2025. Forward Pharma has stated that it expects to file an appeal to the Technical Board of Appeal of the EPO. The settlement and license agreement that we entered with Forward Pharma in January 2017 did not resolve the issues pending in this proceeding and we and Forward Pharma intend to permit the Technical Board of Appeal and the Enlarged Board of Appeal, as applicable, to make a final determination. For additional information regarding this matter, please read Note 7, Intangibles Assets and Goodwill, to these consolidated financial statements. Patent Revocation Matter Swiss Pharma International AG filed actions in the District Court of The Hague (on January 11, 2016), the German Patents Court (on March 3, 2016) and the Commercial Court of Rome (November 2017) to invalidate the Dutch, German and Italian counterparts of our European Patent Number 1 485 127 (“Administration of agents to treat inflammation”) ('127 patent), which was issued in June 2011 and concerns administration of natalizumab (TYSABRI) to treat MS. The patent expires in February 2023. The Dutch counterpart was ruled invalid and we have appealed. In November 2018 Bioeq gmbh (an entity associated with Swiss Pharma and Polpharma) brought an action in the Polish Patent Office seeking to revoke the Polish counterpart of the ‘127 patent. In January 2018 the German court announced that the German counterpart was invalid. No date for a hearing on the merits has yet been set in the Italian action. '755 Patent Litigation On May 28, 2010, Biogen MA Inc. (formerly Biogen Idec MA Inc.) filed a complaint in the U.S. District Court for the District of New Jersey alleging infringement by Bayer Healthcare Pharmaceuticals Inc. (Bayer) (manufacturer, marketer and seller of BETASERON and manufacturer of EXTAVIA), EMD Serono, Inc. (EMD Serono) (manufacturer, marketer and seller of REBIF), Pfizer Inc. (Pfizer) (co-marketer of REBIF) and Novartis Pharmaceuticals Corp. (Novartis) (marketer and seller of EXTAVIA) of our U.S. Patent No. 7,588,755 ('755 Patent), which claims the use of interferon beta for immunomodulation or treating a viral condition, viral disease, cancers or tumors. The complaint seeks monetary damages, including lost profits and royalties. Bayer had previously filed a complaint against us in the same court, on May 27, 2010, seeking a declaratory judgment that it does not infringe the '755 Patent and that the patent is invalid, and seeking monetary relief in the form of attorneys' fees, costs and expenses. Bayer, Pfizer, Novartis and EMD Serono have all filed counterclaims seeking declaratory judgments of patent invalidity and non-infringement, and seeking monetary relief in the form of costs and attorneys' fees. The trial against EMD Serono and Pfizer commenced in mid-January 2018 and is ongoing. A trial date against Bayer and Novartis has not yet been set. Government Matters We have learned that state and federal governmental authorities are investigating our sales and promotional practices and have received related subpoenas. We are cooperating with the government. We have received subpoenas and other requests from the federal government for documents and information relating to our relationship with non-profit organizations that provide assistance to patients taking drugs sold by Biogen and Biogen's co-pay assistance programs. We are cooperating with the government. On July 1, 2016, we received civil investigative demands from the federal government for documents and information relating to our treatment of certain service agreements with wholesalers when calculating and reporting Average Manufacturer Prices in connection with the Medicaid Drug Rebate Program. We are cooperating with the government. In July 2017 we learned that the Prosecution Office of Milan is investigating our interactions with certain healthcare providers in Italy. We are cooperating with the government. 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.

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 these consolidated financial statements.

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

Qui Tam Litigation

On July 6, 2015, a qui tam action filed on behalf of the United States 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. Our motion to dismiss is pending. The United States has not made an intervention decision. An estimate of the possible loss or range of loss cannot be made at this time.

Securities Litigation

We and certain current and former officers are defendants in an action filed by a shareholder on October 20, 2016 in the U.S. District Court for the District of Massachusetts alleging violations of federal securities laws under 15 U.S.C §78j(b) and §78t(a) and 17 C.F.R. §240.10b-5 and seeking 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.

Other Matters

Abbreviated New Drug Application (ANDA) Litigation relating to TECFIDERA

In June, July, and September 2017 and January 2018, we initiated patent infringement proceedings pursuant to the Hatch-Waxman act in the U.S. District Court for the District of Delaware against Amneal Pharmaceuticals LLC, Aurobindo Pharma U.S.A., Inc., Caribe Holdings (Cayman) Co. Ltd. DBA Puracap Caribe, Puracap International LLC, Graviti Pharmaceuticals Pvt. Ltd., Hetero USA, Inc., Impax Laboratories, Inc., Prinston Pharmaceutical Inc., Slayback Pharma LLC, Teva Pharmaceuticals USA, Inc., Alkem Laboratories Ltd., Cipla Limited, Glenmark Pharmaceuticals Ltd., Lupin Atlantis Holdings SA, Macleods Pharmaceuticals, Ltd., MSN Laboratories Pvt. Ltd., Pharmathen S.A., Shipla Medicare Limited, Sun Pharma Global FZE, Torrent Pharmaceuticals Ltd., TWi Pharmaceuticals, Inc., Windlas Healthcare Pvt. Ltd., Accord Healthcare Inc., Par Pharmaceutical Inc., Sandoz Inc., Sawai USA, Inc. and Zydus Pharmaceuticals (USA) Inc. In addition, we initiated patent infringement proceedings pursuant to the Hatch-Waxman act against Stason Pharmaceuticals, Inc. in the U.S. District Court for the Central District of California, Zydus Pharmaceuticals (USA) Inc. in the U.S. District Court for the District of New Jersey, Accord Healthcare Inc. in the U.S. District Court for the Middle District of North Carolina, Par Pharmaceutical Inc. in the U.S. District Court for the Southern District of New York, Sandoz Inc. in the U.S. District Court for the District of Colorado and Mylan Pharmaceuticals Inc. in the U.S. District Court for the Northern District of West Virginia.

The cases against Accord Healthcare Inc., Zydus Pharmaceuticals (USA) Inc. and Sandoz Inc. have been dismissed in the North Carolina, New Jersey and Colorado courts but will continue against those parties in Delaware. The cases against Par Pharmaceutical Inc. in both New York and Delaware have been dismissed because Par Pharmaceutical Inc.’s ANDA application has been withdrawn. The case against Stason Pharmaceuticals, Inc. in California has been dismissed, but the case against its partner Sawai USA, Inc. will proceed in Delaware.

We expect a trial in the Delaware actions in December 2019, and a trial has been set in the West Virginia action in February 2020.

Interference Proceeding with Forward Pharma

In April 2015 the U.S. Patent and Trademark Office (USPTO) declared an interference between Forward Pharma’s pending U.S. Patent Application No. 11/576,871 and our U.S. Patent No. 8,399,514 (the '514 patent). The '514 patent includes claims covering the treatment of MS with 480 mg of dimethyl fumarate as provided for in our TECFIDERA label. In March 2017 the USPTO ruled against Forward Pharma. Forward Pharma has appealed to the U.S. Court of Appeals for the Federal Circuit and the appeal is pending. For additional information regarding this matter, please read Note 7, Intangibles Assets and Goodwill, to these consolidated financial statements.

European Patent Office Oppositions

In 2016 the EPO decided to revoke our European patent number 2 137 537 (the '537 patent), which we have appealed. The '537 patent includes claims covering the treatment of MS with 480 mg of dimethyl fumarate as provided for in our TECFIDERA label.

In January 2018 the EPO announced its decision revoking Forward Pharma’s European Patent No. 2 801 355, which was issued in May 2015 and expires in October 2025. Forward Pharma has stated that it expects to file an appeal to the Technical Board of Appeal of the EPO. The settlement and license agreement that we entered with Forward Pharma in January 2017 did not resolve the issues pending in this proceeding and we and Forward Pharma intend to permit the Technical Board of Appeal and the Enlarged Board of Appeal, as applicable, to make a final determination. For additional information regarding this matter, please read Note 7, Intangibles Assets and Goodwill, to these consolidated financial statements.

Patent Revocation Matter

Swiss Pharma International AG filed actions in the District Court of The Hague (on January 11, 2016), the German Patents Court (on March 3, 2016) and the Commercial Court of Rome (November 2017) to invalidate the Dutch, German and Italian counterparts of our European Patent Number 1 485 127 (“Administration of agents to treat inflammation”) ('127 patent), which was issued in June 2011 and concerns administration of natalizumab (TYSABRI) to treat MS. The patent expires in February 2023. The Dutch counterpart was ruled invalid and we have appealed. In November 2018 Bioeq gmbh (an entity associated with Swiss Pharma and Polpharma) brought an action in the Polish Patent Office seeking to revoke the Polish counterpart of the ‘127 patent. In January 2018 the German court announced that the German counterpart was invalid. No date for a hearing on the merits has yet been set in the Italian action.

'755 Patent Litigation

On May 28, 2010, Biogen MA Inc. (formerly Biogen Idec MA Inc.) filed a complaint in the U.S. District Court for the District of New Jersey alleging infringement by Bayer Healthcare Pharmaceuticals Inc. (Bayer) (manufacturer, marketer and seller of BETASERON and manufacturer of EXTAVIA), EMD Serono, Inc. (EMD Serono) (manufacturer, marketer and seller of REBIF), Pfizer Inc. (Pfizer) (co-marketer of REBIF) and Novartis Pharmaceuticals Corp. (Novartis) (marketer and seller of EXTAVIA) of our U.S. Patent No. 7,588,755 ('755 Patent), which claims the use of interferon beta for immunomodulation or treating a viral condition, viral disease, cancers or tumors. The complaint seeks monetary damages, including lost profits and royalties. Bayer had previously filed a complaint against us in the same court, on May 27, 2010, seeking a declaratory judgment that it does not infringe the '755 Patent and that the patent is invalid, and seeking monetary relief in the form of attorneys' fees, costs and expenses.

Bayer, Pfizer, Novartis and EMD Serono have all filed counterclaims seeking declaratory judgments of patent invalidity and non-infringement, and seeking monetary relief in the form of costs and attorneys' fees. The trial against EMD Serono and Pfizer commenced in mid-January 2018 and is ongoing. A trial date against Bayer and Novartis has not yet been set.

Government Matters

We have learned that state and federal governmental authorities are investigating our sales and promotional practices and have received related subpoenas. We are cooperating with the government.

We have received subpoenas and other requests from the federal government for documents and information relating to our relationship with non-profit organizations that provide assistance to patients taking drugs sold by Biogen and Biogen's co-pay assistance programs. We are cooperating with the government.

On July 1, 2016, we received civil investigative demands from the federal government for documents and information relating to our treatment of certain service agreements with wholesalers when calculating and reporting Average Manufacturer Prices in connection with the Medicaid Drug Rebate Program. We are cooperating with the government.

In July 2017 we learned that the Prosecution Office of Milan is investigating our interactions with certain healthcare providers in Italy. We are cooperating with the government.

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.