Subsequent Events	9 Months Ended
Sep. 30, 2017
Subsequent Events [Abstract]
Subsequent Events	20. Subsequent Events Eisai Co., Ltd. BAN2401 and E2609 Collaboration In March 2014, we entered into a collaboration agreement (Eisai Collaboration Agreement) with Eisai Co., Ltd. (Eisai) to jointly develop and commercialize two Eisai product candidates for the treatment of AD, BAN2401, a monoclonal antibody that targets amyloid-beta aggregates, and E2609, a BACE inhibitor. Under the Eisai Collaboration Agreement, Eisai had an option to jointly develop and commercialize aducanumab, our anti-amyloid beta antibody candidate for AD (Aducanumab Option). On October 23, 2017, Eisai exercised its Aducanumab Option and we entered into a new collaboration agreement (Aducanumab Agreement) for the joint development and commercialization of aducanumab. Under the Aducanumab Agreement, the two companies will co-promote aducanumab with a region-based profit split. We will receive a 55% share of the potential profits (losses) in the U.S., a 68.5% share of the potential profits (losses) in the E.U. and a 20% share of the potential profits (losses) in Japan and Asia, excluding China and South Korea. The companies will continue to share equally in the potential profits (losses) in rest of world markets. Under the Aducanumab Agreement, we will continue to lead the ongoing Phase 3 development of aducanumab and will remain responsible for 100% of development costs for aducanumab incurred in support of the agreement until April 2018. Eisai will then reimburse us for 15% of development expenses for the period April 2018 through December 2018, and 45% thereafter. In addition, both companies will continue to jointly develop BAN2401 and E2609; however, we are no longer required to pay Eisai any milestone payments for products containing BAN2401 and we are no longer entitled to any potential development and commercial milestone payments from Eisai in relation to aducanumab. We and Eisai have also agreed to co-promote AVONEX, TYSABRI and TECFIDERA in Japan in certain settings and Eisai will distribute AVONEX, TYSABRI, TECFIDERA and PLEGRIDY in India and other Asia-Pacific markets (excluding China). For additional information related to our collaboration arrangement with Eisai, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2016 Form 10-K. Neurimmune On October 23, 2017, we amended the terms of our existing collaboration agreement with Neurimmune. Pursuant to the amended agreement, we agreed to make a one-time $150.0 million payment to Neurimmune in exchange for a 15% reduction in royalty rates payable on potential commercial sales of aducanumab. Our royalty rates payable on potential commercial sales of aducanumab will now range from the high single digits to low-teens. This payment will be recognized in the fourth quarter of 2017. Under the amended agreement, we also have an option that will expire in April 2018 to further reduce our royalty rate payable on potential commercial sales of aducanumab by an additional 5% in exchange for a one-time $50.0 million payment to Neurimmune. Under the terms of the Aducanumab Agreement, Eisai may elect to share in the benefit and cost of the royalty reduction. For additional information relating to our collaboration agreement with Neurimmune, please read Note 17, Investments in Variable Interest Entities, to these condensed consolidated financial statements.

20. Subsequent Events

Eisai Co., Ltd.

BAN2401 and E2609 Collaboration

In March 2014, we entered into a collaboration agreement (Eisai Collaboration Agreement) with Eisai Co., Ltd. (Eisai) to jointly develop and commercialize two Eisai product candidates for the treatment of AD, BAN2401, a monoclonal antibody that targets amyloid-beta aggregates, and E2609, a BACE inhibitor. Under the Eisai Collaboration Agreement, Eisai had an option to jointly develop and commercialize aducanumab, our anti-amyloid beta antibody candidate for AD (Aducanumab Option).

On October 23, 2017, Eisai exercised its Aducanumab Option and we entered into a new collaboration agreement (Aducanumab Agreement) for the joint development and commercialization of aducanumab. Under the Aducanumab Agreement, the two companies will co-promote aducanumab with a region-based profit split. We will receive a 55% share of the potential profits (losses) in the U.S., a 68.5% share of the potential profits (losses) in the E.U. and a 20% share of the potential profits (losses) in Japan and Asia, excluding China and South Korea. The companies will continue to share equally in the potential profits (losses) in rest of world markets.

Under the Aducanumab Agreement, we will continue to lead the ongoing Phase 3 development of aducanumab and will remain responsible for 100% of development costs for aducanumab incurred in support of the agreement until April 2018. Eisai will then reimburse us for 15% of development expenses for the period April 2018 through December 2018, and 45% thereafter.

In addition, both companies will continue to jointly develop BAN2401 and E2609; however, we are no longer required to pay Eisai any milestone payments for products containing BAN2401 and we are no longer entitled to any potential development and commercial milestone payments from Eisai in relation to aducanumab.

We and Eisai have also agreed to co-promote AVONEX, TYSABRI and TECFIDERA in Japan in certain settings and Eisai will distribute AVONEX, TYSABRI, TECFIDERA and PLEGRIDY in India and other Asia-Pacific markets (excluding China).

For additional information related to our collaboration arrangement with Eisai, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2016 Form 10-K.

Neurimmune

On October 23, 2017, we amended the terms of our existing collaboration agreement with Neurimmune. Pursuant to the amended agreement, we agreed to make a one-time $150.0 million payment to Neurimmune in exchange for a 15% reduction in royalty rates payable on potential commercial sales of aducanumab. Our royalty rates payable on potential commercial sales of aducanumab will now range from the high single digits to low-teens. This payment will be recognized in the fourth quarter of 2017.

Under the amended agreement, we also have an option that will expire in April 2018 to further reduce our royalty rate payable on potential commercial sales of aducanumab by an additional 5% in exchange for a one-time $50.0 million payment to Neurimmune.

Under the terms of the Aducanumab Agreement, Eisai may elect to share in the benefit and cost of the royalty reduction.

For additional information relating to our collaboration agreement with Neurimmune, please read Note 17, Investments in Variable Interest Entities, to these condensed consolidated financial statements.