Acquisitions	12 Months Ended
Dec. 31, 2017
Business Combinations [Abstract]
Acquisitions	Acquisitions Remedy Pharmaceuticals Inc. In May 2017 we completed an asset purchase of the Phase 3-ready candidate, BIIB093 (intravenous glibencamide) (formerly known as CIRARA), from Remedy. The target indication for BIIB093 is large hemispheric infarction (LHI), a severe form of ischemic stroke where brain swelling (cerebral edema) often leads to a disproportionately large share of stroke-related morbidity and mortality. The U.S. Food and Drug Administration (FDA) recently granted BIIB093 Orphan Drug designation for severe cerebral edema in patients with acute ischemic stroke. The FDA has also granted BIIB093 Fast Track designation. Under this agreement, we are responsible for the future development and commercialization of BIIB093 and Remedy will share in the cost of development for the target indication for BIIB093 in LHI stroke. We accounted for this transaction as an asset acquisition as we did not acquire any employees from Remedy nor did we acquire any significant processes required in the development of BIIB093. In connection with the closing of this transaction, we made an upfront payment of $120.0 million to Remedy, which was recorded as acquired in-process research and development in our consolidated statements of income as BIIB093 has not yet reached technological feasibility. We also have agreed to pay Remedy certain development and sales based milestone payments that are substantially payable upon or after regulatory approval, as well as royalties on future commercial sales. Convergence Pharmaceuticals In February 2015, we completed our acquisition of all of the outstanding stock of Convergence Pharmaceuticals (Convergence), a clinical-stage biopharmaceutical company with a focus on developing product candidates for neuropathic pain. Convergence’s lead candidate was a Phase 2 clinical candidate BIIB074 (formerly known as CNV1014802), which had demonstrated clinical activity in proof-of-concept studies for TGN. Additionally, BIIB074 had potential applicability in several other neuropathic pain states, including lumbosacral radiculopathy and erythromelalgia. The purchase price consisted of a $200.1 million cash payment at closing, plus contingent consideration in the form of development and approval milestones up to a maximum of $450.0 million, of which $350.0 million is associated with the development and approval of BIIB074 for the treatment of TGN. In connection with the closing of this transaction, we recorded a liability of $274.5 million representing the fair value of the contingent consideration resulting in an adjusted purchase price of $474.6 million. The separately identifiable assets and liabilities acquired were primarily comprised of $424.6 million and $128.3 million attributed to in-process research and development and goodwill, respectively. These amounts were partially offset by the establishment of a deferred tax liability for the acquired IPR&D intangible assets which had no tax basis. We attributed the goodwill recognized to the Convergence workforce's expertise in chronic pain research and clinical development and to establishing a deferred tax liability for the acquired IPR&D intangible assets. The goodwill was not tax deductible.

Acquisitions

Remedy Pharmaceuticals Inc.

In May 2017 we completed an asset purchase of the Phase 3-ready candidate, BIIB093 (intravenous glibencamide) (formerly known as CIRARA), from Remedy. The target indication for BIIB093 is large hemispheric infarction (LHI), a severe form of ischemic stroke where brain swelling (cerebral edema) often leads to a disproportionately large share of stroke-related morbidity and mortality. The U.S. Food and Drug Administration (FDA) recently granted BIIB093 Orphan Drug designation for severe cerebral edema in patients with acute ischemic stroke. The FDA has also granted BIIB093 Fast Track designation.

Under this agreement, we are responsible for the future development and commercialization of BIIB093 and Remedy will share in the cost of development for the target indication for BIIB093 in LHI stroke.

We accounted for this transaction as an asset acquisition as we did not acquire any employees from Remedy nor did we acquire any significant processes required in the development of BIIB093. In connection with the closing of this transaction, we made an upfront payment of $120.0 million to Remedy, which was recorded as acquired in-process research and development in our consolidated statements of income as BIIB093 has not yet reached technological feasibility. We also have agreed to pay Remedy certain development and sales based milestone payments that are substantially payable upon or after regulatory approval, as well as royalties on future commercial sales.

Convergence Pharmaceuticals

In February 2015, we completed our acquisition of all of the outstanding stock of Convergence Pharmaceuticals (Convergence), a clinical-stage biopharmaceutical company with a focus on developing product candidates for neuropathic pain. Convergence’s lead candidate was a Phase 2 clinical candidate BIIB074 (formerly known as CNV1014802), which had demonstrated clinical activity in proof-of-concept studies for TGN. Additionally, BIIB074 had potential applicability in several other neuropathic pain states, including lumbosacral radiculopathy and erythromelalgia.

The purchase price consisted of a $200.1 million cash payment at closing, plus contingent consideration in the form of development and approval milestones up to a maximum of $450.0 million, of which $350.0 million is associated with the development and approval of BIIB074 for the treatment of TGN. In connection with the closing of this transaction, we recorded a liability of $274.5 million representing the fair value of the contingent consideration resulting in an adjusted purchase price of $474.6 million. The separately identifiable assets and liabilities acquired were primarily comprised of $424.6 million and $128.3 million attributed to in-process research and development and goodwill, respectively. These amounts were partially offset by the establishment of a deferred tax liability for the acquired IPR&D intangible assets which had no tax basis.

We attributed the goodwill recognized to the Convergence workforce's expertise in chronic pain research and clinical development and to establishing a deferred tax liability for the acquired IPR&D intangible assets. The goodwill was not tax deductible.