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Acquisitions	9 Months Ended
Acquisitions	Sep. 30, 2021
Business Combinations [Abstract]
Acquisitions	BIIB118 Acquisition In March 2020 we acquired BIIB118 (CK1 inhibitor) for the potential treatment of patients with behavioral and neurological symptoms across various psychiatric and neurological diseases from Pfizer Inc. (Pfizer). We are developing BIIB118 for the potential treatment of irregular sleep wake rhythm disorder in Parkinson's disease and plan to develop BIIB118 for the potential treatment of sundowning in Alzheimer's disease. In connection with this acquisition, we made an upfront payment of $75.0 million to Pfizer, which was accounted for as an asset acquisition and recorded as acquired in-process research and development (IPR&D) in our condensed consolidated statements of income as BIIB118 has not yet reached technological feasibility. We may also pay Pfizer up to $635.0 million in potential additional development and commercialization milestone payments as well as tiered royalties in the high single digits to sub-teens.