The costs incurred in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, excluding in-process research and development acquired in a business combination consummated during the period. Excludes software research and development, which has a separate concept.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373

Name:	us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Collaborative and Other Relationships - Bristol Myers Squibb (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Research and development		$ 3,990.9	$ 2,280.6	$ 2,597.2
Bristol-Myers Squibb
Collaborative arrangements and non-collaborative arrangement transactions
Research and development	$ 300.0	62.4	$ 144.0	$ 97.0
Total aducanumab collaboration third party milestone expense		360.0
iPerian
Collaborative arrangements and non-collaborative arrangement transactions
Total aducanumab collaboration third party milestone expense		$ 370.0