Amount of expense for research and development. Includes, but is not limited to, cost for computer software product to be sold, leased, or otherwise marketed and writeoff of research and development assets acquired in transaction other than business combination or joint venture formation or both. Excludes write-down of intangible asset acquired in business combination or from joint venture formation or both, used in research and development activity.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 48
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482785/280-10-55-48

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 985
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481283/985-20-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 730
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482916/730-10-50-1

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 912
-SubTopic 730
-Name Accounting Standards Codification
-Section 25
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479532/912-730-25-1

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

Collaborative and Other Relationships - Eisai (Details) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Mar. 31, 2022	Feb. 28, 2022	Dec. 31, 2024	Dec. 31, 2022	Dec. 31, 2023	Jul. 31, 2023	Mar. 31, 2023
LEQEMBI Collaboration
Collaborative arrangements and non-collaborative arrangement transactions
Share of non-controling interest recognized			50.00%			50.00%
ADUHELM \| Maximum
Collaborative arrangements and non-collaborative arrangement transactions
Research and development expense				$ 335.0
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Accrued payment to termination agreement							$ 31.0
Payments made to termination agreement					$ 16.0
Related cost-sharing reduction (as a percentage)			80.00%
Eisai \| LEQEMBI Collaboration
Collaborative arrangements and non-collaborative arrangement transactions
Term of collaboration agreement	10 years	5 years