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The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

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For an unclassified balance sheet, amounts due from related parties including affiliates, employees, joint ventures, officers and stockholders, immediate families thereof, and pension funds.

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Collaborative and Other Relationships - ADUHELM Collaboration Agreement (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended				12 Months Ended
	Jun. 30, 2021	Mar. 31, 2019	Dec. 31, 2021	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Collaborative arrangements and non-collaborative arrangement transactions
Income (loss) from equity method investments							$ 34.9	$ 3.3	$ (77.4)
Royalty cost of sales							$ 2,109.7	1,805.2	1,955.4
ADUHELM
Collaborative arrangements and non-collaborative arrangement transactions
Royalty cost of sales			$ 164.0
Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of future development costs							45.00%
Loss on contract termination		$ 45.0
Additional milestone payment						$ 75.0	$ 100.0	75.0	0.0
Income (loss) from equity method investments					$ 33.8	$ 33.8	45.0	33.8	$ 0.0
Due from related parties			285.4				285.4	151.1
Due to related parties			$ 46.5				46.5	$ 111.9
Eisai \| Collaboration profit sharing
Collaborative arrangements and non-collaborative arrangement transactions
Income (loss) from equity method investments							$ 233.2
Eisai \| United States
Collaborative arrangements and non-collaborative arrangement transactions
Biogen share of co-promotion profits or losses							55.00%
Eisai \| European Union
Collaborative arrangements and non-collaborative arrangement transactions
Biogen share of co-promotion profits or losses							68.50%
Eisai \| JAPAN
Collaborative arrangements and non-collaborative arrangement transactions
Biogen share of co-promotion profits or losses							20.00%
Eisai \| ADUHELM
Collaborative arrangements and non-collaborative arrangement transactions
Additional milestone payment	$ 100.0
Income (loss) from equity method investments				$ 45.0			$ 45.0