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Collaborative and Other Relationships - Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Total revenues from anti-CD20 therapeutic programs	$ 2,852.6	$ 3,376.1	$ 3,681.6	$ 3,534.3	$ 3,671.3	$ 3,600.1	$ 3,616.7	$ 3,489.8	$ 13,444.6	$ 14,377.9	$ 13,452.9
Revenues from anti-CD20 therapeutic programs
Collaborative arrangements and non-collaborative arrangement transactions
Total revenues from anti-CD20 therapeutic programs	$ 419.0	$ 560.1	$ 478.3	$ 520.4	$ 600.8	$ 595.8	$ 576.4	$ 517.4	1,977.8	2,290.4	1,980.2
Genentech
Collaborative arrangements and non-collaborative arrangement transactions
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA, including the reimbursement of selling and development expenses									1,080.2	1,542.4	1,431.9
Other revenues from anti-CD20 therapeutic programs									$ 897.6	$ 748.0	$ 548.3

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- Definition

Other revenues from anti-CD20 therapeutic programs consists of revenue on sales of RITUXAN outside the U.S., which consist of our share of pre-tax co-promotion profits in Canada and royalty revenue on sales of Ocrevus.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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