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Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 2,775.0	$ 3,681.6	$ 5,469.0	$ 7,215.9
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	178.8	257.5	352.9	598.8
Other revenues from anti-CD20 therapeutic programs	261.2	220.8	476.1	399.9
Revenue from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 440.0	$ 478.3	$ 829.0	$ 998.7

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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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