Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 3,616.7	$ 3,356.5	$ 7,106.5	$ 6,487.6
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA	377.2	359.0	768.0	708.6
Other revenues from anti-CD20 therapeutic programs	199.2	131.4	325.8	225.0
Revenues from anti-CD20 therapeutic programs
Disaggregation of Revenue [Line Items]
Total revenues from anti-CD20 therapeutic programs	$ 576.4	$ 490.4	$ 1,093.8	$ 933.6