Revenues Revenues from Anti-CD20 Therapeutic Programs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Other revenues from anti-cd20 therapeutic programs	$ 2,523.5	$ 2,380.1
Total revenues from anti-CD20 therapeutic programs	443.2	340.6
Genentech
Disaggregation of Revenue [Line Items]
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA, including the reimbursement of selling and development expenses	349.6	323.5
Other revenues from anti-cd20 therapeutic programs	93.6	17.1
Total revenues from anti-CD20 therapeutic programs	$ 443.2	$ 340.6