Collaborative and Other Relationships (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from unconsolidated joint business
Revenue on sales in the rest of world for RITUXAN	$ 2,712.4	$ 2,622.5	$ 2,639.7	$ 2,380.1	$ 2,502.9	$ 2,539.6	$ 2,466.0	$ 2,309.4	$ 10,354.7	$ 9,817.9	$ 9,188.5
Total revenues from anti-CD20 therapeutic programs	$ 415.0	$ 406.5	$ 397.1	$ 340.6	$ 318.2	$ 317.6	$ 349.2	$ 329.5	1,559.2	1,314.5	1,339.2
Roche Group - Genentech
Revenues from unconsolidated joint business
Biogen's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA, including the reimbursement of selling and development expenses									1,316.4	1,249.5	1,269.8
Revenue on sales in the rest of world for RITUXAN									242.8	65.0	69.4
Total revenues from anti-CD20 therapeutic programs									$ 1,559.2	$ 1,314.5	$ 1,339.2
GAZYVA
After First GA101 Threshold Date
Until First GAZYVA Threshold Date	39.00%								39.00%
After First Threshold Date and until Second Threshold Date	37.50%								37.50%
After Second Threshold Date	35.00%								35.00%
RITUXAN
Co-promotion profit sharing formula
Until GAZYVA First Non-CLL FDA Approval	40.00%								40.00%		40.00%
After First GA101 Threshold Date
Until First GAZYVA Threshold Date	39.00%				39.00%				39.00%	39.00%
After First Threshold Date and until Second Threshold Date	37.50%								37.50%
After Second Threshold Date	35.00%								35.00%