Collaborative and Other Relationships (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues from unconsolidated joint business
Revenue on sales in the rest of world for RITUXAN	$ 2,425.9	$ 2,391.7	$ 2,198.6	$ 2,172.3	$ 2,287.0	$ 2,117.3	$ 2,056.3	$ 1,742.8	$ 9,188.5	$ 8,203.4	$ 5,542.3
Total unconsolidated joint business revenues	$ 333.9	$ 337.2	$ 337.5	$ 330.6	$ 304.5	$ 290.7	$ 303.3	$ 296.9	1,339.2	1,195.4	1,126.0
U.S
Revenues from unconsolidated joint business
Revenue on sales in the rest of world for RITUXAN									6,545.8	5,566.7	3,581.0
Roche Group - Genentech
Revenues from unconsolidated joint business
Total unconsolidated joint business revenues									1,339.2	1,195.4	1,126.0
Roche Group - Genentech | U.S
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									$ 1,269.8	1,117.1	1,087.3
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%
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 | Roche Group - Genentech | Outside the U.S
Revenues from unconsolidated joint business
Revenue on sales in the rest of world for RITUXAN									$ 69.4	$ 78.3	$ 38.7