Collaborations (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Co-promotion profit sharing formula
Above $50.0 million	40.00%				40.00%				40.00%	40.00%	40.00%
Revenues from unconsolidated joint business
Revenue on sales of RITUXAN in the rest of world	$ 996,600,000	$ 975,800,000	$ 956,700,000	$ 907,100,000	$ 909,800,000	$ 876,900,000	$ 859,200,000	$ 824,200,000	$ 3,836,117,000	$ 3,470,056,000	$ 3,152,941,000
Total unconsolidated joint business revenues	257,500,000	266,500,000	216,500,000	256,100,000	258,000,000	258,000,000	306,400,000	254,900,000	996,597,000	1,077,244,000	1,094,863,000
U.S [Member]
Revenues from unconsolidated joint business
Revenue on sales of RITUXAN in the rest of world									1,954,800,000	1,744,400,000	1,638,000,000
Roche Group - Genentech [Member]
Revenues from unconsolidated joint business
Biogen Idec's share of pre-tax co-promotion profits in the U.S									872,700,000
Total unconsolidated joint business revenues									996,600,000	1,077,200,000	1,094,900,000
Roche Group - Genentech [Member] | U.S [Member]
Revenues from unconsolidated joint business
Biogen Idec's share of pre-tax co-promotion profits in the U.S										848,000,000	773,600,000
Reimbursement of our selling and development expenses in the U.S									6,100,000	58,300,000	65,600,000
RITUXAN [Member]
Co-promotion profit sharing formula
First $50.0 million	30.00%								30.00%
RITUXAN [Member] | Roche Group - Genentech [Member] | Outside the U.S [Member]
Revenues from unconsolidated joint business
Revenue on sales of RITUXAN in the rest of world									$ 117,800,000	$ 170,900,000	$ 255,700,000
RITUXAN [Member] | After First New Product FDA Approval [Member]
Co-promotion profit sharing formula
First $50.0 million	30.00%								30.00%
Until First GA101 Threshold Date	38.00%								38.00%
After First GA101 Threshold Date
Until First Threshold Date	37.50%								37.50%
After First Threshold Date and until Second Threshold Date	35.00%								35.00%
After Second Threshold Date	30.00%								30.00%
First Non CLL GA 101 FD Approval Occurs First | RITUXAN [Member] | Before First New Product FDA Approval [Member]
Co-promotion profit sharing formula
First $50.0 million	30.00%								30.00%
Until First GA101 Threshold Date	39.00%								39.00%
After First GA101 Threshold Date
Until Second GA101 Threshold Date	37.50%								37.50%
After Second GA101 Threshold Date	35.00%								35.00%
GA 101 CLL Sales Trigger Occurs First [Member] | RITUXAN [Member] | Before First New Product FDA Approval [Member]
Co-promotion profit sharing formula
First $50.0 million	30.00%								30.00%
Above $50.0 million	35.00%								35.00%