Collaborative and Other Relationships (Details) (USD $)	3 Months Ended													12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012		Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenues from unconsolidated joint business
Revenue on sales in the rest of world for RITUXAN	$ 1,607,100,000	[1],[2]	$ 1,453,600,000	$ 1,385,900,000	[3]	$ 1,095,800,000	[4]	$ 1,074,700,000	[5]	$ 1,039,100,000	$ 1,076,800,000	[6]	$ 975,400,000	$ 5,542,331,000	$ 4,166,074,000	$ 3,836,117,000
Total unconsolidated joint business revenues	269,400,000	[1],[2]	303,200,000	288,800,000	[3]	264,600,000	[4]	280,900,000	[5]	287,800,000	284,600,000	[6]	284,600,000	1,126,017,000	1,137,923,000	996,597,000
U.S
Revenues from unconsolidated joint business
Revenue on sales in the rest of world for RITUXAN														3,581,000,000	2,176,800,000	1,954,800,000
Roche Group - Genentech
Revenues from unconsolidated joint business
Total unconsolidated joint business revenues														1,126,000,000	1,137,900,000	996,600,000
Roche Group - Genentech | U.S
Revenues from unconsolidated joint business
Biogen Idec's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA														1,085,200,000	1,031,700,000	872,700,000
Reimbursement of our selling and development expenses in the U.S. for RITUXAN														2,100,000	1,600,000	6,100,000
GAZYVA [Member]
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														$ 38,700,000	$ 104,600,000	$ 117,800,000

[1] Net income and net income attributable to Biogen Idec Inc. for the first quarter of 2012 includes a charge to research and development expense of $29.0 million related to an upfront payment made in connection with our development agreement entered into with Isis.

[2] Net income and net income attributable to Biogen Idec Inc., for the third quarter of 2013, includes a charge to research and development expense of $75.0 million related to an upfront payment made in connection with our collaboration agreement entered into with Isis.

[3] Product revenues and total revenues for the second, third and fourth quarters of 2013 includes 100% of net revenues related to sales of TYSABRI as a result of our acquisition of TYSABRI rights from Elan on April 2, 2013 and net revenues related to sales of TECFIDERA, our new oral first-line treatment for people with relapsing forms of multiple sclerosis (MS), which was approved by the FDA in March 2013 and commenced commercial sales in April 2013.

[4] Our share of revenues from unconsolidated joint business reflects a charge of $41.5 million for damages and interest awarded to Hoechst in Genentech's arbitration with Hoechst for RITUXAN.

[5] Net income and net income attributable to Biogen Idec Inc. for the fourth quarter of 2012 includes the correction of an error that had accumulated over several prior years in our deferred tax accounting for capitalized interest which resulted in an expense of $29.0 million.

[6] Net income and net income attributable to Biogen Idec Inc. for the fourth quarter of 2012 includes a charge to research and development expense of $30.0 million related to an upfront payment made in connection with our development agreement entered into with Isis.