Collaborative and Other Relationships (Details) (USD $)	3 Months Ended													12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011		Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues from unconsolidated joint business
Revenue on sales of RITUXAN in the rest of world	$ 1,074,700,000	[1],[2]	$ 1,039,100,000	$ 1,076,800,000	[3]	$ 975,400,000	[4]	$ 996,600,000	[5]	$ 975,800,000	$ 956,700,000	[6]	$ 907,100,000	$ 4,166,074,000	$ 3,836,117,000	$ 3,470,056,000
Total unconsolidated joint business revenues	280,900,000	[1],[2]	287,800,000	284,600,000	[3]	284,600,000	[4]	257,500,000	[5]	266,500,000	216,500,000	[6]	256,100,000	1,137,923,000	996,597,000	1,077,244,000
U.S
Revenues from unconsolidated joint business
Revenue on sales of RITUXAN in the rest of world														2,176,800,000	1,954,800,000	1,744,400,000
Roche Group - Genentech
Revenues from unconsolidated joint business
Total unconsolidated joint business revenues														1,137,900,000	996,600,000	1,077,200,000
Roche Group - Genentech | U.S
Revenues from unconsolidated joint business
Biogen Idec's share of pre-tax co-promotion profits in the U.S														1,031,700,000	872,700,000	848,000,000
Reimbursement of our selling and development expenses in the U.S														1,600,000	6,100,000	58,300,000
RITUXAN
Co-promotion profit sharing formula
First $50.0 million	30.00%													30.00%
Above $50.0 million	40.00%							40.00%						40.00%	40.00%	40.00%
RITUXAN | Roche Group - Genentech | Outside the U.S
Revenues from unconsolidated joint business
Revenue on sales of RITUXAN in the rest of world														$ 104,600,000	$ 117,800,000	$ 170,900,000
RITUXAN | After First New Product FDA Approval
Co-promotion profit sharing formula
First $50.0 million	30.00%													30.00%
Above $50.0 million	0.00%													0.00%
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%
Until Second GA101 Threshold Date	0.00%													0.00%
After Second GA101 Threshold Date	0.00%													0.00%
First Non CLL GA 101 FD Approval Occurs First | RITUXAN | Before First New Product FDA Approval
Co-promotion profit sharing formula
First $50.0 million	30.00%													30.00%
Above $50.0 million	0.00%													0.00%
Until First GA101 Threshold Date	39.00%													39.00%
After First GA101 Threshold Date
Until First Threshold Date	0.00%													0.00%
After First Threshold Date and until Second Threshold Date	0.00%													0.00%
After Second Threshold Date	0.00%													0.00%
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 | RITUXAN | Before First New Product FDA Approval
Co-promotion profit sharing formula
First $50.0 million	30.00%													30.00%
Above $50.0 million	35.00%													35.00%
Until First GA101 Threshold Date	0.00%													0.00%
After First GA101 Threshold Date
Until First Threshold Date	0.00%													0.00%
After First Threshold Date and until Second Threshold Date	0.00%													0.00%
After Second Threshold Date	0.00%													0.00%
Until Second GA101 Threshold Date	0.00%													0.00%
After Second GA101 Threshold Date	0.00%													0.00%

[1] 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 Pharmaceuticals, Inc.

[2] 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.

[3] Net income and net income attributable to Biogen Idec Inc. for the second quarter of 2012 includes a charge to research and development expense of $12.0 million related to an upfront payment made in connection with our development agreement entered into with Isis Pharmaceuticals, Inc.

[4] 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 Pharmaceuticals, Inc. and a $12.4 million reduction resulting from an increase in our returns reserve and write-offs of unsold inventory due to a voluntary withdrawal of a limited amount of AVONEX product that has demonstrated a trend in oxidation that may lead to expiry earlier than stated on its label.

[5] Net income and net income attributable to Biogen Idec Inc. for the fourth quarter of 2011 includes a charge to research and development expense of $36.8 million related to an upfront payment made in connection with our collaboration and license agreement entered into with Portola Pharmaceuticals, Inc.

[6] Our share of RITUXAN revenues from unconsolidated joint business was reduced by approximately $50.0 million in the second quarter of 2011 as a result of an accrual for estimated compensatory damages (including interest) relating to Genentech’s ongoing arbitration with Hoechst GmbH. For additional information related to this matter, please read Note 22, Litigation to these consolidated financial statements.