Collaborative and Other Relationships (Details Textual)	3 Months Ended											12 Months Ended			3 Months Ended	12 Months Ended		12 Months Ended			3 Months Ended	12 Months Ended							3 Months Ended		12 Months Ended	3 Months Ended		12 Months Ended					12 Months Ended		3 Months Ended	12 Months Ended			12 Months Ended												3 Months Ended	12 Months Ended			3 Months Ended	12 Months Ended			1 Months Ended		12 Months Ended					12 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2014 USD ($)		Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)		Mar. 31, 2014 USD ($)		Dec. 31, 2013 USD ($)	Sep. 30, 2013 USD ($)	Jun. 30, 2013 USD ($)	Mar. 31, 2013 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Jun. 30, 2014 Biogen Idec Hemophilia USD ($)	Dec. 31, 2010 Biogen Idec Hemophilia USD ($)	Jan. 31, 2007 Biogen Idec Hemophilia	Dec. 31, 2013 Genentech USD ($)	Dec. 31, 2013 Elan USD ($)	Dec. 31, 2012 Elan USD ($)	Sep. 30, 2011 Acorda USD ($)	Dec. 31, 2014 Acorda USD ($)	Dec. 31, 2013 Acorda USD ($)	Dec. 31, 2012 Acorda USD ($)	Dec. 31, 2014 Swedish Orphan Biovitrum USD ($)	Dec. 31, 2014 AbbVie USD ($)	Dec. 31, 2013 AbbVie USD ($)	Dec. 31, 2012 AbbVie USD ($)	Sep. 30, 2013 ISIS Pharmaceuticals USD ($)	Dec. 31, 2012 ISIS Pharmaceuticals USD ($)	Dec. 31, 2014 ISIS Pharmaceuticals USD ($)	Jun. 30, 2014 Eisai USD ($)	Mar. 31, 2014 Eisai USD ($)	Dec. 31, 2014 Eisai USD ($)	Dec. 31, 2013 Eisai USD ($)	Dec. 31, 2012 Eisai USD ($)	Dec. 31, 2014 Sangamo BioSciences USD ($)	Mar. 31, 2014 Sangamo BioSciences USD ($)	Dec. 31, 2014 Other research and discovery USD ($)	Dec. 31, 2013 Other research and discovery USD ($)	Dec. 31, 2013 Samsung Biosimilar Agreement USD ($)	Dec. 31, 2014 RITUXAN	Dec. 31, 2013 RITUXAN	Dec. 31, 2012 RITUXAN	Dec. 31, 2014 RITUXAN Maximum	Dec. 31, 2014 RITUXAN Minimum	Dec. 31, 2014 GAZYVA USD ($)	Dec. 31, 2014 New Anti-CD20 Maximum	Dec. 31, 2014 New Anti-CD20 Minimum	Dec. 31, 2014 ELOCTATE USD ($)	Dec. 31, 2013 ELOCTATE USD ($)	Dec. 31, 2012 ELOCTATE USD ($)	Dec. 31, 2014 ELOCTATE Swedish Orphan Biovitrum USD ($)	Dec. 31, 2014 ALPROLIX Swedish Orphan Biovitrum USD ($)	Dec. 31, 2014 Ocrelizumab Maximum	Dec. 31, 2014 Ocrelizumab Minimum	Jun. 30, 2012 DMPK ISIS Pharmaceuticals USD ($)	Dec. 31, 2014 DMPK ISIS Pharmaceuticals USD ($)	Dec. 31, 2013 DMPK ISIS Pharmaceuticals USD ($)	Dec. 31, 2012 DMPK ISIS Pharmaceuticals USD ($)	Mar. 31, 2012 SMNRx ISIS Pharmaceuticals USD ($)	Dec. 31, 2014 SMNRx ISIS Pharmaceuticals USD ($)	Dec. 31, 2013 SMNRx ISIS Pharmaceuticals USD ($)	Dec. 31, 2012 SMNRx ISIS Pharmaceuticals USD ($)	Feb. 29, 2012 Samsung Biosimilar Agreement USD ($)	Feb. 29, 2012 Samsung Biosimilar Agreement KRW	Dec. 31, 2014 Samsung Biosimilar Agreement USD ($)	Dec. 31, 2013 Samsung Biosimilar Agreement USD ($)	Dec. 31, 2012 Samsung Biosimilar Agreement USD ($)	Dec. 31, 2014 Samsung Biosimilar Agreement KRW	Dec. 31, 2013 Samsung Biosimilar Agreement KRW	Dec. 31, 2013 New Anti-CD20 Third Party Anti CD-20 Roche Group - Genentech USD ($)	Dec. 31, 2012 New Anti-CD20 Third Party Anti CD-20 Roche Group - Genentech USD ($)	Mar. 31, 2013 Hoechst USD ($)	Dec. 31, 2013 Hoechst USD ($)	Mar. 31, 2014 Research and development Eisai USD ($)
Collaborative arrangements and non-collaborative arrangement transactions
Royalties Terms Of Collaboration Agreement																																										10% and 12% on sales of RITUXAN outside the U.S. and Canada, with the royalty period lasting 11 years
Collaborative and Other Relationships (Textual)
Percentage of royalties as per collaboration																																													12.00%	10.00%
Period of collaboration agreement																																										11 years
Percentage of future development and commercialization expenses payable related to GAZYVA																																															35.00%
Future royalties percentage to be received on sale of ocrelizumab																																																							24.00%	13.50%
Percentage of Co promotion Operating Profits first fifty million																																										30.00%					35.00%
Co-promotion operating profit threshold for Rituxan in U S and Canada to determine share of co promotion operating profit prior to amendment																																															$ 50,000,000
Limit of gross sale of GAZYVA to be achieved in preceding 12 months under option one																																															150,000,000
Limit of gross sale of GAZYVA to be achieved in any 12 months under option one																																															150,000,000
Sales Trigger Gross Sales Threshold																																															500,000,000
Future percentage of co-promotion operating profits																																																38.00%	30.00%
Percentage Of Co Promotion Operating Losses																																															35.00%
Decrease In Share Of Co Promotion Profits Due To Estimated Compensation Damages																																																																										41,500,000	49,700,000
Revenue on sales in the rest of world for RITUXAN	2,287,000,000	[1]	2,117,300,000	2,056,300,000	[2]	1,742,800,000	[3]	1,607,100,000	1,453,600,000	1,385,900,000	1,095,800,000	8,203,404,000	5,542,331,000	4,166,074,000				41,200,000																																58,400,000	0	0
Biogen Idec's share of pre-tax profits in the U.S. for RITUXAN and GAZYVA																		8,500,000
Percentage of share of Co promotion profits exceeding $50 million																																										40.00%	40.00%	40.00%
Research and development												1,893,422,000	1,444,053,000	1,334,919,000															75,000,000	30,000,000	20,000,000	21,600,000	117,700,000						40,000,000	4,000,000	36,000,000												175,000,000	150,000,000			12,000,000	14,000,000	10,000,000		29,000,000	57,300,000										25,700,000	35,400,000
Reimbursements of research and developments expense from Elan																			11,700,000	43,700,000
Reimbursements of selling general and administrative expenses from Elan																			20,600,000	99,900,000
Amount reflected in collaboration profit sharing line for collaboration																			85,400,000	317,900,000
Total milestone payments																					25,000,000
Expected additional milestone payments for successful achievement of regulatory and commercial sales milestones																						375,000,000
Expected additional milestone payments when certain sales threshold is met																						15,000,000
Foreign sales required to trigger milestone																						100,000,000
Biogen Idec's share of expense reflected within our consolidation statements of income																						29,200,000	24,300,000	20,200,000		67,400,000	71,000,000	65,600,000						29,100,000	0	0	28,900,000																					10,900,000	11,200,000	22,000,000		27,700,000	13,600,000	39,600,000
Stock acquired in acquisition																	100.00%
Consideration per product																									10,000,000
Percentage of reimbursement expenses																									50.00%
Percentage of development reimbursement expenses																									100.00%
Reimbursement cost achieving period																									6 years
Time period for paying remaining balance due																									90 days
Term for revocation option right																									18 months
Estimated additional payments upon achievement of development and commercial milestones.																										60,000,000																																				325,000,000
Expected milestone payment due upon filing with the FDA																										16,000,000
Term of collaboration agreement																													6 years
Total upfront and milestone payments made to collaborative partner																													100,000,000				100,000,000					20,000,000
Prepaid research and discovery services																													25,000,000										3,600,000
License Fee																															70,000,000																											70,000,000
Expected License Fee And Regulatory Milestone Payments																															130,000,000																											130,000,000				150,000,000
Additional milestone payments for product candidate using a different modality																															90,000,000
Additional Milestone Payment																															10,000,000			1,000,000,000			300,000,000																					59,000,000				100,000,000					85,000,000
Amount recorded to reflect fair value of options granted																																																																												17,700,000
Milestone Payments Made During Period															20,000,000	40,000,000																35,000,000
Investments and other assets	618,795,000							594,350,000				618,795,000	594,350,000																											35,000,000
Investments By Third Party In Joint Venture As Per Agreement.																																																																	250,000,000	280,500,000,000
Joint Venture Owner Ship Percentage By Third Party.																																																																	85.00%	85.00%
Percentage of equity interest to the portion of total capital stock																																																																	15.00%	15.00%	10.00%
Equity Method Investment Ownership Percentage Maximum																																																																	49.90%	49.90%
Equity Method Investments, Expected Profit Share																																									50.00%
Equity Method Investments																																																																	45,000,000	49,500,000,000	8,600,000	23,900,000		9,100,000,000	25,200,000,000
Income (Loss) from Equity Method Investments												(15,126,000)	(17,224,000)	(4,518,000)
Other revenues from external customers	$ 49,200,000	[1]	$ 103,400,000	$ 61,900,000	[2]	$ 90,100,000	[3]	$ 89,400,000	$ 71,000,000	$ 48,800,000	$ 54,700,000	$ 304,531,000	$ 263,851,000	$ 212,464,000																																																					$ 58,500,000	$ 43,100,000	$ 13,300,000
Royalty Period For Substantially All Remaining Royalty Bearing Sales Rest Of World																																										expire through 2012

[1] 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.

[2] 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 all remaining rights to TYSABRI from Elan on April 2, 2013 and net revenues related to sales of TECFIDERA, which was approved by the FDA in March 2013 and commenced commercial sales in April 2013.

[3] 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.