Collaborative and Other Relationships (Details Textual)	3 Months Ended													12 Months Ended				12 Months Ended								1 Months Ended	12 Months Ended				3 Months Ended	12 Months Ended					3 Months Ended		1 Months Ended			3 Months Ended						1 Months Ended	3 Months Ended		12 Months Ended			12 Months Ended					1 Months Ended		12 Months Ended				12 Months Ended			3 Months Ended		12 Months Ended	3 Months Ended			12 Months Ended
	Dec. 31, 2013 USD ($)		Sep. 30, 2013 USD ($)	Jun. 30, 2013 USD ($)		Mar. 31, 2013 USD ($)		Dec. 31, 2012 USD ($)		Sep. 30, 2012 USD ($)	Jun. 30, 2012 USD ($)		Mar. 31, 2012 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Jan. 31, 2007 Biogen Idec Hemophilia	Dec. 31, 2013 U.S USD ($)	Dec. 31, 2012 U.S USD ($)	Dec. 31, 2011 U.S USD ($)	Dec. 31, 2013 Maximum	Dec. 31, 2013 Minimum	Dec. 31, 2013 Roche Group - Genentech U.S USD ($)	Dec. 31, 2012 Roche Group - Genentech U.S USD ($)	Dec. 31, 2011 Roche Group - Genentech U.S USD ($)	Jun. 30, 2012 Genentech USD ($)	Dec. 31, 2013 Genentech USD ($)	Dec. 31, 2013 Elan USD ($)	Dec. 31, 2012 Elan USD ($)	Dec. 31, 2011 Elan USD ($)	Sep. 30, 2011 Acorda USD ($)	Dec. 31, 2013 Acorda USD ($)	Dec. 31, 2013 Swedish Orphan Biovitrum USD ($)	Dec. 31, 2013 Swedish Orphan Biovitrum Maximum USD ($)	Dec. 31, 2013 Swedish Orphan Biovitrum Minimum USD ($)	Dec. 31, 2013 AbbVie USD ($)	Dec. 31, 2011 Portola Pharmaceuticals USD ($)	Dec. 31, 2013 Portola Pharmaceuticals USD ($)	Dec. 31, 2012 ISIS Pharmaceuticals USD ($)	Jun. 30, 2012 ISIS Pharmaceuticals USD ($)	Jan. 31, 2012 ISIS Pharmaceuticals USD ($)	Dec. 31, 2013 ISIS Pharmaceuticals USD ($)	Sep. 30, 2013 ISIS Pharmaceuticals USD ($)	Dec. 31, 2012 ISIS Pharmaceuticals USD ($)	Jun. 30, 2012 ISIS Pharmaceuticals USD ($)	Mar. 31, 2012 ISIS Pharmaceuticals USD ($)	Jan. 31, 2014 ISIS Pharmaceuticals USD ($)	Jan. 31, 2014 Proteostasis Therapeutics USD ($)	Dec. 31, 2013 Proteostasis Therapeutics USD ($)	Dec. 31, 2013 Other research and discovery USD ($)	Dec. 31, 2013 RITUXAN USD ($)	Dec. 31, 2012 RITUXAN	Dec. 31, 2011 RITUXAN	Dec. 31, 2013 RITUXAN Maximum	Dec. 31, 2013 RITUXAN Minimum	Dec. 31, 2013 GAZYVA [Member] USD ($)	Dec. 31, 2013 Ocrelizumab Maximum	Dec. 31, 2013 Ocrelizumab Minimum	Feb. 29, 2012 Samsung Biosimilar Agreement USD ($)	Feb. 29, 2012 Samsung Biosimilar Agreement KRW	Dec. 31, 2013 Samsung Biosimilar Agreement USD ($)	Dec. 31, 2013 Samsung Biosimilar Agreement KRW	Dec. 31, 2012 Samsung Biosimilar Agreement USD ($)	Dec. 31, 2012 Samsung Biosimilar Agreement KRW	Dec. 31, 2013 New Anti-CD20 Third Party Anti CD-20 Roche Group - Genentech USD ($)	Dec. 31, 2012 New Anti-CD20 Third Party Anti CD-20 Roche Group - Genentech USD ($)	Dec. 31, 2011 New Anti-CD20 Third Party Anti CD-20 Roche Group - Genentech USD ($)	Mar. 31, 2013 Hoechst USD ($)	Mar. 31, 2013 Hoechst EUR (€)	Dec. 31, 2013 Hoechst USD ($)	Jun. 30, 2011 Hoechst RITUXAN Genentech USD ($)	Dec. 31, 2013 Cash and cash equivalents Samsung Biosimilar Agreement USD ($)	Dec. 31, 2013 Accrued Liabilities Samsung Biosimilar Agreement USD ($)	Dec. 31, 2011 Strategic Investments USD ($)	Dec. 31, 2013 Strategic Investments Portola Pharmaceuticals 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
Royalty Period For Substantially All Remaining Royalty Bearing Sales Rest Of World																																																			expire through 2012
Percentage Of Co Promotion Operating Profits Greater Than First Fifty Million Option Two Sub Option Three																																																			35.00%					35.00%
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%
Estimates of compensatory damages and interest																										125,000,000
Decrease In Share Of Co Promotion Profits Due To Estimated Compensation Damages																																																																				41,500,000		49,700,000	50,000,000
Compensatory damages and interest																																																																					108,000,000
Prejudgment interest																																																																					54,000,000
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		3,581,000,000	2,176,800,000	1,954,800,000							41,200,000
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		8,500,000
Percentage of share of Co promotion profits exceeding $50 million																																																			40.00%	40.00%	40.00%
Research and development																																		130,000,000	120,000,000		36,800,000					10,000,000	75,000,000	30,000,000	12,000,000	29,000,000			2,500,000	4,000,000															25,700,000	35,400,000	26,900,000					21,000,000	15,000,000
Reimbursements of research and developments expense from Elan																												11,700,000	43,700,000	47,500,000
Reimbursements of selling general and administrative expenses from Elan																												20,600,000	99,900,000	77,300,000
Amount reflected in collaboration profit sharing line for collaboration																												85,400,000	317,900,000	317,800,000
Total milestone payments																															25,000,000
Expected additional milestone payments for successful achievement of regulatory and commercial sales milestones																																375,000,000						406,800,000
Expected additional milestone payments when certain sales threshold is met																																15,000,000
Foreign sales required to trigger milestone																																100,000,000
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
Amount paid to purchase Portola equity																																					8,200,000											5,000,000
Gain Loss From Sale Of Strategic Equity Investments																																																																										13,500,000	7,100,000
Term of collaboration agreement																																											6 years
Total upfront and milestone payments made to collaborative partner																																											100,000,000
Prepaid research and discovery services																																											25,000,000
License Fee																																							70,000,000	70,000,000	75,000,000		70,000,000
Expected License Fee And Regulatory Milestone Payments																																							130,000,000	130,000,000	150,000,000		130,000,000	130,000,000	130,000,000
Additional milestone payments for product candidate using a different modality																																											90,000,000
Additional Milestone Payment																																							10,000,000	59,000,000				10,000,000	59,000,000		40,000,000		197,500,000												85,000,000
Investments By Third Party In Joint Venture As Per Agreement.																																																											250,000,000	280,500,000,000
Other Assets, Noncurrent	594,350,000							274,054,000						594,350,000	274,054,000																																			35,000,000
Joint Venture Owner Ship Percentage By Third Party.																																																											85.00%	85.00%
Equity Method Investments																																																											45,000,000	49,500,000,000	23,900,000	25,200,000,000	27,800,000	29,700,000,000
Percentage of equity interest to the portion of total capital stock																																																											15.00%	15.00%
Equity Method Investment Ownership Percentage Maximum																																																											49.90%	49.90%
Payments to Acquire Equity Method Investments																																																													12,400,000	13,500,000,000
Income (Loss) from Equity Method Investments														(17,224,000)	(4,518,000)	0																																													(17,200,000)		(4,500,000)
Other revenues from external customers	$ 89,400,000	[1],[2]	$ 71,000,000	$ 48,800,000	[3]	$ 54,700,000	[4]	$ 62,300,000	[5]	$ 58,600,000	$ 59,600,000	[6]	$ 32,000,000	$ 263,851,000	$ 212,464,000	$ 215,920,000		$ 193,500,000	$ 170,200,000	$ 187,000,000																																									$ 43,100,000		$ 13,300,000

[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.