Accounts Receivable (Details Textual)	3 Months Ended											12 Months Ended			3 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 ($)	Mar. 31, 2014 Spain USD ($)	Jun. 30, 2014 Italy USD ($)	Dec. 31, 2011 Italy EUR (€)	Dec. 31, 2014 Italy USD ($)	Dec. 31, 2014 Italy EUR (€)	Jun. 30, 2013 Italy EUR (€)
Accounts Receivable (Textual)
Accounts receivable outstanding for greater than one year	$ 14,200,000							$ 45,900,000				$ 14,200,000	$ 45,900,000
Accounts Receivable, Payments Received, Receivables Aged Greater Than One And Two Years															59,600,000
Payment in case of any unfavorable determination received																	30,700,000		35,600,000	33,300,000
Reimbursement Limit Stated In Resolution																	24 months
Loss contingency accrual																				15,400,000
Loss Contingency, Adjustment Related To Claim Amount, Percent																				50.00%
Product, net	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		53,500,000
Deferred revenue																		$ 77,600,000

[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.