Quarterly Financial Data (Unaudited) (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
Selected Quarterly Financial Information [Abstract]
Product, net	$ 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
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
Other	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
Total revenues	1,965,900,000	[1],[2]	1,827,800,000	1,723,500,000	[3]	1,415,100,000	[4]	1,417,900,000	[5]	1,385,500,000	1,421,000,000	[6]	1,292,000,000	6,932,199,000	5,516,461,000	5,048,634,000
Gross Profit	1,707,300,000	[1],[2]	1,593,100,000	1,492,800,000	[3]	1,281,400,000	[4]	1,284,100,000	[5]	1,246,200,000	1,281,800,000	[6]	1,158,800,000	6,074,500,000	4,971,000,000
Net income	457,300,000	[1],[2]	487,600,000	490,700,000	[3]	426,700,000	[4]	292,100,000	[5]	398,400,000	387,100,000	[6]	302,400,000	1,862,341,000	1,380,033,000	1,266,686,000
Net income attributable to Biogen Idec Inc	$ 457,300,000	[1],[2]	$ 487,600,000	$ 490,700,000	[3]	$ 426,700,000	[4]	$ 292,100,000	[5]	$ 398,400,000	$ 386,800,000	[6]	$ 302,700,000	$ 1,862,341,000	$ 1,380,033,000	$ 1,234,428,000
Basic earnings per share attributable to Biogen Idec Inc.	$ 1.94	[1],[2]	$ 2.06	$ 2.07	[3]	$ 1.80	[4]	$ 1.24	[5]	$ 1.68	$ 1.62	[6]	$ 1.26	$ 7.86	$ 5.80	$ 5.09
Diluted earnings per share attributable to Biogen Idec Inc.	$ 1.92	[1],[2]	$ 2.05	$ 2.06	[3]	$ 1.79	[4]	$ 1.23	[5]	$ 1.67	$ 1.61	[6]	$ 1.25	$ 7.81	$ 5.76	$ 5.04

[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.