Quarterly Financial Data (Unaudited) (Details) (USD $)	3 Months Ended											12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Abstract]
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
Unconsolidated joint business revenues	304,500,000	[1]	290,700,000	303,300,000	[2]	296,900,000	[3]	269,400,000	303,200,000	288,800,000	264,600,000	1,195,389,000	1,126,017,000	1,137,923,000
Other	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
Total revenues	2,640,700,000	[1]	2,511,400,000	2,421,500,000	[2]	2,129,800,000	[3]	1,965,900,000	1,827,800,000	1,723,500,000	1,415,100,000	9,703,324,000	6,932,199,000	5,516,461,000
Gross Profit	2,343,400,000	[1]	2,208,800,000	2,129,600,000	[2]	1,850,500,000	[3]	1,707,300,000	1,593,100,000	1,492,800,000	1,281,400,000	8,532,300,000	6,074,500,000
Net income	882,600,000	[1]	856,100,000	723,100,000	[2]	479,700,000	[3]	457,300,000	487,600,000	490,700,000	426,700,000	2,941,565,000	1,862,341,000	1,380,033,000
Net income attributable to Biogen Idec Inc	$ 883,500,000	[1]	$ 856,900,000	$ 714,500,000	[2]	$ 480,000,000	[3]	$ 457,300,000	$ 487,600,000	$ 490,700,000	$ 426,700,000	$ 2,934,784,000	$ 1,862,341,000	$ 1,380,033,000
Basic earnings per share attributable to Biogen Idec Inc.	$ 3.75	[1]	$ 3.63	$ 3.02	[2]	$ 2.03	[3]	$ 1.94	$ 2.06	$ 2.07	$ 1.80	$ 12.42	$ 7.86	$ 5.80
Diluted earnings per share attributable to Biogen Idec Inc.	$ 3.74	[1]	$ 3.62	$ 3.01	[2]	$ 2.02	[3]	$ 1.92	$ 2.05	$ 2.06	$ 1.79	$ 12.37	$ 7.81	$ 5.76

[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.