Segment Information (Details 1) (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
Geographic information
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
Revenues from unconsolidated joint business													1,195,400,000		1,126,000,000		1,137,900,000
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
Long-lived assets	1,765,700,000							1,750,700,000					1,765,700,000		1,750,700,000		1,742,200,000
U.S
Geographic information
Product, net													5,566,700,000		3,581,000,000		2,176,800,000
Revenues from unconsolidated joint business													1,117,100,000		1,087,300,000		1,033,300,000
Other revenues from external customers													212,600,000		193,500,000		170,200,000
Long-lived assets	1,055,500,000							984,400,000					1,055,500,000		984,400,000		996,600,000
Europe
Geographic information
Product, net													1,383,900,000	[4]	1,170,200,000	[4]	1,216,200,000	[4]
Revenues from unconsolidated joint business													7,700,000	[4]	1,600,000	[4]	14,300,000	[4]
Other revenues from external customers													31,600,000	[4]	26,100,000	[4]	27,900,000	[4]
Long-lived assets	701,900,000	[4]						758,300,000	[4]				701,900,000	[4]	758,300,000	[4]	738,600,000	[4]
Germany
Geographic information
Product, net													811,800,000		417,700,000		409,200,000
Revenues from unconsolidated joint business													0		0		0
Other revenues from external customers													1,800,000		1,200,000		1,100,000
Long-lived assets	2,500,000							2,500,000					2,500,000		2,500,000		1,900,000
Asia
Geographic information
Product, net													112,800,000		93,200,000		93,200,000
Revenues from unconsolidated joint business													0		3,200,000		27,500,000
Other revenues from external customers													58,500,000		43,100,000		13,300,000
Long-lived assets	2,600,000							2,100,000					2,600,000		2,100,000		2,900,000
Other
Geographic information
Product, net													328,200,000		280,200,000		270,700,000
Revenues from unconsolidated joint business													70,600,000		33,900,000		62,800,000
Other revenues from external customers													0		0		0
Long-lived assets	$ 3,200,000							$ 3,300,000					$ 3,200,000		$ 3,300,000		$ 2,200,000

[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.

[4] Represents amounts related to Europe less those attributable to Germany.