Segment Information (Details 1) (USD $)	3 Months Ended													12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011		Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Geographic information
Product, net	$ 1,074,700,000	[1],[2]	$ 1,039,100,000	$ 1,076,800,000	[3]	$ 975,400,000	[4]	$ 996,600,000	[5]	$ 975,800,000	$ 956,700,000	[6]	$ 907,100,000	$ 4,166,074,000		$ 3,836,117,000		$ 3,470,056,000
Revenues from unconsolidated joint business														1,137,900,000		996,600,000		1,077,200,000
Other revenues from external customers	62,300,000	[1],[2]	58,600,000	59,600,000	[3]	32,000,000	[4]	72,600,000	[5]	67,700,000	35,500,000	[6]	40,100,000	212,464,000		215,920,000		169,123,000
Long-lived assets	1,742,200,000							1,838,400,000						1,742,200,000		1,838,400,000		1,826,200,000
Asia [Member]
Geographic information
Product, net														93,200,000		88,700,000		69,000,000
Revenues from unconsolidated joint business														27,500,000		30,700,000		26,000,000
Other revenues from external customers														13,300,000		0		0
Long-lived assets	2,900,000							5,300,000						2,900,000		5,300,000		5,400,000
Europe [Member]
Geographic information
Product, net														1,216,200,000	[7]	1,163,300,000	[7]	1,090,700,000	[7]
Revenues from unconsolidated joint business														14,300,000	[7]	29,900,000	[7]	95,300,000	[7]
Other revenues from external customers														27,900,000	[7]	28,300,000	[7]	32,600,000	[7]
Long-lived assets	738,600,000	[7]						816,600,000	[7]					738,600,000	[7]	816,600,000	[7]	717,400,000	[7]
Other [Member]
Geographic information
Product, net														270,700,000		251,800,000		203,600,000
Revenues from unconsolidated joint business														62,800,000		57,200,000		49,600,000
Other revenues from external customers														0		0		0
Long-lived assets	2,200,000							2,400,000						2,200,000		2,400,000		1,600,000
U.S
Geographic information
Product, net														2,176,800,000		1,954,800,000		1,744,400,000
Revenues from unconsolidated joint business														1,033,300,000		878,800,000		906,300,000
Other revenues from external customers														170,200,000		187,000,000		136,000,000
Long-lived assets	996,600,000							1,012,500,000						996,600,000		1,012,500,000		1,100,300,000
Germany [Member]
Geographic information
Product, net														409,200,000		377,500,000		362,400,000
Revenues from unconsolidated joint business														0		0		0
Other revenues from external customers														1,100,000		600,000		500,000
Long-lived assets	$ 1,900,000							$ 1,600,000						$ 1,900,000		$ 1,600,000		$ 1,500,000

[1] 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 Pharmaceuticals, Inc.

[2] 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.

[3] Net income and net income attributable to Biogen Idec Inc. for the second quarter of 2012 includes a charge to research and development expense of $12.0 million related to an upfront payment made in connection with our development agreement entered into with Isis Pharmaceuticals, Inc.

[4] 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 Pharmaceuticals, Inc. and a $12.4 million reduction resulting from an increase in our returns reserve and write-offs of unsold inventory due to a voluntary withdrawal of a limited amount of AVONEX product that has demonstrated a trend in oxidation that may lead to expiry earlier than stated on its label.

[5] Net income and net income attributable to Biogen Idec Inc. for the fourth quarter of 2011 includes a charge to research and development expense of $36.8 million related to an upfront payment made in connection with our collaboration and license agreement entered into with Portola Pharmaceuticals, Inc.

[6] Our share of RITUXAN revenues from unconsolidated joint business was reduced by approximately $50.0 million in the second quarter of 2011 as a result of an accrual for estimated compensatory damages (including interest) relating to Genentech’s ongoing arbitration with Hoechst GmbH. For additional information related to this matter, please read Note 22, Litigation to these consolidated financial statements.

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