Segment Information (Details) (USD $) In Thousands, unless otherwise specified	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
Revenue by product
Product, net	$ 1,074,700	[1],[2]	$ 1,039,100	$ 1,076,800	[3]	$ 975,400	[4]	$ 996,600	[5]	$ 975,800	$ 956,700	[6]	$ 907,100	$ 4,166,074	$ 3,836,117	$ 3,470,056
U.S
Revenue by product
Product, net														2,176,800	1,954,800	1,744,400
Rest of world [Member]
Revenue by product
Product, net														1,989,300	1,881,300	1,725,700
AVONEX [Member]
Revenue by product
Product, net														2,913,100	2,686,600	2,518,400
AVONEX [Member] | U.S
Revenue by product
Product, net														1,793,700	1,628,300	1,491,600
AVONEX [Member] | Rest of world [Member]
Revenue by product
Product, net														1,119,400	1,058,300	1,026,800
TYSABRI [Member]
Revenue by product
Product, net														1,135,900	1,079,500	900,200
TYSABRI [Member] | U.S
Revenue by product
Product, net														383,100	326,500	252,800
TYSABRI [Member] | Rest of world [Member]
Revenue by product
Product, net														752,800	753,000	647,400
Other products [Member]
Revenue by product
Product, net														117,100	70,000	51,500
Other products [Member] | U.S
Revenue by product
Product, net														0	0	0
Other products [Member] | Rest of world [Member]
Revenue by product
Product, net														$ 117,100	$ 70,000	$ 51,500

[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.