Earnings per Share (Details) (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
Numerator:
Net income attributable to Biogen Idec Inc	$ 292,100,000	[1],[2]	$ 398,400,000	$ 386,800,000	[3]	$ 302,700,000	[4]	$ 300,200,000	[5]	$ 351,800,000	$ 288,000,000	[6]	$ 294,300,000	$ 1,380,033,000	$ 1,234,428,000	$ 1,005,273,000
Adjustment for net income allocable to preferred stock														0	(500,000)	(2,000,000)
Net income used in calculating basic and diluted earnings per share														$ 1,380,000,000	$ 1,233,900,000	$ 1,003,300,000
Denominator:
Weighted average number of common shares outstanding														237,938,000	242,395,000	252,307,000
Effect of dilutive securities:
Dilutive potential common shares														1,800,000	2,600,000	2,600,000
Shares used in calculating diluted earnings per share														239,740,000	245,033,000	254,867,000
Earnings per share (Textual)
Repurchase of common stock														5,800,000	6,000,000	40,300,000
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock options and employee stock purchase plan														500,000	1,000,000	900,000
Time-vested restricted stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan														1,000,000	1,300,000	1,600,000
Market stock units
Effect of dilutive securities:
Stock options and employee stock purchase plan														300,000	300,000	100,000

[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.