Consolidated Statements of Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Total Biogen Idec Inc. shareholders' equity	Preferred stock	Common stock	Additional paid-in capital	Accumulated other comprehensive income (loss)	Retained earnings	Treasury stock	Noncontrolling interests
Beginning Balance at Dec. 31, 2008	$ 5,833,952	$ 5,806,083	$ 0	$ 149	$ 6,073,957	$ (11,106)	$ 270,180	$ (527,097)	$ 27,869
Beginning Balance, shares at Dec. 31, 2008			8	297,253				(9,207)
Comprehensive income:
Net income	977,062	970,132					970,132		6,930
Unrealized gains (losses) recognized on securities available for sale
Unrealized gains (losses) recognized during the period, net of tax of $133, $6,345, $4,136 for 2011, 2010 and 2009, respectively	7,191	7,191				7,191
Less: reclassification adjustment for gains included in net income, net of tax of $7,155, $5,656, $3,756 for 2011, 2010 and 2009, respectively	(6,396)	(6,396)				(6,396)
Net unrealized gains (losses) on securities available for sale, net of tax of $7,288, $689, $380 for 2011, 2010 and 2009, respectively	795	795				795
Unrealized gains (losses) on foreign currency forward contracts
Unrealized gains (losses) recognized during the period, net of tax of $3,647, $1,268, $254 for 2011, 2010 and 2009, respectively	1,452	1,452				1,452
Less: reclassification adjustment for (gains) losses included in net income, net of tax of $1,268, $304, $3,836 for 2011, 2010 and 2009, respectively	40,216	40,216				40,216
Unrealized gains (losses) on foreign currency forward contracts , net of tax of $4,915, $964, $3,582 for 2011, 2010 and 2009, respectively	41,668	41,668				41,668
Unrealized gains (losses) on pension benefit obligation
Unrealized gains (losses) on pension benefit obligation, net of tax of $0, $0, $67 for 2011, 2010 and 2009, respectively	501	501				501
Translation adjustment	19,835	18,638				18,638			1,197
Total comprehensive income	1,039,861	1,031,734							8,127
Distributions to noncontrolling interests	(2,832)								(2,832)
Capital contributions from noncontrolling interests	7,188								7,188
Repurchase of common stock for Treasury pursuant to the 2011, 2010 and 2009 share repurchase plans, at cost	(751,170)	(751,170)						(751,170)
Repurchase of common stock for Treasury pursuant to the 2011, 2010 and 2009 stock repurchase plans, at cost, shares								(15,982)
Retirement of common stock pursuant to the 2009 and 2010 share repurchase plans				(5)	(422,415)			422,420
Retirement of common stock pursuant to the 2009 and 2010 share repurchase plans, shares				(8,759)				8,759
Issuance of common and treasury stock under stock option and stock purchase plans	47,810	47,810					(27,191)	75,001
Issuance of common and treasury stock under stock option and stock purchase plans, shares								1,181
Issuance of common stock under stock award plan	(43,305)	(43,305)					(144,231)	100,926
Issuance of common under stock award plan, shares								1,610
Compensation expense related to share-based payments	167,207	167,207			167,207
Tax benefit from share-based payments	(36,829)	(36,829)			(36,829)
Ending Balance at Dec. 31, 2009	6,261,882	6,221,530	0	144	5,781,920	50,496	1,068,890	(679,920)	40,352
Ending Balance, shares at Dec. 31, 2009			8	288,494				(13,639)
Comprehensive income:
Net income	898,573	1,005,273					1,005,273		(106,700)
Unrealized gains (losses) recognized on securities available for sale
Unrealized gains (losses) recognized during the period, net of tax of $133, $6,345, $4,136 for 2011, 2010 and 2009, respectively	10,775	10,775				10,775
Less: reclassification adjustment for gains included in net income, net of tax of $7,155, $5,656, $3,756 for 2011, 2010 and 2009, respectively	(9,631)	(9,631)				(9,631)
Net unrealized gains (losses) on securities available for sale, net of tax of $7,288, $689, $380 for 2011, 2010 and 2009, respectively	1,144	1,144				1,144
Unrealized gains (losses) on foreign currency forward contracts
Unrealized gains (losses) recognized during the period, net of tax of $3,647, $1,268, $254 for 2011, 2010 and 2009, respectively	(9,767)	(9,767)				(9,767)
Less: reclassification adjustment for (gains) losses included in net income, net of tax of $1,268, $304, $3,836 for 2011, 2010 and 2009, respectively	(1,502)	(1,502)				(1,502)
Unrealized gains (losses) on foreign currency forward contracts , net of tax of $4,915, $964, $3,582 for 2011, 2010 and 2009, respectively	(11,269)	(11,269)				(11,269)
Unrealized gains (losses) on pension benefit obligation
Unrealized gains (losses) on pension benefit obligation, net of tax of $0, $0, $67 for 2011, 2010 and 2009, respectively	(1,942)	(1,942)				(1,942)
Translation adjustment	(62,279)	(60,039)				(60,039)			(2,240)
Total comprehensive income	824,227	933,167							(108,940)
Fair value of assets and liabilities acquired and assigned to noncontrolling interests (Note19)	145,000								145,000
Distributions to noncontrolling interests	(33,891)								(33,891)
Capital contributions from noncontrolling interests	2,488								2,488
Termination of relationship with less than majority owned subsidiary	7,928								7,928
Repurchase of common stock for Treasury pursuant to the 2011, 2010 and 2009 share repurchase plans, at cost	(2,077,579)	(2,077,579)						(2,077,579)
Repurchase of common stock for Treasury pursuant to the 2011, 2010 and 2009 stock repurchase plans, at cost, shares								(40,294)
Retirement of common stock pursuant to the 2009 and 2010 share repurchase plans				(20)	(2,077,559)			2,077,579
Retirement of common stock pursuant to the 2009 and 2010 share repurchase plans, shares				(40,294)				40,294
Issuance of common and treasury stock under stock option and stock purchase plans	183,486	183,486					(28,632)	212,118
Issuance of common and treasury stock under stock option and stock purchase plans, shares								4,020
Issuance of common stock under stock award plan	(54,840)	(54,840)					(173,050)	118,210
Issuance of common under stock award plan, shares								1,957
Compensation expense related to share-based payments	171,435	171,435			171,435
Tax benefit from share-based payments	19,307	19,307			19,307
Ending Balance at Dec. 31, 2010	5,449,443	5,396,506	0	124	3,895,103	(21,610)	1,872,481	(349,592)	52,937
Ending Balance, shares at Dec. 31, 2010			8	248,200				(7,662)
Comprehensive income:
Net income	1,266,686	1,234,428					1,234,428		32,258
Unrealized gains (losses) recognized on securities available for sale
Unrealized gains (losses) recognized during the period, net of tax of $133, $6,345, $4,136 for 2011, 2010 and 2009, respectively	(224)	(224)				(224)
Less: reclassification adjustment for gains included in net income, net of tax of $7,155, $5,656, $3,756 for 2011, 2010 and 2009, respectively	(12,184)	(12,184)				(12,184)
Net unrealized gains (losses) on securities available for sale, net of tax of $7,288, $689, $380 for 2011, 2010 and 2009, respectively	(12,408)	(12,408)				(12,408)
Unrealized gains (losses) on foreign currency forward contracts
Unrealized gains (losses) recognized during the period, net of tax of $3,647, $1,268, $254 for 2011, 2010 and 2009, respectively	32,830	32,830				32,830
Less: reclassification adjustment for (gains) losses included in net income, net of tax of $1,268, $304, $3,836 for 2011, 2010 and 2009, respectively	9,767	9,767				9,767
Unrealized gains (losses) on foreign currency forward contracts , net of tax of $4,915, $964, $3,582 for 2011, 2010 and 2009, respectively	42,597	42,597				42,597
Unrealized gains (losses) on pension benefit obligation
Unrealized gains (losses) on pension benefit obligation, net of tax of $0, $0, $67 for 2011, 2010 and 2009, respectively	(9,280)	(9,280)				(9,280)
Translation adjustment	(20,931)	(25,884)				(25,834)			4,903
Total comprehensive income	1,266,664	1,229,503							37,161
Distributions to noncontrolling interests	(27,062)	(148)					(148)		(26,914)
Acquisitions of noncontrolling interests	(187,337)	(125,641)			(125,641)				(61,696)
Repurchase of common stock for Treasury pursuant to the 2011, 2010 and 2009 share repurchase plans, at cost	(497,975)	(497,975)						(497,975)
Repurchase of common stock for Treasury pursuant to the 2011, 2010 and 2009 stock repurchase plans, at cost, shares								(6,018)
Issuance of common and treasury stock under stock option and stock purchase plans	314,649	314,649		3	306,982			7,664
Issuance of common and treasury stock under stock option and stock purchase plans, shares				5,458				162
Issuance of common stock under stock award plan	(50,953)	(50,953)		1	(50,954)
Issuance of common under stock award plan, shares	5,235,000			1,482
Conversion of preferred stock	0
Conversion of preferred stock, shares			(8)	493
Compensation expense related to share-based payments	117,347	117,347			117,347
Recharacterization of share-based awards from equity to cash-settled due to restructuring	(8,172)	(8,172)			(8,172)
Tax benefit from share-based payments	50,383	50,383			50,383
Ending Balance at Dec. 31, 2011	$ 6,426,987	$ 6,425,499	$ 0	$ 128	$ 4,185,048	$ (26,535)	$ 3,106,761	$ (839,903)	$ 1,488
Ending Balance, shares at Dec. 31, 2011			0	255,633				(13,518)