Consolidated Statements of Equity (USD $) In Thousands, except Share data	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, 2009	$ 6,261,882	$ 6,221,530	$ 0	$ 144	$ 5,781,920	$ 50,496	$ 1,068,890	$ (679,920)	$ 40,352
Beginning Balance, shares at Dec. 31, 2009			8,000	288,494,000				(13,639,000)
Net income	898,573	1,005,273					1,005,273		(106,700)
Other Comprehensive Income (Loss), Net of Tax	(74,346)	(72,106)				(72,106)			(2,240)
Distributions to noncontrolling interests	(33,891)	0							(33,891)
Capital contributions from noncontrolling interests	2,488	0							2,488
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								(40,294,000)
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								4,020,000
Issuance of common stock under stock award plan	(54,840)	(54,840)					(173,050)	118,210
Issuance of common stock under stock award plan, shares								1,957,000
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
Fair value of assets and liabilities acquired and assigned to noncontrolling interests (Note 20)	145,000	0							145,000
Termination of relationship with less than majority owned subsidiary	7,928	0							7,928
Retirement of common stock pursuant to the 2009 and 2010 share repurchase plans	0	0		(20)	(2,077,559)			2,077,579
Retirement of common stock pursuant to the 2009 and 2010 share repurchase plans, shares				(40,294,000)				40,294,000
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			8,000	248,200,000				(7,662,000)
Net income	1,266,686	1,234,428					1,234,428		32,258
Other Comprehensive Income (Loss), Net of Tax	(22)	(4,925)				(4,925)			4,903
Distributions to noncontrolling interests	(27,062)	(148)					(148)		(26,914)
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	(6,000,000)							(6,018,000)
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				5,458,000				162,000
Issuance of common stock under stock award plan	(50,953)	(50,953)		1	(50,954)
Issuance of common stock under stock award plan, shares				1,482,000
Compensation expense related to share-based payments	117,347	117,347			117,347
Tax benefit from share-based payments	50,383	50,383			50,383
Acquisitions of noncontrolling interests	(187,337)	(125,641)			(125,641)				(61,696)
Conversion of preferred stock	0	0		0	0
Conversion of preferred stock, shares			(8,000)	493,000
Recharacterization of share-based awards from equity to cash-settled due to restructuring	(8,172)	(8,172)			(8,172)
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	255,633,000				(13,518,000)
Net income	1,380,033	1,380,033					1,380,033		0
Other Comprehensive Income (Loss), Net of Tax	(28,705)	(28,770)				(28,770)			65
Distributions to noncontrolling interests	(1,199)	0							1,199
Capital contributions from noncontrolling interests	73	0							73
Noncontrolling Interest, Decrease from Deconsolidation	(556)	(3)			(3)				(553)
Repurchase of common stock for Treasury pursuant to the 2011, 2010 and 2009 share repurchase plans, at cost	(984,715)	(984,715)						(984,715)
Repurchase of common stock for Treasury pursuant to the 2011, 2010 and 2009 stock repurchase plans, at cost, shares	(7,800,000)							(7,811,000)
Issuance of common and treasury stock under stock option and stock purchase plans	67,493	67,493		0	67,493
Issuance of common and treasury stock under stock option and stock purchase plans, shares				1,039,000
Issuance of common stock under stock award plan	(71,357)	(71,357)		1	(71,358)
Issuance of common stock under stock award plan, shares	765,000			1,239,000
Compensation expense related to share-based payments	123,956	123,956			123,956
Tax benefit from share-based payments	49,387	49,387			49,387
Retirement of common stock pursuant to the 2009 and 2010 share repurchase plans	0	0		(2)	(499,998)			500,000
Retirement of common stock pursuant to the 2009 and 2010 share repurchase plans, shares				(3,674,000)				3,674,000
Ending Balance at Dec. 31, 2012	$ 6,963,795	$ 6,961,523	$ 0	$ 127	$ 3,854,525	$ (55,305)	$ 4,486,794	$ (1,324,618)	$ 2,272
Ending Balance, shares at Dec. 31, 2012			0	254,237,000				(17,655,000)