Consolidated Statements of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues:
Product, net	$ 4,166,074	$ 3,836,117	$ 3,470,056
Unconsolidated joint business	1,137,923	996,597	1,077,244
Other	212,464	215,920	169,123
Total revenues	5,516,461	5,048,634	4,716,423
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	545,494	466,780	400,262
Research and development	1,334,919	1,219,602	1,248,604
Selling, general and administrative	1,277,465	1,056,133	1,031,540
Collaboration profit sharing	317,895	317,771	258,071
Amortization of acquired intangible assets	202,204	208,566	208,928
Fair value adjustment of contingent consideration	27,202	36,065	0
Restructuring charge	2,225	19,026	75,153
Acquired in-process research and development	0	0	244,976
Total cost and expenses	3,707,404	3,323,943	3,467,534
Gain on sale of rights	46,792	0	0
Income from operations	1,855,849	1,724,691	1,248,889
Other income (expense), net	(744)	(13,477)	(18,983)
Income before income tax expense and equity in loss of investee, net of tax	1,855,105	1,711,214	1,229,906
Income tax expense	470,554	444,528	331,333
Equity in loss of investee, net of tax	4,518	0	0
Net income	1,380,033	1,266,686	898,573
Net income (loss) attributable to noncontrolling interests, net of tax	0	32,258	(106,700)
Net income attributable to Biogen Idec Inc.	$ 1,380,033	$ 1,234,428	$ 1,005,273
Net income per share:
Basic earnings per share attributable to Biogen Idec Inc.	$ 5.80	$ 5.09	$ 3.98
Diluted earnings per share attributable to Biogen Idec Inc.	$ 5.76	$ 5.04	$ 3.94
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Idec Inc.	237,938	242,395	252,307
Diluted earnings per share attributable to Biogen Idec Inc.	239,740	245,033	254,867