Condensed Consolidated Statements of Income (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenues:
Product, net	$ 1,453,554	$ 1,039,110	$ 3,935,251	$ 3,091,398
Unconsolidated joint business	303,210	287,792	856,601	856,975
Other	71,016	58,652	174,497	150,147
Total revenues	1,827,780	1,385,554	4,966,349	4,098,520
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	234,696	139,358	599,173	411,666
Research and development	410,017	304,217	1,021,820	989,738
Selling, general and administrative	405,584	299,631	1,189,194	901,488
Amortization of acquired intangible assets	99,998	53,013	233,524	151,256
Collaboration profit sharing	0	75,545	85,357	239,951
(Gain) loss on fair value remeasurement of contingent consideration	(97)	9,456	(2,983)	23,573
Restructuring charges	0	803	0	2,225
Total cost and expenses	1,150,198	882,023	3,126,085	2,719,897
Gain on sale of rights	6,949	31,719	17,319	31,719
Income from operations	684,531	535,250	1,857,583	1,410,342
Other income (expense), net	(4,640)	(4,548)	(29,525)	13,546
Income before income tax expense and equity in loss of investee, net of tax	679,891	530,702	1,828,058	1,423,888
Income tax expense	186,105	131,044	410,753	334,213
Equity in loss of investee, net of tax	6,170	1,258	12,270	1,769
Net income	487,616	398,400	1,405,035	1,087,906
Net income attributable to noncontrolling interests, net of tax	0	0	0	0
Net income attributable to Biogen Idec Inc.	$ 487,616	$ 398,400	$ 1,405,035	$ 1,087,906
Net income per share:
Basic earnings per share attributable to Biogen Idec Inc.	$ 2.06	$ 1.68	$ 5.93	$ 4.56
Diluted earnings per share attributable to Biogen Idec Inc.	$ 2.05	$ 1.67	$ 5.89	$ 4.53
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Idec Inc.	237,070	236,474	237,131	238,331
Diluted earnings per share attributable to Biogen Idec Inc.	238,349	238,125	238,508	240,137