Condensed Consolidated Statements of Income (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Revenues:
Product, net	$ 1,385,918	$ 1,076,800	$ 2,481,697	$ 2,052,288
Unconsolidated joint business	288,785	284,630	553,391	569,183
Other	48,770	59,521	103,481	91,495
Total revenues	1,723,473	1,420,951	3,138,569	2,712,966
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	230,728	139,112	364,477	272,308
Research and development	327,463	329,559	611,803	685,521
Selling, general and administrative	431,012	301,767	783,610	601,856
Amortization of acquired intangible assets	82,225	52,282	133,526	98,243
Collaboration profit sharing	0	78,511	85,357	164,406
(Gain) loss on fair value remeasurement of contingent consideration	(5,163)	12,858	(2,886)	14,117
Restructuring charge	0	1,139	0	1,422
Total cost and expenses	1,066,265	915,228	1,975,887	1,837,873
Gain on sale of rights	5,319	0	10,370	0
Income from operations	662,527	505,723	1,173,052	875,093
Other income (expense), net	(10,428)	2,950	(24,885)	18,094
Income before income tax expense and equity in loss of investee, net of tax	652,099	508,673	1,148,167	893,187
Income tax expense	159,140	121,021	224,648	203,169
Equity in loss of investee, net of tax	2,289	511	6,100	511
Net income	490,670	387,141	917,419	689,507
Net income attributable to noncontrolling interests, net of tax	0	295	0	0
Net income attributable to Biogen Idec Inc.	$ 490,670	$ 386,846	$ 917,419	$ 689,507
Net income per share:
Basic earnings per share attributable to Biogen Idec Inc.	$ 2.07	$ 1.62	$ 3.87	$ 2.88
Diluted earnings per share attributable to Biogen Idec Inc.	$ 2.06	$ 1.61	$ 3.85	$ 2.86
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Idec Inc.	237,484	238,988	237,162	239,389
Diluted earnings per share attributable to Biogen Idec Inc.	238,743	240,622	238,543	241,245