Condensed Consolidated Statements of Income (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Revenues:
Product, net	$ 1,095,779	$ 975,488
Unconsolidated joint business	264,606	284,552
Other	54,711	31,974
Total revenues	1,415,096	1,292,014
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	133,749	133,197
Research and development	284,340	355,962
Selling, general and administrative	352,598	300,089
Collaboration profit sharing	85,357	85,894
Amortization of acquired intangible assets	51,301	45,961
Fair value adjustment of contingent consideration	2,277	1,258
Restructuring charge	0	283
Total cost and expenses	909,622	922,644
Gain on sale of rights	5,051	0
Income from operations	510,525	369,370
Other income (expense), net	(14,457)	15,144
Income before income tax expense and equity in loss of investee, net of tax	496,068	384,514
Tax benefit	65,508	82,148
Equity in loss of investee, net of tax	3,811	0
Net income	426,749	302,366
Net loss attributable to noncontrolling interests, net of tax	0	(295)
Net income attributable to Biogen Idec Inc.	$ 426,749	$ 302,661
Net income per share:
Basic earnings per share attributable to Biogen Idec Inc.	$ 1.80	$ 1.26
Diluted earnings per share attributable to Biogen Idec Inc.	$ 1.79	$ 1.25
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Idec Inc.	236,837	239,754
Diluted earnings per share attributable to Biogen Idec Inc.	238,304	241,828