Quarterly Financial Data (Unaudited) (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Selected Quarterly Financial Information [Abstract]
Revenue on sales of RITUXAN in the rest of world	$ 996,600,000	$ 975,800,000	$ 956,700,000	$ 907,100,000	$ 909,800,000	$ 876,900,000	$ 859,200,000	$ 824,200,000	$ 3,836,117,000	$ 3,470,056,000	$ 3,152,941,000
Unconsolidated joint business revenues	257,500,000	266,500,000	216,500,000	256,100,000	258,000,000	258,000,000	306,400,000	254,900,000	996,597,000	1,077,244,000	1,094,863,000
Other	72,600,000	67,700,000	35,500,000	40,100,000	51,400,000	41,000,000	47,100,000	29,700,000	215,920,000	169,123,000	129,544,000
Total revenues	1,326,700,000	1,309,900,000	1,208,600,000	1,203,300,000	1,219,000,000	1,175,800,000	1,212,700,000	1,108,900,000	5,048,634,000	4,716,423,000	4,377,348,000
Gross Profit	1,187,100,000	1,186,400,000	1,108,100,000	1,100,200,000	1,118,800,000	1,079,900,000	1,105,700,000	1,011,800,000	4,581,900,000	4,316,200,000
Net income	300,200,000	353,700,000	304,000,000	308,800,000	271,800,000	112,200,000	294,600,000	220,000,000	1,266,686,000	898,573,000	977,062,000
Net income attributable to Biogen Idec Inc	$ 300,200,000	$ 351,800,000	$ 288,000,000	$ 294,300,000	$ 240,300,000	$ 254,100,000	$ 293,400,000	$ 217,400,000	$ 1,234,428,000	$ 1,005,273,000	$ 970,132,000
Basic earnings per share attributable to Biogen Idec Inc.	$ 1.24	$ 1.45	$ 1.19	$ 1.22	$ 1.00	$ 1.06	$ 1.13	$ 0.80	$ 5.09	$ 3.98	$ 3.37
Diluted earnings per share attributable to Biogen Idec Inc.	$ 1.22	$ 1.43	$ 1.18	$ 1.20	$ 0.99	$ 1.05	$ 1.12	$ 0.80	$ 5.04	$ 3.94	$ 3.35