Segment Information (Details) (USD $) In Thousands, unless otherwise specified	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
Revenue by product
Product	$ 996,600	$ 975,800	$ 956,700	$ 907,100	$ 909,800	$ 876,900	$ 859,200	$ 824,200	$ 3,836,117	$ 3,470,056	$ 3,152,941
U.S [Member]
Revenue by product
Product									1,954,800	1,744,400	1,638,000
Rest of world [Member]
Revenue by product
Product									1,881,300	1,725,700	1,514,900
AVONEX [Member]
Revenue by product
Product									2,686,600	2,518,400	2,322,900
AVONEX [Member] | U.S [Member]
Revenue by product
Product									1,628,300	1,491,600	1,406,200
AVONEX [Member] | Rest of world [Member]
Revenue by product
Product									1,058,300	1,026,800	916,700
TYSABRI [Member]
Revenue by product
Product									1,079,500	900,200	776,000
TYSABRI [Member] | U.S [Member]
Revenue by product
Product									326,500	252,800	231,800
TYSABRI [Member] | Rest of world [Member]
Revenue by product
Product									753,000	647,400	544,200
Other products [Member]
Revenue by product
Product									70,000	51,500	54,000
Other products [Member] | Rest of world [Member]
Revenue by product
Product									$ 70,000	$ 51,500	$ 54,000