Inventory (Details 1) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Components of inventory [Line Items]
Total finished goods and work in process	$ 243,000,000	$ 230,100,000
Raw materials	83,800,000	59,000,000
Total inventory	326,843,000	289,066,000
Inventory (Textual) [Abstract]
Write-downs on excess, obsolete or unmarketable inventory	25,446,000	11,808,000	16,924,000
AVONEX [Member]
Components of inventory [Line Items]
Total finished goods and work in process	113,300,000	87,000,000
TYSABRI [Member]
Components of inventory [Line Items]
Total finished goods and work in process	114,700,000	117,000,000
Other products [Member]
Components of inventory [Line Items]
Total finished goods and work in process	$ 15,000,000	$ 26,100,000