Financial Instruments (Details Textual) (USD $)	6 Months Ended		12 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Strategic Investments [Member]	Jun. 30, 2012 Strategic Investments [Member]	Jun. 30, 2013 Strategic Investments [Member]	Jun. 30, 2012 Strategic Investments [Member]	Jun. 30, 2013 TYSABRI product [Member]
Financial Instruments (Textual) [Abstract]
Acquisition of TYSABRI rights	$ 3,262,719,000	$ 0						$ 3,250,000,000
Original maturities of commercial paper and short-term debt securities	less than 90 days
Average maturity of marketable securities, months	11 months		14 months
Impairment charges				$ 1,400,000	$ 800,000	$ 1,700,000	$ 1,300,000