Unaudited Quarterly Results (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Revenues	$ 2,968	$ 867	$ 1,801	$ 2,226	$ 8,885	$ 1,117	$ 1,820	$ 2,226	$ 7,862	$ 14,048	$ 84,796
Net income (loss)	(8,551)	(10,500)	(12,263)	(11,258)	(324)	(11,301)	(10,720)	(13,086)	(42,572)	(35,431)	10,381
Net income (loss) per share - basic and diluted (in dollars per share)	$ (0.14)	$ (0.17)	$ (0.24)	$ (0.22)	$ (0.01)	$ (0.33)	$ (0.32)	$ (0.39)	$ (0.76)	$ (1.02)
MedImmune [Member]
Segment Reporting Information [Line Items]
Revenues	5,000
CytoDyn [Member]
Segment Reporting Information [Line Items]
Revenues	$ 2,800