STATEMENTS OF OPERATIONS (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
STATEMENTS OF OPERATIONS [Abstract]
Government contracts	$ 7,504	$ 4,268	$ 20,217	$ 10,985
Research and development collaborations	710	534	3,718	1,182
Total revenue	8,214	4,802	23,935	12,167
Costs and expenses:
Cost of government contracts revenue	4,027	2,276	12,150	5,619
Research and development	19,219	13,948	48,940	33,989
General and administrative	4,757	3,857	14,871	10,740
Total costs and expenses	28,003	20,081	75,961	50,348
Loss from operations	(19,789)	(15,279)	(52,026)	(38,181)
Other income (expense):
Interest income	128	53	160	149
Interest expense	(47)	(64)	(150)	(132)
Other income (expense)	(19)	(10)		(10)
Change in fair value of warrant liability				267
Realized gains on investments			615
Loss from operations before income tax	(19,727)	(15,300)	(51,401)	(37,907)
Income tax expense				22
Net loss	$ (19,727)	$ (15,300)	$ (51,401)	$ (37,929)
Basic and diluted net loss per share	$ (0.08)	$ (0.09)	$ (0.23)	$ (0.24)
Basic and diluted weighted average number of common shares outstanding	238,304	168,537	221,578	156,555