STATEMENTS OF OPERATIONS (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
STATEMENTS OF OPERATIONS [Abstract]
Government contracts	$ 7,241	$ 3,276	$ 12,713	$ 6,717
Research and development collaborations	1,018	255	3,008	648
Total revenue	8,259	3,531	15,721	7,365
Costs and expenses:
Cost of government contracts revenue	5,102	1,632	8,123	3,344
Research and development	15,202	10,785	29,720	20,041
General and administrative	5,806	4,012	10,114	6,882
Total costs and expenses	26,110	16,429	47,957	30,267
Loss from operations	(17,851)	(12,898)	(32,236)	(22,902)
Other income (expense):
Interest income	20	48	32	95
Interest expense	(51)	(45)	(103)	(68)
Other income	18		18
Change in fair value of warrant liability		267		267
Realized gains on investments			615
Loss from operations before income tax	(17,864)	(12,628)	(31,674)	(22,608)
Income tax expense		5		22
Net loss	$ (17,864)	$ (12,633)	$ (31,674)	$ (22,630)
Basic and diluted net loss per share	$ (0.08)	$ (0.08)	$ (0.15)	$ (0.15)
Basic and diluted weighted average number of common shares outstanding	217,178	152,312	213,075	150,391