Condensed Consolidated Statements of Operations - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Grant revenue	$ 5,379,000	$ 14,457,000	$ 18,279,000	$ 20,057,000
Licensing and collaboration revenue	103,000	888,000	334,000	3,008,000
Total operating revenues	5,482,000	15,345,000	18,613,000	23,065,000
Operating expenses:
Research and development	18,480,000	24,463,000	52,405,000	73,810,000
General and administrative	7,907,000	7,749,000	25,038,000	26,043,000
Total operating expenses	26,387,000	32,212,000	77,443,000	99,853,000
Loss from operations	(20,905,000)	(16,867,000)	(58,830,000)	(76,788,000)
Other income and expense:
Miscellaneous income (expense)	12,000	(44,000)	(101,000)	(44,000)
Interest income	151,000	68,000	353,000	180,000
Interest expense	(3,000)	(5,000)	(116,000)	(18,000)
Other income, net	160,000	19,000	136,000	118,000
Net loss before taxes	(20,745,000)	(16,848,000)	(58,694,000)	(76,670,000)
Income tax benefit	119,000	1,308,000	429,000	5,021,000
Net loss	$ (20,626,000)	$ (15,540,000)	$ (58,265,000)	$ (71,649,000)
Basic and diluted loss per share (in dollars per share)	$ (0.69)	$ (0.54)	$ (1.98)	$ (2.48)
Basic and diluted average shares outstanding (in shares)	29,939,823	28,983,561	29,462,226	28,911,042