Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Contract and grant revenue	$ 250	$ 229	$ 387
Operating expenses:
Research and development	97,744	60,127	28,452
General and administrative	21,582	15,948	5,885
Total operating expenses	119,326	76,075	34,337
Loss from operations	(119,076)	(75,846)	(33,950)
Other income (expense):
Interest income	897	97	3
Interest expense		(1)
Other expense	(2)	(27)
Total other income, net	895	69	3
Net loss	$ (118,181)	$ (75,777)	$ (33,947)
Net loss per share-basic and diluted	$ (3.32)	$ (2.43)	$ (5.59)
Weighted-average number of common shares outstanding used in net loss per share-basic and diluted	35,619,506	31,135,694	6,067,679