Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Revenue	$ 2,061	$ 1,184	$ 5,397	$ 1,343
Operating expenses:
Research and development	10,372	3,217	25,238	8,157
General and administrative	2,334	451	6,115	1,536
Total operating expenses	12,706	3,668	31,353	9,693
Loss from operations	(10,645)	(2,484)	(25,956)	(8,350)
Interest income (expense), net	(22)		(72)
Net loss and comprehensive loss	(10,667)	(2,484)	(26,028)	(8,350)
Accretion of convertible preferred stock to redemption value		(18)	(23)	(52)
Net loss attributable to common stockholders	$ (10,667)	$ (2,502)	$ (26,051)	$ (8,402)
Net loss per share attributable to common stockholders - basic and diluted	$ (2.73)	$ (0.70)	$ (6.71)	$ (2.35)
Weighted average common shares outstanding - basic and diluted	3,907,196	3,585,603	3,880,779	3,582,213