Consolidated 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
Statement Of Income And Comprehensive Income [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 0
Operating expenses:
Research and development expenses	9,850	2,466	24,195	5,658
General and administrative expenses	4,711	1,113	10,873	2,211
Total operating expenses	14,561	3,579	35,068	7,869
Loss from operations	(14,561)	(3,579)	(35,068)	(7,869)
Other income (expense):
Interest income (expense), net	(59)	(61)	(71)	(154)
Revaluation of preferred stock warrant liability		(521)	(7)	(504)
Total other income (expense), net	(59)	(582)	(78)	(658)
Net loss	(14,620)	(4,161)	(35,146)	(8,527)
Accretion of convertible preferred stock to redemption value		(461)		(1,019)
Net loss attributable to common stockholders	$ (14,620)	$ (4,622)	$ (35,146)	$ (9,546)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.38)	$ (0.68)	$ (1.92)	$ (1.42)
Weighted average common shares outstanding, basic and diluted	38,980,839	6,767,951	18,292,002	6,731,724
Other comprehensive loss:
Unrealized loss on investments, net of tax of $0	$ (33)		$ (10)
Total other comprehensive loss	(33)		(10)
Comprehensive loss	$ (14,653)	$ (4,622)	$ (35,156)	$ (9,546)