Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Operating expenses:
Research and development	$ 12,323	$ 8,295	$ 28,074	$ 18,329
General and administrative	2,960	2,738	10,768	6,053
Total operating expenses	15,283	11,033	38,842	24,382
Loss from operations	(15,283)	(11,033)	(38,842)	(24,382)
Other income (expense), net:
Change in fair value of preferred stock warrants		(7,487)		(17,443)
Other income	88	29	259	48
Total other income (expense), net	88	(7,458)	259	(17,395)
Loss before provision for income taxes	(15,195)	(18,491)	(38,583)	(41,777)
Provision for income taxes	14	14	36	25
Net loss	(15,209)	(18,505)	(38,619)	(41,802)
Adjustment to redemption value on redeemable convertible preferred stock		(222,130)		(374,015)
Net loss attributable to common stockholders	$ (15,209)	$ (266,544)	$ (38,619)	$ (441,726)
Net loss per share attributable to common stockholders, basic and diluted (Note 3)	$ (0.50)	$ (11.03)	$ (1.28)	$ (48.36)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (Note 3)	30,331,616	24,167,238	30,198,134	9,133,978
Series B and B-1 [Member]
Other income (expense), net:
Redeemable convertible preferred stock dividend		$ (5,543)		$ (5,543)
Series C and D [Member]
Other income (expense), net:
Redeemable convertible preferred stock dividend		$ (20,366)		$ (20,366)