Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating expenses:
Research and development	$ 33,895	$ 26,356	$ 19,078
General and administrative	14,180	9,472	3,500
Total operating expenses	48,075	35,828	22,578
Loss from operations	(48,075)	(35,828)	(22,578)
Other income (expense), net:
Change in fair value of preferred stock warrants		(17,443)	(1,380)
Other income	351	66	87
Total other income (expense), net	351	(17,377)	(1,293)
Loss before provision for income taxes	(47,724)	(53,205)	(23,871)
Provision for income taxes	143	55	2
Net loss	(47,867)	(53,260)	(23,873)
Adjustment to redemption value on redeemable convertible preferred stock		(374,015)	(49,849)
Net loss attributable to common stockholders	$ (47,867)	$ (453,184)	$ (73,722)
Net loss per share attributable to common stockholders, basic and diluted	$ (1.58)	$ (31.47)	$ (69.08)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted	30,240,258	14,399,506	1,067,259
Series B and B-1 [Member]
Other income (expense), net:
Redeemable convertible preferred stock dividend		$ (5,543)
Series C and D [Member]
Other income (expense), net:
Redeemable convertible preferred stock dividend		$ (20,366)