Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 30,157	$ 33,895	$ 26,356
General and administrative	12,462	14,180	9,472
Total operating expenses	42,619	48,075	35,828
Loss from operations	(42,619)	(48,075)	(35,828)
Other income (expense), net:
Other income	760	351	66
Change in fair value of preferred stock warrants			(17,443)
Total other income (expense), net	760	351	(17,377)
Loss before provision for income taxes	(41,859)	(47,724)	(53,205)
Provision for income taxes	156	143	55
Net loss and comprehensive loss	(42,015)	(47,867)	(53,260)
Adjustment to redemption value on redeemable convertible preferred stock			(374,015)
Net loss attributable to common stockholders	$ (42,015)	$ (47,867)	$ (453,184)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.84)	$ (1.58)	$ (31.47)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted	49,899,299	30,240,258	14,399,506
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)