Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Stockholder Note Receivable [Member]	Other Comprehensive Loss [Member]	Accumulated Deficit [Member]	Redeemable Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member]
Redeemable Convertible Preferred Stock, Beginning balance at Dec. 31, 2014							$ 95,619
Redeemable Convertible Preferred Stock, shares, Beginning balance at Dec. 31, 2014							73,487,415
Beginning balance at Dec. 31, 2014	$ (91,010)	$ 1	$ 19,209	$ (69)		$ (110,151)
Beginning balance, Shares at Dec. 31, 2014		909,880
Issuance of Series E redeemable convertible preferred stock for cash							$ 75,650
Issuance of Series E redeemable convertible preferred stock for cash, Shares							68,166,894
Conversion of redeemable convertible preferred stock in connection with initial public offering	171,284	$ 16	171,268				$ (171,284)
Conversion of redeemable convertible preferred stock in connection with IPO, Shares		16,279,859					(141,654,309)
Issuance of common stock through initial public offering, net	75,903	$ 6	75,897
Issuance of common stock through initial public offering, net, Shares		6,164,000
Repayment of stockholder note receivable	60		(9)	$ 69
Exercise of common stock options	535	$ 1	534
Exercise of common stock options, Shares		196,500
Reclassification of preferred stock warrant liability to additional paid-in-capital	290		290
Issuance of common stock to The Scripps Research Institute	1,411		1,411
Issuance of common stock to The Scripps Research Institute, Shares		119,840
Changes in share repurchase liability	(120)		(120)
Stock-based compensation	4,856		4,856
Accretion to redemption value of redeemable convertible preferred stock	(15)		(15)				$ 15
Net unrealized gain (loss) on investments, net of tax	(171)				$ (171)
Net loss	(47,973)					(47,973)
Ending balance at Dec. 31, 2015	115,050	$ 24	273,321		(171)	(158,124)
Ending balance, Shares at Dec. 31, 2015		23,670,079
Exercise of common stock options and release of restricted stock units	20		20
Exercise of common stock options and release of restricted stock units, Shares		17,972
Issuance of common stock pursuant to employee stock purchase plan	143		143
Issuance of common stock pursuant to employee stock purchase plan, Shares		56,781
Issuance of warrants related to term loan	217		217
Changes in share repurchase liability	102		102
Stock-based compensation	5,029		5,029
Net unrealized gain (loss) on investments, net of tax	95				95
Net loss	(57,855)					(57,855)
Ending balance at Dec. 31, 2016	62,801	$ 24	278,832		(76)	(215,979)
Ending balance, Shares at Dec. 31, 2016		23,744,832
Exercise of common stock options and release of restricted stock units	$ 186		186
Exercise of common stock options and release of restricted stock units, Shares		111,039
Exercise of common stock options, Shares	75,289
Issuance of common stock pursuant to employee stock purchase plan	$ 175		175
Issuance of common stock pursuant to employee stock purchase plan, Shares		61,171
Issuance of common stock and preferred stock from private placement, net of offering costs	42,239	$ 6	42,231					$ 2
Issuance of common stock and preferred stock from private placement, net of offering costs, Shares		5,872,120						2,285,952
Issuance of warrants related to term loan	263		263
Changes in share repurchase liability	48		48
Stock-based compensation	6,784		6,784
Net unrealized gain (loss) on investments, net of tax	(44)				(44)
Net loss	(48,207)					(48,207)
Ending balance at Dec. 31, 2017	$ 64,245	$ 30	$ 328,519		$ (120)	$ (264,186)		$ 2
Ending balance, Shares at Dec. 31, 2017		29,789,162						2,285,952