Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Redeemable Convertible Preferred Stock.	Series A-2	Series B redeemable convertible	Common Stock. Follow-on Offering	Common Stock. Public Offering	Common Stock.	Additional Paid-In Capital Follow-on Offering	Additional Paid-In Capital Public Offering	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss	Follow-on Offering	Public Offering	Total
Balance, beginning of period at Dec. 31, 2014	$ 20,772
Balance, beginning of period (in shares) at Dec. 31, 2014	21,260,000
Statements of Redeemable Convertible Preferred Stock
Issuance of redeemable convertible preferred stock, net of issuance costs		$ 21,989	$ 119,722
Issuance of redeemable convertible preferred stock, net of issuance costs (in shares)		16,890,699	26,666,660
Reclassification of tranche rights upon issuance of redeemable convertible preferred stock	$ 37,038													$ 37,038
Accretion of redeemable convertible preferred stock to redemption value	394													394
Balance, end of period at Dec. 31, 2015	$ 199,915
Balance, end of period (in shares) at Dec. 31, 2015	64,817,359
Balance, beginning of period at Dec. 31, 2014									$ 156	$ (15,448)				(15,292)
Balance, beginning of period (in shares) at Dec. 31, 2014						1,863,169
Statement of Stockholders' Deficit
Accretion of redeemable convertible preferred stock to redemption value									(394)					(394)
Issuance of common stock to licensors upon settlement of antidilution protection liability									1,936					1,936
Issuance of common stock to licensors upon settlement of antidilution protection liability (in shares)						327,970
Exercise of stock options									6					6
Exercise of stock options (in shares)						28,651
Vesting of restricted common stock and common stock subject to repurchase									17					17
Vesting of restricted common stock and common stock subject to repurchase (in shares)						653,272
Vesting of founders shares									2,345					2,345
Vesting of founders shares (in shares)						360,576
Stock-based compensation expense									1,168					1,168
Net loss										(72,900)				(72,900)
Balance, end of period at Dec. 31, 2015									5,234	(88,348)				(83,114)
Balance, end of period (in shares) at Dec. 31, 2015						3,233,638
Statements of Redeemable Convertible Preferred Stock
Accretion of redeemable convertible preferred stock to redemption value	$ 47													47
Conversion of redeemable convertible preferred stock into common stock upon closing of the initial public offering	$ (199,962)
Conversion of redeemable convertible preferred stock into common stock upon closing of the initial public offering (in shares)	(64,817,359)
Statement of Stockholders' Deficit
Accretion of redeemable convertible preferred stock to redemption value									(47)					(47)
Conversion of redeemable convertible preferred stock into common stock upon closing of the initial public offering						$ 3			199,954	5				199,962
Conversion of redeemable convertible preferred stock into common stock upon closing of the initial public offering (in shares)						24,929,709
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering									376					376
Issuance of common stock from public offering, net of issuance costs						$ 1			97,487					97,488
Issuance of common stock from public offering, net of issuance costs (in shares)						6,785,000
Exercise of common stock warrant (in shares)						19,271
Exercise of stock options									233					233
Exercise of stock options (in shares)						58,915
Vesting of restricted common stock and common stock subject to repurchase									11					11
Vesting of restricted common stock and common stock subject to repurchase (in shares)						431,018
Vesting of founders shares									8,315					8,315
Vesting of founders shares (in shares)						360,580
Stock-based compensation expense									8,566					8,566
Net loss										(97,183)				(97,183)
Balance, end of period at Dec. 31, 2016						$ 4			320,129	(185,526)				134,607
Balance, end of period (in shares) at Dec. 31, 2016						35,818,131
Statement of Stockholders' Deficit
Issuance of common stock from public offering, net of issuance costs							$ 96,685	$ 57,223				$ 96,685	$ 57,223
Issuance of common stock from public offering, net of issuance costs (in shares)				4,600,000	2,265,500
Issuance of common stock for repayment of notes payable									14,823					14,823
Issuance of common stock for repayment of notes payable (in shares)						750,617
Exercise of stock options									1,768					1,768
Exercise of stock options (in shares)						272,210
Vesting of restricted common stock and common stock subject to repurchase									4,096					4,096
Vesting of restricted common stock and common stock subject to repurchase (in shares)						561,118
Vesting of founders shares									3,989					3,989
Vesting of founders shares (in shares)						240,384
Stock-based compensation expense									15,289					15,289
Unrealized losses on marketable securities											$ (76)			(76)
Net loss										(120,324)				(120,324)
Balance, end of period at Dec. 31, 2017						$ 4			$ 514,002	$ (305,850)	$ (76)			$ 208,080
Balance, end of period (in shares) at Dec. 31, 2017						44,507,960