Consolidated Statements of Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Series B Convertible Preferred Stock	Series C Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss
Balance at Dec. 31, 2015	$ (2,171)					$ 11	$ (2,182)
Balance, shares at Dec. 31, 2015		12,556,791
Balance at Dec. 31, 2015		$ 49,637
Balance, shares at Dec. 31, 2015					2,798,146
Issuance of convertible preferred stock, net of issuance costs			$ 69,817
Issuance of convertible preferred stock, net of issuance costs, shares			8,750,852
Vesting of early exercised stock options	63					63
Vesting of early exercised stock options, shares					137,050
Stock-based compensation	90					90
Other comprehensive loss	(26)							$ (26)
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares	$ 20					20
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares	637,107				47,576
Net loss	$ (17,970)					0	(17,970)
Balance at Dec. 31, 2016	(19,994)					184	(20,152)	(26)
Balance, shares at Dec. 31, 2016		21,307,643
Balance at Dec. 31, 2016		$ 119,454
Balance, shares at Dec. 31, 2016					2,982,772
Issuance of convertible preferred stock, net of issuance costs				$ 106,742
Issuance of convertible preferred stock, net of issuance costs, shares				9,151,931
Vesting of early exercised stock options and restricted stock	235					235
Vesting of early exercised stock options and restricted stock, shares					269,752
Stock-based compensation	495					495
Other comprehensive loss	(16)							(16)
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares	$ 34					34
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares	643,024				25,605
Net loss	$ (53,082)						(53,082)
Balance at Dec. 31, 2017	$ (72,328)					948	(73,234)	(42)
Balance, shares at Dec. 31, 2017	30,459,574	30,459,574
Balance at Dec. 31, 2017	$ 226,196	$ 226,196
Balance, shares at Dec. 31, 2017	4,090,898				3,278,129
Conversion of preferred stock to common stock	$ 226,198				$ 3	226,195
Conversion of preferred stock to common stock, shares		(30,459,574)
Conversion of preferred stock to common stock		$ (226,196)
Conversion of preferred stock to common stock, shares					30,459,574
Issuance of common stock upon IPO, net of issuance costs	124,735				$ 1	124,734
Issuance of common stock upon IPO, net of issuance costs, shares					9,200,000
Vesting of early exercised stock options and restricted stock	1,276					1,276
Vesting of early exercised stock options and restricted stock, shares					528,374
Issuance of common stock under Employee Stock Purchase Plan	751					751
Issuance of common stock under Employee Stock Purchase Plan, shares					77,397
Stock-based compensation	3,874					3,874
Other comprehensive loss	(65)							(65)
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares	$ 95					95
Issuance of common stock upon exercise of stock options, net of amounts related to unvested shares, shares	720,756				67,349
Net loss	$ (49,594)						(49,594)
Balance at Dec. 31, 2018	$ 234,942				$ 4	$ 357,873	$ (122,828)	$ (107)
Balance, shares at Dec. 31, 2018	0	0
Balance at Dec. 31, 2018	$ 0
Balance, shares at Dec. 31, 2018	44,537,946				43,610,823