Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 154,496	$ (151,197)
Conversion of convertible securities		237,259
Stock compensation expense	8,645	5,096
Exercise of common stock options	1,418	(154)
Issuance of common stock under employee stock purchase plan	739
Sale of common stock upon consummation of initial public offering, net of issuance costs	59,917	125,471
Foreign currency translation adjustment	54	(12)
Net loss	(106,658)	(48,902)
Ending balance	$ 118,611	$ 154,496
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding (in shares)	37,802,556	4,067,717
Beginning balance	$ 38	$ 4
Conversion of convertible securities (in shares)		25,068,556
Conversion of convertible securities		$ 25
Exercise of common stock options (in shares)	482,190	41,283
Exercise of warrants (in shares)	14,647
Issuance of common stock under employee stock purchase plan (in shares)	53,137
Sale of common stock upon consummation of initial public offering, net of issuance costs (in shares)	2,875,000	8,625,000
Sale of common stock upon consummation of initial public offering, net of issuance costs	$ 3	$ 9
Shares outstanding (in shares)	41,227,530	37,802,556
Ending balance	$ 41	$ 38
Additional Paid -in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	365,838	0
Conversion of convertible securities		237,234
Stock compensation expense	8,645	5,096
Exercise of common stock options	1,418	(154)
Issuance of common stock under employee stock purchase plan	739
Sale of common stock upon consummation of initial public offering, net of issuance costs	59,914	125,462
Ending balance	436,554	365,838
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(211,269)	(151,102)
Net loss	(106,658)	(48,902)
Ending balance	(317,927)	(211,269)
Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(111)	(99)
Foreign currency translation adjustment	54	(12)
Ending balance	$ (57)	$ (111)
Redeemable Convertible Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding (in shares)	0	6,875,514
Beginning balance	$ 0	$ 168,173
Conversion of convertible securities (in shares)		8,680,566
Conversion of convertible securities		$ (237,259)
Shares outstanding (in shares)		0
Ending balance		$ 0
Redeemable Convertible Preferred Stock, Series C2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of convertible securities (in shares)		687,474
Conversion of convertible securities		$ 19,527
Redeemable Convertible Preferred Stock, Series D
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Sale of Series D preferred stock, net of issuance costs (in shares)		1,117,578
Sale of Series D preferred stock, net of issuance costs		$ 36,494
Redeemable Convertible Preferred Stock, Series C, C1, D
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accretion of preferred stock	0	1,096
Accretion of preferred stock		(1,096)
Redeemable Convertible Preferred Stock, Series C, C1, D | Additional Paid -in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accretion of preferred stock		(1,096)
Redeemable Convertible Preferred Stock, Series C, C1, C2, and D
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accretion of preferred stock	$ 0	11,969
Accretion of preferred stock		(11,969)
Redeemable Convertible Preferred Stock, Series C, C1, C2, and D | Additional Paid -in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accretion of preferred stock		(704)
Redeemable Convertible Preferred Stock, Series C, C1, C2, and D | Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accretion of preferred stock		$ (11,265)