Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) Equity - USD ($) $ in Thousands	Common stock	Additional paid-in capital	Accumulated deficit	Series A Preferred Stock	Series B Preferred Stock	Series C Preferred Stock	Series D Preferred Stock	Total
Beginning balance at Dec. 31, 2015	$ 2		$ (88,642)	$ 23,898	$ 15,178	$ 34,369		$ (88,640)
Beginning balance (in shares) at Dec. 31, 2015	1,976,992			14,400,001	5,624,106	8,605,944
Increase (Decrease) in Stockholders' Equity
Issuance of stock, net of issuance costs							$ 45,428
Issuance of stock, net of issuance costs (in shares)							12,420,262
Exercise of preferred stock warrants				$ 3,901	$ 1,374
Exercise of preferred stock warrants (in shares)				1,300,000	397,530
Exercise of common stock options		$ 213						213
Exercise of common stock options (in shares)	90,883
Vesting of restricted stock (in shares)	247,621
Accretion of preferred stock to redemption value		(1,141)	(3,296)	$ 1,180	$ 907	$ 2,309	$ 41	(4,437)
Stock-based compensation expense		928						928
Net loss			(23,173)					(23,173)
Ending Balance at Dec. 31, 2016	$ 2		(115,111)	$ 28,979	$ 17,459	$ 36,678	$ 45,469	(115,109)
Ending Balance (in shares) at Dec. 31, 2016	2,315,496			15,700,001	6,021,636	8,605,944	12,420,262
Increase (Decrease) in Stockholders' Equity
Issuance of stock, net of issuance costs	$ 5	65,570					$ 8,423	65,575
Issuance of stock, net of issuance costs (in shares)	4,916,480						2,113,902
Exercise of preferred stock warrants				$ 2,078		$ 138
Exercise of preferred stock warrants (in shares)				700,000		31,283
Exercise of common stock options and vesting restricted stock		204						204
Exercise of common stock options and vesting restricted stock (in shares)	289,321
Cumulative-effect of adoption of ASU		141	(141)
Accretion of preferred stock to redemption value		(2,029)	(2,081)	$ 1,080	$ 840	$ 2,140	$ 50	(4,110)
Conversion of preferred stock to common stock	$ 15	143,319		$ (32,137)	$ (18,299)	$ (38,956)	$ (53,942)	143,334
Conversion of preferred stock to common stock (in shares)	14,185,744			(16,400,001)	(6,021,636)	(8,637,227)	(14,534,164)
Warrant Liability reclassified to equity upon IPO		823						823
Stock-based compensation expense		2,168						2,168
Net loss			(27,019)					(27,019)
Ending Balance at Dec. 31, 2017	$ 22	210,196	(144,352)					65,866
Ending Balance (in shares) at Dec. 31, 2017	21,707,041
Increase (Decrease) in Stockholders' Equity
Exercise of common stock options (in shares)	16,718
Exercise of common stock options and vesting restricted stock		1,871						1,871
Exercise of common stock options and vesting restricted stock (in shares)	645,277
Common stock issuance offering costs		(20)						(20)
Stock-based compensation expense		4,884						4,884
Net loss			(31,536)					(31,536)
Ending Balance at Dec. 31, 2018	$ 22	$ 216,931	$ (175,888)					$ 41,065
Ending Balance (in shares) at Dec. 31, 2018	22,369,036