Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss
Balances at Dec. 31, 2015	$ (22,207)				$ 1,373	$ (23,579)	$ (1)
Balances (in shares) at Dec. 31, 2015				757,000
Temporary Equity Balances at Dec. 31, 2015		$ 13,573	$ 44,738
Temporary Equity Balances (in shares) at Dec. 31, 2015		2,173,000	5,000,000
Conversion of preferred stock to common stock upon initial public offering	58,311	$ (13,573)	$ (44,738)	$ 1	58,310
Conversion of preferred stock to common stock upon initial public offering (in shares)		(2,173,000)	(5,000,000)	7,173,000
Issuance of common stock in connection with initial public offering/ follow-on offering/ at-the-market offerings, net of offering costs	47,266				47,266
Issuance of common stock in connection with initial public offering/ follow-on offering/ at-the-market offerings, net of offering costs				5,482,000
Issuance of common stock in connection with employee stock purchase plan	76				76
Issuance of common stock in connection with employee stock purchase plan (in shares)				19,000
Stock-based compensation expense	1,221				1,221
Unrealized gain (loss) on marketable securities	(3)						(3)
Net loss	(21,698)					(21,698)
Balances at Dec. 31, 2016	62,966			$ 1	108,246	(45,277)	(4)
Balances (in shares) at Dec. 31, 2016				13,431,000
Issuance of common stock in connection with initial public offering/ follow-on offering/ at-the-market offerings, net of offering costs	11,380			$ 1	11,379
Issuance of common stock in connection with initial public offering/ follow-on offering/ at-the-market offerings, net of offering costs				3,000,000
Issuance of common stock in connection with employee stock purchase plan	131				131
Issuance of common stock in connection with employee stock purchase plan (in shares)				39,000
Issuance of common stock in connection with exercise of stock options	702				702
Issuance of common stock in connection with exercise of stock options (in shares)				200,000
Stock-based compensation expense	2,492				2,492
Unrealized gain (loss) on marketable securities	(98)						(98)
Net loss	(27,236)					(27,236)
Balances at Dec. 31, 2017	50,337			$ 2	122,950	(72,513)	(102)
Balances (in shares) at Dec. 31, 2017				16,670,000
Issuance of common stock in connection with initial public offering/ follow-on offering/ at-the-market offerings, net of offering costs	54,048				54,048
Issuance of common stock in connection with initial public offering/ follow-on offering/ at-the-market offerings, net of offering costs				6,893,000
Issuance of common stock in connection with employee stock purchase plan	207				207
Issuance of common stock in connection with employee stock purchase plan (in shares)				56,000
Issuance of common stock in connection with exercise of stock options	$ 2,829				2,829
Issuance of common stock in connection with exercise of stock options (in shares)	521,665			521,000
Stock-based compensation expense	$ 4,280				4,280
Unrealized gain (loss) on marketable securities	75						75
Net loss	(44,348)					(44,348)
Balances at Dec. 31, 2018	$ 67,428			$ 2	$ 184,314	$ (116,861)	$ (27)
Balances (in shares) at Dec. 31, 2018				24,140,000