Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ (83,292)		$ 2	$ 2,470	$ (1)	$ (85,763)
Temporary equity, Beginning balance, shares at Dec. 31, 2017		17,510,045
Temporary equity, Beginning balance at Dec. 31, 2017		$ 137,469
Beginning balance, shares at Dec. 31, 2017			1,627,032
Exercise of Series A-1 warrants		$ 312
Temporary equity, Exercise of Series A-1 warrants, Shares		52,169
Issuance of common stock warrants	104			104
Exercise of stock options	215			215
Exercise of stock options, Shares			143,422
Unrealized gain on available-for-sale securities	1				$ 1
Vesting of early exercised options	22			22
Stock-based compensation	273			273
Net loss	(9,594)					(9,594)
Ending balance at Mar. 31, 2018	(92,271)		$ 2	3,084		(95,357)
Temporary equity, Ending balance, shares at Mar. 31, 2018		17,562,214
Temporary equity, Ending balance at Mar. 31, 2018		$ 137,781
Ending balance, shares at Mar. 31, 2018			1,770,454
Beginning balance at Dec. 31, 2018	(122,588)		$ 2	4,275		(126,865)
Temporary equity, Beginning balance, shares at Dec. 31, 2018		18,670,328
Temporary equity, Beginning balance at Dec. 31, 2018	152,806	$ 152,806
Beginning balance, shares at Dec. 31, 2018			1,824,852
Exercise of common stock warrants for cash	110			110
Exercise of common stock warrants for cash, shares			50,331
Issuance of common stock upon net exercise of warrants	133			133
Issuance of common stock upon net exercise of warrants, shares			101,744
Conversion of preferred stock to common stock upon initial public offering	152,806		$ 18	152,788
temporary equity conversion of preferred stock shares to common stock upon initial public offering, shares		(18,670,328)
temporary equity conversion of preferred stock value to common stock upon initial public offering		$ (152,806)
Conversion of preferred to common stock upon initial public offering, Shares			18,670,328
Conversion of Series A-1 warrants to common stock warrants upon initial public offering	789			789
Issuance of common stock in connection with initial public offering, net of issuance costs of $11.3 million	100,139		$ 7	100,132
Issuance of common stock in connection with initial public offering, net of issuance costs, Shares			6,555,000
Issuance of common stock in connection with private placement	10,000		$ 1	9,999
issuance of common stock in connection with private placement, shares			588,235
Exercise of stock options	$ 169			169
Exercise of stock options, Shares	80,515		80,515
Vesting of early exercised options	$ 18			18
Stock-based compensation	412			412
Adjustment for fractional shares resulting from reverse stock split	(3)			(3)
Adjustment for fractional shares resulting from reverse stock split			(114)
Net loss	(12,799)					(12,799)
Ending balance at Mar. 31, 2019	$ 129,186		$ 28	$ 268,822		$ (139,664)
Ending balance, shares at Mar. 31, 2019			27,870,891