Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Deficit (Unaudited) - USD ($) $ in Thousands	Total	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Common Stock	Additional Paid in Capital	Other Comprehensive Income / (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ (20,280)			$ 3	$ 66		$ (20,349)
Beginning balance, shares at Dec. 31, 2017		9,008,919
Beginning balance at Dec. 31, 2017		$ 17,178
Beginning balance, shares at Dec. 31, 2017				3,361,016
Stock based compensation	47				47
Proceeds from Series B redeemable convertible preferred stock, net of issuance costs, shares			7,890,466
Proceeds from Series B redeemable convertible preferred stock, net of issuance costs			$ 75,688
Issuance of Series B redeemable convertible preferred stock in connection with the conversion of convertible promissory notes and accrued interest			$ 11,027
Issuance of Series B redeemable convertible preferred stock in connection with the conversion of convertible promissory notes and accrued interest, shares			1,147,205
Vesting of Series B redeemable convertible preferred stock in lieu of rent			$ 61
Issuance of Series B redeemable convertible preferred stock in connection with the facility lease agreement, shares			114,437
Other comprehensive income	(16)					$ (16)
Net loss	(9,006)						(9,006)
Ending balance at Sep. 30, 2018	(29,255)			$ 3	113	(16)	(29,355)
Ending balance, shares at Sep. 30, 2018		9,008,919	9,152,108
Ending balance at Sep. 30, 2018		$ 17,178	$ 86,776
Ending balance, shares at Sep. 30, 2018				3,361,016
Beginning balance at Jun. 30, 2018	(26,738)			$ 3	93		(26,834)
Beginning balance, shares at Jun. 30, 2018		9,008,919	9,037,671
Beginning balance at Jun. 30, 2018		$ 17,178	$ 86,715
Beginning balance, shares at Jun. 30, 2018				3,361,016
Stock based compensation	20				20
Vesting of Series B redeemable convertible preferred stock in lieu of rent			$ 61
Issuance of Series B redeemable convertible preferred stock in connection with the facility lease agreement, shares			114,437
Other comprehensive income	(16)					(16)
Net loss	(2,521)						(2,521)
Ending balance at Sep. 30, 2018	(29,255)			$ 3	113	(16)	(29,355)
Ending balance, shares at Sep. 30, 2018		9,008,919	9,152,108
Ending balance at Sep. 30, 2018		$ 17,178	$ 86,776
Ending balance, shares at Sep. 30, 2018				3,361,016
Beginning balance at Dec. 31, 2018	(32,626)			$ 3	245	(49)	(32,825)
Beginning balance, shares at Dec. 31, 2018		9,008,919	9,152,108
Beginning balance at Dec. 31, 2018		$ 17,178	$ 86,868
Beginning balance, shares at Dec. 31, 2018				3,412,366
Exercise of stock options	69			$ 1	68
Exercise of stock options, shares				166,015
Stock based compensation	1,479				1,479
Vesting of Series B redeemable convertible preferred stock in lieu of rent			$ 948
Conversion of redeemable convertible preferred stock to common stock	104,994			$ 18	104,976
Conversion of redeemable convertible preferred stock to common stock, shares		(9,008,919)	(9,152,108)
Conversion of redeemable convertible preferred stock to common stock		$ (17,178)	$ (87,816)
Conversion of redeemable convertible preferred stock to common stock, shares				18,161,027
Initial public offering of common stock, net of issuance costs	77,827			$ 5	77,822
Initial public offering of common stock, net of issuance costs, shares				5,073,800
Exercise of stock warrant, shares				27,941
Other comprehensive income	145					145
Net loss	(24,601)						(24,601)
Ending balance at Sep. 30, 2019	127,287			$ 27	184,590	96	(57,426)
Ending balance, shares at Sep. 30, 2019		0	0
Ending balance, shares at Sep. 30, 2019				26,841,149
Beginning balance at Jun. 30, 2019	136,217			$ 27	183,678	80	(47,568)
Beginning balance, shares at Jun. 30, 2019				26,841,149
Stock based compensation	912				912
Other comprehensive income	16					16
Net loss	(9,858)						(9,858)
Ending balance at Sep. 30, 2019	$ 127,287			$ 27	$ 184,590	$ 96	$ (57,426)
Ending balance, shares at Sep. 30, 2019		0	0
Ending balance, shares at Sep. 30, 2019				26,841,149