Condensed Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Redeemable Convertible Preferred Stock	Series A Preferred Stock	Common Stock	Treasury Stock	Additional Paid-In-Capital	Accumulated Deficit	Total
Balance at beginning at Dec. 31, 2017	$ 5,000					$ (4,564)	$ (4,564)
Balance at beginning (in shares) at Dec. 31, 2017	5,000,000		226,400
Issuance of treasury stock as founders’ stock (in shares)			75,467	(75,467)
Net loss and comprehensive loss						(421)	(421)
Balance at ending at Mar. 31, 2018	$ 5,000					(4,985)	(4,985)
Balance at ending (in shares) at Mar. 31, 2018	5,000,000		226,400
Balance at beginning at Dec. 31, 2017	$ 5,000					(4,564)	(4,564)
Balance at beginning (in shares) at Dec. 31, 2017	5,000,000		226,400
Net loss and comprehensive loss							(20,242)
Balance at ending at Sep. 30, 2018	$ 14,108					(25,093)	(25,093)
Balance at ending (in shares) at Sep. 30, 2018	22,653,333		226,400
Repurchase of founders’ stock (in shares)			(75,467)	75,467
Balance at beginning at Mar. 31, 2018	$ 5,000					(4,985)	(4,985)
Balance at beginning (in shares) at Mar. 31, 2018	5,000,000		226,400
Issuance of stock		$ 8,787
Issuance of stock (in shares)		17,653,333
Accretion of redeemable convertible preferred stock to redemption value	$ 70					70	70
Net loss and comprehensive loss						(8,673)	(8,673)
Balance at ending at Jun. 30, 2018	$ 13,857					(13,728)	(13,728)
Balance at ending (in shares) at Jun. 30, 2018	22,653,333		226,400
Stock-based compensation expense					$ 34		34
Accretion of redeemable convertible preferred stock to redemption value					34
Accretion of redeemable convertible preferred stock to redemption value	$ 251					217	251
Net loss and comprehensive loss						(11,148)	(11,148)
Balance at ending at Sep. 30, 2018	$ 14,108					(25,093)	(25,093)
Balance at ending (in shares) at Sep. 30, 2018	22,653,333		226,400
Balance at beginning at Dec. 31, 2018	$ 124,728				36,646	(86,565)	(49,919)
Balance at beginning (in shares) at Dec. 31, 2018	64,730,410		238,986
Stock-based compensation expense					215		215
Net loss and comprehensive loss						(5,362)	(5,362)
Balance at ending at Mar. 31, 2019	$ 124,728				36,861	(91,927)	(55,066)
Balance at ending (in shares) at Mar. 31, 2019	64,730,410		238,986
Balance at beginning at Dec. 31, 2018	$ 124,728				36,646	(86,565)	(49,919)
Balance at beginning (in shares) at Dec. 31, 2018	64,730,410		238,986
Conversion of convertible preferred stock into common stock upon closing of initial public offering							$ 124,728
Issuance of restricted common stock upon early exercise of stock options (in shares)			383,320
Exercise of stock options (in shares)							651,031
Net loss and comprehensive loss							$ (28,144)
Balance at ending at Sep. 30, 2019			$ 3		258,090	(114,709)	143,384
Balance at ending (in shares) at Sep. 30, 2019			28,558,653
Balance at beginning at Mar. 31, 2019	$ 124,728				36,861	(91,927)	(55,066)
Balance at beginning (in shares) at Mar. 31, 2019	64,730,410		238,986
Conversion of convertible preferred stock into common stock upon closing of initial public offering	$ (124,728)		$ 2		124,726		124,728
Conversion of convertible preferred stock into common stock upon closing of initial public offering (in shares)	(64,730,410)		21,056,136
Issuance of stock			$ 1		95,452		95,453
Issuance of stock (in shares)			6,612,500
Issuance of restricted common stock upon early exercise of stock options (in shares)			487,933
Exercise of stock options					100		100
Exercise of stock options (in shares)			159,824
Stock-based compensation expense					402		402
Net loss and comprehensive loss						(7,228)	(7,228)
Balance at ending at Jun. 30, 2019			$ 3		257,541	(99,155)	158,389
Balance at ending (in shares) at Jun. 30, 2019			28,555,379
Vesting of restricted common stock					66		66
Issuance of restricted common stock upon early exercise of stock options (in shares)			3,274
Stock-based compensation expense					483		483
Net loss and comprehensive loss						(15,554)	(15,554)
Balance at ending at Sep. 30, 2019			$ 3		$ 258,090	$ (114,709)	$ 143,384
Balance at ending (in shares) at Sep. 30, 2019			28,558,653