Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) - USD ($) $ in Thousands	Total	Series Seed Redeemable Convertible Preferred Stock [Member]	Series A Redeemable Convertible Preferred Stock [Member]	Series B Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017	$ (23,135)	$ 1,500	$ 25,000			$ 339	$ (23,474)
Beginning balance, shares at Dec. 31, 2017		3,333,333	31,450,499		2,305,173
Issuance of redeemable convertible preferred stock,				$ 58,257
Issuance of redeemable convertible preferred stock, shares				28,285,557
Issuance of series B redeemable convertible preferred stock to settled accrued liability of license cost				$ 500
Issuance of series B redeemable convertible preferred stock to settle accrued liability of license cost, shares				233,765
Accretion of issuance costs related to redeemable convertible preferred stock	(2,243)			$ 2,243		(339)	(1,904)
Stock-based compensation expense	109					109
Vesting of restricted stock awards					$ 30,753
Net loss	(8,242)						(8,242)
Ending balance at Mar. 31, 2018	(33,511)	$ 1,500	$ 25,000	$ 61,000		109	(33,620)
Ending balance, shares at Mar. 31, 2018		3,333,333	31,450,499	28,519,322	2,335,926
Beginning balance at Dec. 31, 2017	(23,135)	$ 1,500	$ 25,000			339	(23,474)
Beginning balance, shares at Dec. 31, 2017		3,333,333	31,450,499		2,305,173
Net loss	(18,749)
Ending balance at Jun. 30, 2018	148,318				$ 2	192,443	(44,127)
Ending balance, shares at Jun. 30, 2018					23,722,049
Beginning balance at Mar. 31, 2018	(33,511)	$ 1,500	$ 25,000	$ 61,000		109	(33,620)
Beginning balance, shares at Mar. 31, 2018		3,333,333	31,450,499	28,519,322	2,335,926
Issuance of common stock upon IPO	104,182				$ 1	104,181
Issuance of common stock upon IPO, shares					6,035,151
Stock-based compensation expense	457					457
Vesting of restricted stock awards	0	$ 0	$ 0	$ 0	$ 0	0	0
Vesting of restricted stock awards, shares					30,759
Net loss	(10,507)						(10,507)
Conversion of redeemable convertible preferred stock into common stock	87,500	$ (1,500)	$ (25,000)	$ (61,000)	$ 1	87,499
Conversion of redeemable convertible preferred stock into common stock, shares		(3,333,333)	(31,450,499)	(28,519,322)	15,320,213
Reclassification of warrants to purchase shares of redeemable convertible preferred stock into warrants to purchase common stock	197					197
Ending balance at Jun. 30, 2018	148,318				$ 2	192,443	(44,127)
Ending balance, shares at Jun. 30, 2018					23,722,049
Beginning balance at Dec. 31, 2018	122,184				$ 2	193,921	(71,739)
Beginning balance, shares at Dec. 31, 2018					23,806,628
Stock-based compensation expense	1,455					1,455
Exercise of stock options	252					252
Exercise of stock options, shares					116,859
Vesting of restricted stock awards	0	$ 0	$ 0	$ 0	$ 0	0	0
Vesting of restricted stock awards, shares					30,753
Net loss	(17,103)						(17,103)
Ending balance at Mar. 31, 2019	106,788				$ 2	195,628	(88,842)
Ending balance, shares at Mar. 31, 2019					23,954,240
Beginning balance at Dec. 31, 2018	$ 122,184				$ 2	193,921	(71,739)
Beginning balance, shares at Dec. 31, 2018					23,806,628
Exercise of stock options, shares	227,149
Net loss	$ (33,158)
Ending balance at Jun. 30, 2019	92,634				$ 2	197,529	(104,897)
Ending balance, shares at Jun. 30, 2019					24,095,287
Beginning balance at Mar. 31, 2019	106,788				$ 2	195,628	(88,842)
Beginning balance, shares at Mar. 31, 2019					23,954,240
Stock-based compensation expense	1,663					1,663
Exercise of stock options	238					238
Exercise of stock options, shares					110,290
Vesting of restricted stock awards	0	$ 0	$ 0	$ 0	$ 0	0	0
Vesting of restricted stock awards, shares					30,757
Net loss	(16,055)						(16,055)
Ending balance at Jun. 30, 2019	$ 92,634				$ 2	$ 197,529	$ (104,897)
Ending balance, shares at Jun. 30, 2019					24,095,287