Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) - USD ($) $ in Thousands	Total	July 2019 Follow-on Offering [Member]	IPO [Member]	Series Seed Redeemable Convertible Preferred Stock [Member]	Series A Redeemable Convertible Preferred Stock [Member]	Series B Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] July 2019 Follow-on Offering [Member]	Common Stock [Member] IPO [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] July 2019 Follow-on Offering [Member]	Additional Paid-in Capital [Member] IPO [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	(30,340)
Ending balance at Sep. 30, 2018	137,109						$ 2			192,825			(55,718)
Ending balance, shares at Sep. 30, 2018							23,758,898
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 common stock on public offering, net of issuance or offering costs			$ 104,182						$ 1			$ 104,181
Issuance common stock on public offering, net of issuance or offering costs, 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
Exercise of common stock warrant, shares							6,091
IPO issuance cost difference	(168)									(168)
Stock-based compensation expense	550									550
Vesting of restricted stock awards	0			$ 0	$ 0	$ 0	$ 0			0			0
Vesting of restricted stock awards, shares							30,758
Net loss	(11,591)												(11,591)
Ending balance at Sep. 30, 2018	137,109						$ 2			192,825			(55,718)
Ending balance, shares at Sep. 30, 2018							23,758,898
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
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	232,758
Net loss	$ (50,303)
Ending balance at Sep. 30, 2019	206,760						$ 3			328,799			(122,042)
Ending balance, shares at Sep. 30, 2019							31,606,648
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
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
Issuance common stock on public offering, net of issuance or offering costs		$ 129,465						$ 1			$ 129,464
Issuance common stock on public offering, net of issuance or offering costs, shares								7,475,000
Stock-based compensation expense	1,797									1,797
Exercise of stock options	9									9
Exercise of stock options, shares							5,609
Vesting of restricted stock awards	0						$ 0			0			0
Vesting of restricted stock awards, shares							30,752
Net loss	(17,145)												(17,145)
Ending balance at Sep. 30, 2019	$ 206,760						$ 3			$ 328,799			$ (122,042)
Ending balance, shares at Sep. 30, 2019							31,606,648