CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Thousands	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated deficit	Convertible preferred stock	Total
Balances at Beginning of period (in shares) at Dec. 31, 2017	14,977,317				43,933,006
Balances at Beginning of period at Dec. 31, 2017	$ 15	$ 17,277		$ (60,979)	$ 139,790	$ (43,687)
Issuance of common stock upon exercise of stock options (in shares)	19,750
Issuance of common stock upon exercise of stock options		4				4
Stock-based compensation expense		2,613				2,613
Unrealized gains (losses) on investments			$ (45)			(45)
Net loss				(14,411)		(14,411)
Balances at End of period (in shares) at Mar. 31, 2018	14,997,067				51,845,438
Balances at End of period at Mar. 31, 2018	$ 15	19,802	(45)	(75,298)	$ 240,776	(55,526)
Convertible preferred stock
Issuance of stock, net of issuance costs (in shares)					7,912,432
Issuance of stock, net of issuance costs					$ 100,986
Balances at Beginning of period (in shares) at Dec. 31, 2017	14,977,317				43,933,006
Balances at Beginning of period at Dec. 31, 2017	$ 15	17,277		(60,979)	$ 139,790	(43,687)
Net loss						(62,012)
Balances at End of period (in shares) at Sep. 30, 2018	79,042,603
Balances at End of period at Sep. 30, 2018	$ 79	533,846	(19)	(122,899)		411,007
Cumulative effect adjustment for adoption | ASU 2018-07		(92)		92
Balances at Beginning of period (in shares) at Mar. 31, 2018	14,997,067				51,845,438
Balances at Beginning of period at Mar. 31, 2018	$ 15	19,802	(45)	(75,298)	$ 240,776	(55,526)
Issuance of common stock upon exercise of stock options (in shares)	12,750
Issuance of common stock upon exercise of stock options		14				14
Stock-based compensation expense		5,902				5,902
Vesting of restricted common stock		122				122
Unrealized gains (losses) on investments			18			18
Net loss				(21,239)		(21,239)
Balances at End of period (in shares) at Jun. 30, 2018	15,009,817				51,845,438
Balances at End of period at Jun. 30, 2018	$ 15	25,840	(27)	(96,537)	$ 240,776	(70,709)
Shares issued	12,055,450
Issuance of common stock, initial public offering, net of issuance cost $3,548	$ 12	254,306				254,318
Cashless exercise of warrants	131,273
Issuance of common stock upon exercise of stock options (in shares)	625
Stock-based compensation expense		9,887				9,887
Vesting of restricted common stock		36				36
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering		3,053				3,053
Conversion of redeemable convertible preferred stock to common stock (in shares)	51,845,438				(51,845,438)
Conversion of redeemable convertible preferred stock to common stock	$ 52	240,724			$ (240,776)	240,776
Unrealized gains (losses) on investments			8			8
Net loss				(26,362)		(26,362)
Balances at End of period (in shares) at Sep. 30, 2018	79,042,603
Balances at End of period at Sep. 30, 2018	$ 79	533,846	(19)	(122,899)		411,007
Balances at Beginning of period (in shares) at Dec. 31, 2018	79,234,853
Balances at Beginning of period at Dec. 31, 2018	$ 79	543,040	(29)	(150,082)		393,008
Issuance of common stock upon exercise of stock options (in shares)	619,342
Issuance of common stock upon exercise of stock options	$ 1	798				799
Stock-based compensation expense		9,478				9,478
Vesting of restricted common stock		22				22
Unrealized gains (losses) on investments			110			110
Net loss				(32,581)		(32,581)
Balances at End of period (in shares) at Mar. 31, 2019	79,854,195
Balances at End of period at Mar. 31, 2019	$ 80	553,338	81	(181,863)		$ 371,636
Balances at Beginning of period (in shares) at Dec. 31, 2018						0
Balances at Beginning of period (in shares) at Dec. 31, 2018	79,234,853
Balances at Beginning of period at Dec. 31, 2018	$ 79	543,040	(29)	(150,082)		$ 393,008
Net loss						(118,986)
Balances at End of period (in shares) at Sep. 30, 2019	79,777,733
Balances at End of period at Sep. 30, 2019	$ 80	575,365	166	(268,268)		$ 307,343
Balances at Beginning of period (in shares) at Dec. 31, 2018						0
Balances at End of period (in shares) at Sep. 30, 2019						0
Cumulative effect adjustment for adoption | ASU 2016-02				800		$ 800
Balances at Beginning of period (in shares) at Mar. 31, 2019	79,854,195
Balances at Beginning of period at Mar. 31, 2019	$ 80	553,338	81	(181,863)		371,636
Issuance of common stock upon exercise of stock options (in shares)	223,484
Issuance of common stock upon exercise of stock options		572				572
Stock-based compensation expense		10,604				10,604
Vesting of restricted common stock		13				13
Unrealized gains (losses) on investments			209			209
Net loss				(39,390)		(39,390)
Balances at End of period (in shares) at Jun. 30, 2019	80,077,679
Balances at End of period at Jun. 30, 2019	$ 80	564,527	290	(221,253)		343,644
Issuance of common stock upon exercise of stock options (in shares)	367,971
Issuance of common stock upon exercise of stock options	$ 1	471				472
Stock-based compensation expense		10,358				10,358
Repurchase of unvested restricted common stock (in shares)	(667,917)
Repurchase of unvested restricted common stock	$ (1)					(1)
Vesting of restricted common stock		9				9
Unrealized gains (losses) on investments			(124)			(124)
Net loss				(47,015)		(47,015)
Balances at End of period (in shares) at Sep. 30, 2019	79,777,733
Balances at End of period at Sep. 30, 2019	$ 80	$ 575,365	$ 166	$ (268,268)		$ 307,343
Balances at End of period (in shares) at Sep. 30, 2019						0