Condensed Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ (52,033)		$ 1	$ 10,153		$ (62,187)
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2017		36,001,203
Convertible preferred stock, beginning balance at Dec. 31, 2017		$ 77,485
Beginning balance (in shares) at Dec. 31, 2017			13,776,153
Forfeiture of restricted common stock (in shares)			(11,324)
Stock-based compensation	1,124			1,124
Accumulated other comprehensive income	(154)				$ (154)
Net loss	(8,437)					(8,437)
Ending balance at Mar. 31, 2018	(59,500)		$ 1	11,277	(154)	(70,624)
Convertible preferred stock, ending balance (in shares) at Mar. 31, 2018		36,001,203
Ending balance (in shares) at Mar. 31, 2018			13,764,829
Convertible preferred stock, ending balance at Mar. 31, 2018		$ 77,485
Beginning balance at Dec. 31, 2017	(52,033)		$ 1	10,153		(62,187)
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2017		36,001,203
Convertible preferred stock, beginning balance at Dec. 31, 2017		$ 77,485
Beginning balance (in shares) at Dec. 31, 2017			13,776,153
Net loss	(19,558)
Ending balance at Jun. 30, 2018	(69,398)		$ 1	12,438	(92)	(81,745)
Convertible preferred stock, ending balance (in shares) at Jun. 30, 2018		40,943,028
Ending balance (in shares) at Jun. 30, 2018			13,764,829
Convertible preferred stock, ending balance at Jun. 30, 2018		$ 147,569
Beginning balance at Mar. 31, 2018	(59,500)		$ 1	11,277	(154)	(70,624)
Convertible preferred stock, beginning balance (in shares) at Mar. 31, 2018		36,001,203
Convertible preferred stock, beginning balance at Mar. 31, 2018		$ 77,485
Beginning balance (in shares) at Mar. 31, 2018			13,764,829
Issuance of Series E convertible preferred stock, net of issuance costs		$ 70,084
Issuance of Series E convertible preferred stock, net of costs (in shares)		4,941,825
Stock-based compensation	1,161			1,161
Accumulated other comprehensive income	62				62
Net loss	(11,121)					(11,121)
Ending balance at Jun. 30, 2018	(69,398)		$ 1	12,438	(92)	(81,745)
Convertible preferred stock, ending balance (in shares) at Jun. 30, 2018		40,943,028
Ending balance (in shares) at Jun. 30, 2018			13,764,829
Convertible preferred stock, ending balance at Jun. 30, 2018		$ 147,569
Beginning balance at Dec. 31, 2018	$ (97,398)		$ 1	17,078	(42)	(114,435)
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2018	45,374,836	45,374,836
Convertible preferred stock, beginning balance at Dec. 31, 2018	$ 210,520	$ 210,520
Beginning balance (in shares) at Dec. 31, 2018	13,764,829		13,764,829
Conversion of convertible preferred stock into common stock	$ 210,521	$ (210,520)	$ 5	210,516
Conversion of convertible preferred stock into common stock (in shares)		(45,374,836)	45,374,836
Issuance of common stock upon initial public offering, net of issuance costs	168,223		$ 1	168,222
Issuance of common stock upon initial public offering, net of issuance costs (in shares)			9,739,541
Exercise of stock options	5			5
Exercise of stock options (in shares)			625
Forfeiture of restricted common stock (in shares)			(8,039)
Stock-based compensation	3,245			3,245
Accumulated other comprehensive income	150				150
Net loss	(18,560)					(18,560)
Ending balance at Mar. 31, 2019	266,186		$ 7	399,066	108	(132,995)
Ending balance (in shares) at Mar. 31, 2019			68,871,792
Beginning balance at Dec. 31, 2018	$ (97,398)		$ 1	17,078	(42)	(114,435)
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2018	45,374,836	45,374,836
Convertible preferred stock, beginning balance at Dec. 31, 2018	$ 210,520	$ 210,520
Beginning balance (in shares) at Dec. 31, 2018	13,764,829		13,764,829
Net loss	$ (43,120)
Ending balance at Jun. 30, 2019	$ 245,716		$ 7	402,823	441	(157,555)
Convertible preferred stock, ending balance (in shares) at Jun. 30, 2019	0
Ending balance (in shares) at Jun. 30, 2019	68,831,857		68,831,857
Beginning balance at Mar. 31, 2019	$ 266,186		$ 7	399,066	108	(132,995)
Beginning balance (in shares) at Mar. 31, 2019			68,871,792
Exercise of stock options	63		$ 8,999	63
Forfeiture of restricted common stock (in shares)			(48,934)
Stock-based compensation	3,694			3,694
Accumulated other comprehensive income	333				333
Net loss	(24,560)					(24,560)
Ending balance at Jun. 30, 2019	$ 245,716		$ 7	$ 402,823	$ 441	$ (157,555)
Convertible preferred stock, ending balance (in shares) at Jun. 30, 2019	0
Ending balance (in shares) at Jun. 30, 2019	68,831,857		68,831,857