Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A, B and C Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017		$ 92,439
Beginning balance at Dec. 31, 2017	$ (42,118)		$ 2	$ 3,091		$ (45,211)
Beginning balance, shares at Dec. 31, 2017		49,178,527
Beginning balance, shares at Dec. 31, 2017			2,351,247
Issuance of Series C redeemable convertible preferred stock, net of issuance cost of $88		$ 52,212
Issuance of Series C redeemable convertible preferred stock, net of issuance cost of $88, shares		11,223,102
Accretion of Series C redeemable convertible preferred stock to redemption value	(88)	$ 88		(88)
Conversion of preferred stock to common stock	144,739	$ (144,739)	$ 23	144,716
Conversion of preferred stock to common stock, shares		(60,401,629)	23,375,405
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs	89,906		$ 7	89,899
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs, shares			6,666,667
Vesting of restricted stock			$ 1	(1)
Vesting of restricted stock, shares			704,696
Issuance of common stock upon exercise of stock options	29			29
Issuance of common stock upon exercise of stock options, shares			4,152
Stock-based compensation expense	8,152			8,152
Net loss	(40,836)					(40,836)
Ending balance at Sep. 30, 2018	159,784		$ 33	245,798		(86,047)
Ending balance, shares at Sep. 30, 2018			33,102,167
Beginning balance at Dec. 31, 2017		$ 92,439
Beginning balance at Dec. 31, 2017	(42,118)		$ 2	3,091		(45,211)
Beginning balance, shares at Dec. 31, 2017		49,178,527
Beginning balance, shares at Dec. 31, 2017			2,351,247
Net loss	(57,500)
Ending balance at Dec. 31, 2018	145,648		$ 33	248,349	$ (8)	(102,726)
Ending balance, shares at Dec. 31, 2018			33,305,033
Beginning balance at Jun. 30, 2018	172,217		$ 33	242,216		(70,032)
Beginning balance, shares at Jun. 30, 2018			32,889,056
Initial public offering costs	(123)			(123)
Vesting of restricted stock, shares			208,959
Issuance of common stock upon exercise of stock options	29			29
Issuance of common stock upon exercise of stock options, shares			4,152
Stock-based compensation expense	3,676			3,676
Net loss	(16,015)					(16,015)
Ending balance at Sep. 30, 2018	159,784		$ 33	245,798		(86,047)
Ending balance, shares at Sep. 30, 2018			33,102,167
Beginning balance at Dec. 31, 2018	145,648		$ 33	248,349	(8)	(102,726)
Beginning balance, shares at Dec. 31, 2018			33,305,033
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs	60,276		$ 5	60,271
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs, shares			4,887,500
Vesting of restricted stock			$ 1	(1)
Vesting of restricted stock, shares			592,849
Issuance of common stock upon exercise of stock options	1,107			1,107
Issuance of common stock upon exercise of stock options, shares			157,337
Stock-based compensation expense	7,939			7,939
Unrealized gains (losses) on marketable securities	91				91
Net loss	(53,532)					(53,532)
Ending balance at Sep. 30, 2019	161,529		$ 39	317,665	83	(156,258)
Ending balance, shares at Sep. 30, 2019			38,942,719
Beginning balance at Jun. 30, 2019	179,556		$ 39	314,646	139	(135,268)
Beginning balance, shares at Jun. 30, 2019			38,710,858
Vesting of restricted stock, shares			197,570
Issuance of common stock upon exercise of stock options	257			257
Issuance of common stock upon exercise of stock options, shares			34,291
Stock-based compensation expense	2,762			2,762
Unrealized gains (losses) on marketable securities	(56)				(56)
Net loss	(20,990)					(20,990)
Ending balance at Sep. 30, 2019	$ 161,529		$ 39	$ 317,665	$ 83	$ (156,258)
Ending balance, shares at Sep. 30, 2019			38,942,719