Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	Total	IPO [Member]	Private Placement [Member]	Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] IPO [Member]	Common Stock [Member] Private Placement [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] IPO [Member]	Additional Paid-in Capital [Member] Private Placement [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balances at Dec. 31, 2017	$ (48,846)			$ 77,151	$ 10			$ 2,499			$ (16)	$ (51,339)
Beginning Balances, Shares at Dec. 31, 2017				20,771,850	10,201,690
Issuance of common stock upon exercise of stock options	28							28
Issuance of common stock upon exercise of stock options, Shares					6,368
Stock-based compensation expense	479							479
Unrealized gains (losses) on marketable securities	9										9
Accretion of redeemable convertible preferred stock to redemption value				$ 16
Accretion of redeemable convertible preferred stock to redemption value	(16)							(16)
Net loss	(6,735)											(6,735)
Ending Balances at Mar. 31, 2018	(61,269)			$ 77,167	$ 10			2,990			(7)	(64,262)
Ending Balances, Shares at Mar. 31, 2018				20,771,850	10,208,058
Beginning Balances at Dec. 31, 2017	(48,846)			$ 77,151	$ 10			2,499			(16)	(51,339)
Beginning Balances, Shares at Dec. 31, 2017				20,771,850	10,201,690
Stock-based compensation expense	2,186
Unrealized gains (losses) on marketable securities	(8)
Net loss	(25,926)
Ending Balances at Sep. 30, 2018	67,531				$ 30			150,978			(24)	(83,453)
Ending Balances, Shares at Sep. 30, 2018					29,924,859
Beginning Balances at Dec. 31, 2017	(48,846)			$ 77,151	$ 10			2,499			(16)	(51,339)
Beginning Balances, Shares at Dec. 31, 2017				20,771,850	10,201,690
Net loss	(34,500)
Ending Balances at Dec. 31, 2018	60,234				$ 30			152,275			(12)	(92,059)
Ending Balances, Shares at Dec. 31, 2018					30,057,970
Adjustment to retained earnings for change in accounting policy	(6,188)											(6,188)
Beginning Balances at Mar. 31, 2018	(61,269)			$ 77,167	$ 10			2,990			(7)	(64,262)
Beginning Balances, Shares at Mar. 31, 2018				20,771,850	10,208,058
Conversion of redeemable convertible preferred stock to common stock	77,167			$ (77,167)	$ 13			77,154
Conversion of redeemable convertible preferred stock to common stock, Shares				(20,771,850)	13,229,362
Issuance of common stock sold in initial public offering, net of underwriting discounts, commissions and offering costs		$ 63,948	$ 5,000			$ 6	$ 1		$ 63,942	$ 4,999
Issuance of common stock sold in initial public offering, net of underwriting discounts, commissions and offering costs, Shares						5,985,000	416,666
Issuance of common stock upon exercise of stock options	11							11
Issuance of common stock upon exercise of stock options, Shares					28,162
Stock-based compensation expense	847							847
Unrealized gains (losses) on marketable securities	(6)										(6)
Net loss	(9,023)											(9,023)
Ending Balances at Jun. 30, 2018	76,675				$ 30			149,943			(13)	(73,285)
Ending Balances, Shares at Jun. 30, 2018					29,867,248
Issuance of common stock upon exercise of stock options	175							175
Issuance of common stock upon exercise of stock options, Shares					57,611
Stock-based compensation expense	860							860
Unrealized gains (losses) on marketable securities	(11)										(11)
Net loss	(10,168)											(10,168)
Ending Balances at Sep. 30, 2018	67,531				$ 30			150,978			(24)	(83,453)
Ending Balances, Shares at Sep. 30, 2018					29,924,859
Beginning Balances at Dec. 31, 2018	60,234				$ 30			152,275			(12)	(92,059)
Beginning Balances, Shares at Dec. 31, 2018					30,057,970
Issuance of common stock upon exercise of stock options	11							11
Issuance of common stock upon exercise of stock options, Shares					60,852
Stock-based compensation expense	726							726
Unrealized gains (losses) on marketable securities	10										10
Net loss	(11,691)											(11,691)
Ending Balances at Mar. 31, 2019	49,290				$ 30			153,012			(2)	(103,750)
Ending Balances, Shares at Mar. 31, 2019					30,118,822
Beginning Balances at Dec. 31, 2018	60,234				$ 30			152,275			(12)	(92,059)
Beginning Balances, Shares at Dec. 31, 2018					30,057,970
Stock-based compensation expense	2,399
Unrealized gains (losses) on marketable securities	12
Net loss	(34,130)
Ending Balances at Sep. 30, 2019	28,623				$ 30			154,782				(126,189)
Ending Balances, Shares at Sep. 30, 2019					30,663,054
Beginning Balances at Mar. 31, 2019	49,290				$ 30			153,012			(2)	(103,750)
Beginning Balances, Shares at Mar. 31, 2019					30,118,822
Issuance of common stock upon exercise of stock options	97							97
Issuance of common stock upon exercise of stock options, Shares					541,732
Stock-based compensation expense	885							885
Unrealized gains (losses) on marketable securities	2										$ 2
Net loss	(10,516)											(10,516)
Ending Balances at Jun. 30, 2019	39,758				$ 30			153,994				(114,266)
Ending Balances, Shares at Jun. 30, 2019					30,660,554
Issuance of common stock upon exercise of stock options, Shares					2,500
Stock-based compensation expense	788							788
Net loss	(11,923)											(11,923)
Ending Balances at Sep. 30, 2019	$ 28,623				$ 30			$ 154,782				$ (126,189)
Ending Balances, Shares at Sep. 30, 2019					30,663,054