Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Convertible preferred stock (as converted)	Total
Balances at beginning at Dec. 31, 2017					$ 89,362
Balances at beginning (in shares) at Dec. 31, 2017					8,306,934
Balances at beginning at Dec. 31, 2017		$ 2,130	$ (14)	$ (58,682)		$ (56,566)
Balances at beginning (in shares) at Dec. 31, 2017	2,092,040
Issuance of convertible preferred stock					$ 120,313
Issuance of convertible preferred stock (in shares)					8,941,325
Issuance of common stock upon initial public offering, net (in shares)	22,923
Issuance of common stock upon exercise of options		30				30
Stock-based compensation		833				833
Unrealized gain (loss) on fair value of investments			27			27
Unrealized gain (loss) on currency translation			(14)			(14)
Net loss				(25,956)		(25,956)
Balances at end at Sep. 30, 2018					$ 209,675
Balances at end (in shares) at Sep. 30, 2018					17,248,259
Balances at end at Sep. 30, 2018		2,993	(1)	(84,638)		(81,646)
Balances at end (in shares) at Sep. 30, 2018	2,114,963
Balances at beginning at Jun. 30, 2018					$ 89,362
Balances at beginning (in shares) at Jun. 30, 2018					8,306,934
Balances at beginning at Jun. 30, 2018		2,629	(17)	(75,006)		(72,394)
Balances at beginning (in shares) at Jun. 30, 2018	2,110,540
Issuance of convertible preferred stock					$ 120,313
Issuance of convertible preferred stock (in shares)					8,941,325
Issuance of common stock upon exercise of options		21				21
Issuance of common stock upon exercise of options (in shares)	4,423
Stock-based compensation		343				343
Unrealized gain (loss) on fair value of investments			6			6
Unrealized gain (loss) on currency translation			10			10
Net loss				(9,632)		(9,632)
Balances at end at Sep. 30, 2018					$ 209,675
Balances at end (in shares) at Sep. 30, 2018					17,248,259
Balances at end at Sep. 30, 2018		2,993	(1)	(84,638)		(81,646)
Balances at end (in shares) at Sep. 30, 2018	2,114,963
Balances at beginning at Dec. 31, 2018					$ 209,668
Balances at beginning (in shares) at Dec. 31, 2018					17,248,259
Balances at beginning at Dec. 31, 2018		3,593	(4)	(96,622)		(93,033)
Balances at beginning (in shares) at Dec. 31, 2018	2,119,872
Conversion of convertible preferred stock					$ (209,668)	209,669
Conversion of convertible preferred stock (in shares)					(17,248,259)
Conversion of convertible preferred stock	$ 2	209,666				209,668
Conversion of convertible preferred stock (in shares)	17,248,259
Issuance of common stock upon initial public offering, net	$ 1	130,785				130,786
Issuance of common stock upon initial public offering, net (in shares)	8,452,500
Exercise of warrants (in shares)	62,936
Issuance of common stock upon exercise of options		241				241
Issuance of common stock upon exercise of options (in shares)	67,244
Vesting of early exercised stock options		8				8
Issuance of common stock under the Employee Stock Purchase Plan		133				133
Issuance of common stock under the Employee Stock Purchase Plan (shares)	9,232
Reclassification of redeemable convertible preferred stock warrant liability to additional paid-in capital		503				503
Stock-based compensation		4,013				4,013
Unrealized gain (loss) on fair value of investments			48			48
Unrealized gain (loss) on currency translation			(1)			(1)
Net loss				(47,371)		(47,371)
Balances at end at Sep. 30, 2019	$ 3	348,942	43	(143,993)		204,995
Balances at end (in shares) at Sep. 30, 2019	27,960,043
Balances at beginning at Jun. 30, 2019	$ 3	346,915	86	(128,122)		218,882
Balances at beginning (in shares) at Jun. 30, 2019	27,947,201
Issuance of common stock upon exercise of options		10				10
Issuance of common stock upon exercise of options (in shares)	3,610
Vesting of early exercised stock options		6				6
Issuance of common stock under the Employee Stock Purchase Plan		133				133
Issuance of common stock under the Employee Stock Purchase Plan (shares)	9,232
Stock-based compensation		1,878				1,878
Unrealized gain (loss) on fair value of investments			(41)			(41)
Unrealized gain (loss) on currency translation			(2)			(2)
Net loss				(15,871)		(15,871)
Balances at end at Sep. 30, 2019	$ 3	$ 348,942	$ 43	$ (143,993)		$ 204,995
Balances at end (in shares) at Sep. 30, 2019	27,960,043