CONDENSED CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS EQUITY (DEFICIT) (unaudited) - USD ($) $ in Thousands	Total	Common Stock and Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss	Redeemable Convertible Preferred Stock
Total redeemable convertible preferred stock, beginning balances at Dec. 31, 2017	$ 75,995
Conversion of redeemable convertible preferred stock to common stock		$ 0			$ 0
Convertible Notes conversion on September 20, 2018 (see Note 6), net of issuance cost	13,409
Total redeemable convertible preferred stock, ending balances at Sep. 30, 2018	89,404
Beginning balances at Dec. 31, 2017		3,026	$ (95,619)	$ (10)
Equity component credited to additional paid-in capital upon Convertible Notes modification on May 31, 2018 and August 20, 2018 (see Note 6)		4,690
Proceeds from initial public offering, net of expenses		0
Conversion of redeemable convertible preferred stock warrants to common stock warrants		0
Proceeds from exercise of common stock warrant		0
Issuance of common stock warrants		0
Proceeds from exercise of stock options		36
Stock-based compensation		924
Net loss	(16,331)		(16,331)
Foreign currency translation adjustment	(3)			(3)
Unrealized loss on available-for-sale debt securities				0
Ending balances at Sep. 30, 2018	(103,287)	8,676	(111,950)	(13)
Total redeemable convertible preferred stock, beginning balances at Jun. 30, 2018	75,995
Conversion of redeemable convertible preferred stock to common stock		0			0
Convertible Notes conversion on September 20, 2018 (see Note 6), net of issuance cost	13,409
Total redeemable convertible preferred stock, ending balances at Sep. 30, 2018	89,404
Beginning balances at Jun. 30, 2018		7,523	(108,309)	(12)
Equity component credited to additional paid-in capital upon Convertible Notes modification on May 31, 2018 and August 20, 2018 (see Note 6)		800
Proceeds from initial public offering, net of expenses		0
Conversion of redeemable convertible preferred stock warrants to common stock warrants		0
Proceeds from exercise of common stock warrant		0
Issuance of common stock warrants		0
Proceeds from exercise of stock options		12
Stock-based compensation		341
Net loss	(3,641)		(3,641)
Foreign currency translation adjustment	(1)			(1)
Unrealized loss on available-for-sale debt securities				0
Ending balances at Sep. 30, 2018	(103,287)	8,676	(111,950)	(13)
Total redeemable convertible preferred stock, beginning balances at Dec. 31, 2018	89,404
Conversion of redeemable convertible preferred stock to common stock		89,404			(89,404)
Convertible Notes conversion on September 20, 2018 (see Note 6), net of issuance cost	0
Total redeemable convertible preferred stock, ending balances at Sep. 30, 2019	0
Beginning balances at Dec. 31, 2018	(106,388)	9,132	(115,505)	(15)
Equity component credited to additional paid-in capital upon Convertible Notes modification on May 31, 2018 and August 20, 2018 (see Note 6)		0
Proceeds from initial public offering, net of expenses		139,828
Conversion of redeemable convertible preferred stock warrants to common stock warrants		2,086
Proceeds from exercise of common stock warrant		8
Issuance of common stock warrants		572
Proceeds from exercise of stock options		678
Stock-based compensation		3,657
Net loss	(18,439)		(18,439)
Foreign currency translation adjustment	(2)			(2)
Unrealized loss on available-for-sale debt securities	(12)			(12)
Ending balances at Sep. 30, 2019	111,392	245,365	(133,944)	(29)
Total redeemable convertible preferred stock, beginning balances at Jun. 30, 2019	0
Conversion of redeemable convertible preferred stock to common stock		0			$ 0
Convertible Notes conversion on September 20, 2018 (see Note 6), net of issuance cost	0
Total redeemable convertible preferred stock, ending balances at Sep. 30, 2019	0
Beginning balances at Jun. 30, 2019		244,092	(127,059)	(15)
Equity component credited to additional paid-in capital upon Convertible Notes modification on May 31, 2018 and August 20, 2018 (see Note 6)		0
Proceeds from initial public offering, net of expenses		(197)
Conversion of redeemable convertible preferred stock warrants to common stock warrants		0
Proceeds from exercise of common stock warrant		0
Issuance of common stock warrants		0
Proceeds from exercise of stock options		68
Stock-based compensation		1,402
Net loss	(6,885)		(6,885)
Foreign currency translation adjustment	(2)			(2)
Unrealized loss on available-for-sale debt securities	(12)			(12)
Ending balances at Sep. 30, 2019	$ 111,392	$ 245,365	$ (133,944)	$ (29)