Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders’ Deficit - USD ($) $ in Thousands	Series A Redeemable convertible preferred stock	Series B Redeemable convertible preferred stock	Common stock	Additional paid in capital [Member]	Accumulated other comprehensive loss	Accumulated deficit	Total
Balance at Beginning of period at Dec. 31, 2017						$ (108,171)	$ (108,171)
Balance at Beginning of period (in shares) at Dec. 31, 2017			293,791
Vesting of common stock previously subject to repurchase				$ 3			3
Vesting of common stock previously subject to repurchase (in shares)			448
Exercise of stock options				6			6
Exercise of stock options (in shares)			1,157
Stock-based compensation expense				169			169
Accretion of redeemable convertible preferred stock to redemption value				(178)		(6,670)	(6,848)
Net loss						(13,701)	(13,701)
Balance at Ending period at Sep. 30, 2018						(128,542)	(128,542)
Balance at Ending period (in shares) at Sep. 30, 2018			295,396
Balance at Beginning of period at Dec. 31, 2017	$ 30,940	$ 80,409
Balance at Beginning of period (in shares) at Dec. 31, 2017	22,501,174	59,425,431
Increase (Decrease) in Temporary Equity [Roll Forward]
Sale of Series B redeemable convertible preferred stock, net of stock issue costs of $26, $141, $151		$ 1,349
Sale of Series B redeemable convertible preferred stock, net of stock issue costs of $26, $141, $151 (in shares)		1,294,069
Accretion of redeemable convertible preferred stock to redemption value	$ 1,718	$ 5,130
Balance at Ending period at Sep. 30, 2018	$ 32,658	$ 86,888
Balance at Ending period (in shares) at Sep. 30, 2018	22,501,174	60,719,500
Balance at Beginning of period at Jun. 30, 2018						(123,928)	(123,928)
Balance at Beginning of period (in shares) at Jun. 30, 2018			295,125
Vesting of common stock previously subject to repurchase				1			1
Vesting of common stock previously subject to repurchase (in shares)			126
Exercise of stock options				1			1
Exercise of stock options (in shares)			145
Stock-based compensation expense				61			61
Accretion of redeemable convertible preferred stock to redemption value				63		(1,808)	(1,871)
Net loss						(2,806)	(2,806)
Balance at Ending period at Sep. 30, 2018						(128,542)	(128,542)
Balance at Ending period (in shares) at Sep. 30, 2018			295,396
Balance at Beginning of period at Jun. 30, 2018	$ 32,205	$ 85,470
Balance at Beginning of period (in shares) at Jun. 30, 2018	22,501,174	60,719,500
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable convertible preferred stock to redemption value	$ 453	$ 1,418
Balance at Ending period at Sep. 30, 2018	$ 32,658	$ 86,888
Balance at Ending period (in shares) at Sep. 30, 2018	22,501,174	60,719,500
Balance at Beginning of period at Dec. 31, 2018						(137,860)	(137,860)
Balance at Beginning of period (in shares) at Dec. 31, 2018			295,717
Vesting of common stock previously subject to repurchase				3			3
Vesting of common stock previously subject to repurchase (in shares)			508
Exercise of stock options				12			$ 12
Exercise of stock options (in shares)			1,892				1,892
Foreign currency translation adjustment					$ (2)		$ (2)
Stock-based compensation expense				183			183
Accretion of redeemable convertible preferred stock to redemption value				(198)		(6,645)	(6,843)
Net loss						(15,893)	(15,893)
Balance at Ending period at Sep. 30, 2019					(2)	(160,398)	(160,400)
Balance at Ending period (in shares) at Sep. 30, 2019			298,117
Balance at Beginning of period at Dec. 31, 2018	$ 33,112	$ 91,038					124,150
Balance at Beginning of period (in shares) at Dec. 31, 2018	22,501,174	63,032,500
Increase (Decrease) in Temporary Equity [Roll Forward]
Sale of Series B redeemable convertible preferred stock, net of stock issue costs of $26, $141, $151		$ 14,391
Sale of Series B redeemable convertible preferred stock, net of stock issue costs of $26, $141, $151 (in shares)		12,527,956
Accretion of redeemable convertible preferred stock to redemption value	$ 1,346	$ 5,497
Balance at Ending period at Sep. 30, 2019	$ 34,458	$ 110,926					145,384
Balance at Ending period (in shares) at Sep. 30, 2019	22,501,174	75,560,456
Balance at Beginning of period at Jun. 30, 2019					(3)	(153,744)	(153,747)
Balance at Beginning of period (in shares) at Jun. 30, 2019			297,502
Vesting of common stock previously subject to repurchase				2			2
Vesting of common stock previously subject to repurchase (in shares)			202
Exercise of stock options				4			4
Exercise of stock options (in shares)			413
Foreign currency translation adjustment					1		1
Stock-based compensation expense				64			64
Accretion of redeemable convertible preferred stock to redemption value				$ (70)		(1,988)	(2,058)
Net loss						(4,666)	(4,666)
Balance at Ending period at Sep. 30, 2019					$ (2)	$ (160,398)	(160,400)
Balance at Ending period (in shares) at Sep. 30, 2019			298,117
Balance at Beginning of period at Jun. 30, 2019	$ 34,005	$ 107,058
Balance at Beginning of period (in shares) at Jun. 30, 2019	22,501,174	73,587,014
Increase (Decrease) in Temporary Equity [Roll Forward]
Sale of Series B redeemable convertible preferred stock, net of stock issue costs of $26, $141, $151		$ 2,263
Sale of Series B redeemable convertible preferred stock, net of stock issue costs of $26, $141, $151 (in shares)		1,973,442
Accretion of redeemable convertible preferred stock to redemption value	$ 453	$ 1,605
Balance at Ending period at Sep. 30, 2019	$ 34,458	$ 110,926					$ 145,384
Balance at Ending period (in shares) at Sep. 30, 2019	22,501,174	75,560,456