CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Temporary equity, beginning of period (in shares) at Dec. 31, 2017	11,871,578
Temporary equity, beginning of period at Dec. 31, 2017	$ 118,548
Temporary equity, end of period (in shares) at Mar. 31, 2018	11,871,578
Temporary equity, end of period at Mar. 31, 2018	$ 118,548
Stockholders' equity (deficit), beginning of period (in shares) at Dec. 31, 2017		3,603,140
Stockholders' equity (deficit), beginning of period at Dec. 31, 2017	(129,378)	$ 1	$ 9,943	$ 402	$ (139,724)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		4,207
Issuance of common stock upon exercise of stock options	18		18
Stock-based compensation	391		391
Vesting of early exercised stock options	20		20
Foreign currency translation	(21)			(21)
Net loss	(4,201)				(4,201)
Stockholders' equity (deficit), ending of period (in shares) at Mar. 31, 2018		3,607,347
Stockholders' equity (deficit), ending of period at Mar. 31, 2018	$ (133,171)	$ 1	10,372	381	(143,925)
Temporary equity, beginning of period (in shares) at Dec. 31, 2017	11,871,578
Temporary equity, beginning of period at Dec. 31, 2017	$ 118,548
Temporary equity, end of period (in shares) at Sep. 30, 2018	11,871,578
Temporary equity, end of period at Sep. 30, 2018	$ 118,548
Stockholders' equity (deficit), beginning of period (in shares) at Dec. 31, 2017		3,603,140
Stockholders' equity (deficit), beginning of period at Dec. 31, 2017	(129,378)	$ 1	9,943	402	(139,724)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Foreign currency translation	29
Unrealized gain (loss) on marketable securities	0
Net loss	(12,140)
Stockholders' equity (deficit), ending of period (in shares) at Sep. 30, 2018		3,912,907
Stockholders' equity (deficit), ending of period at Sep. 30, 2018	$ (139,429)	$ 1	12,003	431	(151,864)
Temporary equity, beginning of period (in shares) at Mar. 31, 2018	11,871,578
Temporary equity, beginning of period at Mar. 31, 2018	$ 118,548
Temporary equity, end of period (in shares) at Jun. 30, 2018	11,871,578
Temporary equity, end of period at Jun. 30, 2018	$ 118,548
Stockholders' equity (deficit), beginning of period (in shares) at Mar. 31, 2018		3,607,347
Stockholders' equity (deficit), beginning of period at Mar. 31, 2018	(133,171)	$ 1	10,372	381	(143,925)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		44,195
Issuance of common stock upon exercise of stock options	190		190
Issuance of common stock upon exercises of unvested stock options (in shares)		55,726
Issuance of common stock upon exercises of unvested stock options	0
Stock-based compensation	363		363
Vesting of early exercised stock options	8		8
Foreign currency translation	54			54
Net loss	(3,147)				(3,147)
Stockholders' equity (deficit), ending of period (in shares) at Jun. 30, 2018		3,707,268
Stockholders' equity (deficit), ending of period at Jun. 30, 2018	$ (135,703)	$ 1	10,933	435	(147,072)
Temporary equity, end of period (in shares) at Sep. 30, 2018	11,871,578
Temporary equity, end of period at Sep. 30, 2018	$ 118,548
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		205,639
Issuance of common stock upon exercise of stock options	707		707
Stock-based compensation	327		327
Vesting of early exercised stock options	36		36
Foreign currency translation	(4)			(4)
Unrealized gain (loss) on marketable securities	0
Net loss	(4,792)				(4,792)
Stockholders' equity (deficit), ending of period (in shares) at Sep. 30, 2018		3,912,907
Stockholders' equity (deficit), ending of period at Sep. 30, 2018	(139,429)	$ 1	12,003	431	(151,864)
Stockholders' equity (deficit), beginning of period (in shares) at Dec. 31, 2018		24,450,757
Stockholders' equity (deficit), beginning of period at Dec. 31, 2018	90,192	$ 3	246,927	439	(157,177)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		46,809
Issuance of common stock upon exercise of stock options	125		125
Stock-based compensation	1,871		1,871
Vesting of early exercised stock options	66		66
Additional accrual of IPO related cost	(160)		(160)
Foreign currency translation	(19)			(19)
Unrealized gain (loss) on marketable securities	25			25
Net loss	(9,345)				(9,345)
Stockholders' equity (deficit), ending of period (in shares) at Mar. 31, 2019		24,497,566
Stockholders' equity (deficit), ending of period at Mar. 31, 2019	82,755	$ 3	248,829	445	(166,522)
Stockholders' equity (deficit), beginning of period (in shares) at Dec. 31, 2018		24,450,757
Stockholders' equity (deficit), beginning of period at Dec. 31, 2018	$ 90,192	$ 3	246,927	439	(157,177)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)	359,200
Foreign currency translation	$ (11)
Unrealized gain (loss) on marketable securities	47
Net loss	(29,305)
Stockholders' equity (deficit), ending of period (in shares) at Sep. 30, 2019		24,965,512
Stockholders' equity (deficit), ending of period at Sep. 30, 2019	68,961	$ 3	254,965	475	(186,482)
Stockholders' equity (deficit), beginning of period (in shares) at Mar. 31, 2019		24,497,566
Stockholders' equity (deficit), beginning of period at Mar. 31, 2019	82,755	$ 3	248,829	445	(166,522)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		137,185
Issuance of common stock upon exercise of stock options	442		442
Issuance of common stock related to employee stock purchase plan (in shares)		99,086
Issuance of common stock related to employee stock purchase plan	1,263		1,263
Issuance of common stock upon vesting of restricted stock units (in shares)		27,320
Issuance of common stock upon vesting of restricted stock units	0
Stock-based compensation	1,814		1,814
Vesting of early exercised stock options	56		56
Foreign currency translation	12			12
Unrealized gain (loss) on marketable securities	59			59
Net loss	(8,654)				(8,654)
Stockholders' equity (deficit), ending of period (in shares) at Jun. 30, 2019		24,761,157
Stockholders' equity (deficit), ending of period at Jun. 30, 2019	77,747	$ 3	252,404	516	(175,176)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		175,070
Issuance of common stock upon exercise of stock options	611		611
Issuance of common stock upon vesting of restricted stock units (in shares)		38,115
Issuance of common stock upon vesting of restricted stock units	0
Repurchase of unvested early exercised stock options (in shares)		(8,830)
Repurchase of unvested early exercised stock options	0
Stock-based compensation	1,911		1,911
Vesting of early exercised stock options	39		39
Foreign currency translation	(4)			(4)
Unrealized gain (loss) on marketable securities	(37)			(37)
Net loss	(11,306)				(11,306)
Stockholders' equity (deficit), ending of period (in shares) at Sep. 30, 2019		24,965,512
Stockholders' equity (deficit), ending of period at Sep. 30, 2019	$ 68,961	$ 3	$ 254,965	$ 475	$ (186,482)