CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK, REDEEMABLE COMMON STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Series Seed, A and B Convertible Preferred	Redeemable Common Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Convertible preferred shares, beginning balance at Dec. 31, 2017		$ 60,903
Convertible preferred shares, beginning balance, shares at Dec. 31, 2017		38,018,968
Convertible preferred shares, ending balance at Mar. 31, 2018		$ 60,386
Convertible preferred shares, ending balance, shares at Mar. 31, 2018		37,419,993
Beginning balance at Dec. 31, 2017	$ (44,545)		$ 734	$ 0	$ 1,385	$ 0	$ (45,930)
Beginning balance, shares at Dec. 31, 2017			107,364	350,607
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of Series Seed convertible preferred stock, shares		(598,975)
Repurchase of Series Seed convertible preferred stock, net of issuance costs of $11	(22)	$ (517)					(22)
Stock-based compensation expense	128				128
Net loss	(7,367)						(7,367)
Ending balance, shares at Mar. 31, 2018			107,364	350,607
Ending balance at Mar. 31, 2018	(51,806)		$ 734	$ 0	1,513	0	(53,319)
Convertible preferred shares, beginning balance at Dec. 31, 2017		$ 60,903
Convertible preferred shares, beginning balance, shares at Dec. 31, 2017		38,018,968
Convertible preferred shares, ending balance at Sep. 30, 2018		$ 64,675
Convertible preferred shares, ending balance, shares at Sep. 30, 2018		40,079,567
Beginning balance at Dec. 31, 2017	(44,545)		$ 734	$ 0	1,385	0	(45,930)
Beginning balance, shares at Dec. 31, 2017			107,364	350,607
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of redeemable common stock into common stock	0
Repurchase of Series Seed convertible preferred stock, net of issuance costs of $11	(22)
Net loss	(22,003)
Ending balance, shares at Sep. 30, 2018			107,364	351,652
Ending balance at Sep. 30, 2018	(66,092)		$ 734	$ 0	1,863	0	(67,955)
Convertible preferred shares, beginning balance at Mar. 31, 2018		$ 60,386
Convertible preferred shares, beginning balance, shares at Mar. 31, 2018		37,419,993
Convertible preferred shares, ending balance at Jun. 30, 2018		$ 60,386
Convertible preferred shares, ending balance, shares at Jun. 30, 2018		37,419,993
Beginning balance at Mar. 31, 2018	(51,806)		$ 734	$ 0	1,513	0	(53,319)
Beginning balance, shares at Mar. 31, 2018			107,364	350,607
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options, shares				1,045
Exercise of stock options	7				7
Stock-based compensation expense	145				145
Net loss	(6,034)						(6,034)
Ending balance, shares at Jun. 30, 2018			107,364	351,652
Ending balance at Jun. 30, 2018	(57,688)		$ 734	$ 0	1,665	0	(59,353)
Convertible preferred shares, ending balance at Sep. 30, 2018		$ 64,675
Convertible preferred shares, ending balance, shares at Sep. 30, 2018		40,079,567
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of Series Seed convertible preferred stock, net of issuance costs of $11	0
Issuance of Series B convertible preferred stock, net of issuance costs of $539, shares		2,659,574
Issuance of Series B convertible preferred stock, net of issuance costs of $539		$ 4,289
Stock-based compensation expense	198				198
Net loss	(8,602)						(8,602)
Ending balance, shares at Sep. 30, 2018			107,364	351,652
Ending balance at Sep. 30, 2018	(66,092)		$ 734	$ 0	1,863	0	(67,955)
Convertible preferred shares, beginning balance at Dec. 31, 2018	$ 64,675	$ 64,675
Convertible preferred shares, beginning balance, shares at Dec. 31, 2018	40,079,567	40,079,567
Increase (Decrease) in Temporary Equity [Roll Forward]
Conversion of convertible preferred shares into common stock	$ 64,675	$ (64,675)		$ 4	64,671
Conversion of convertible preferred shares into common stock, shares		(40,079,567)		3,808,430
Beginning balance at Dec. 31, 2018	(77,086)		$ 734	$ 0	2,151	0	(79,237)
Beginning balance, shares at Dec. 31, 2018			107,364	351,652
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of redeemable common stock into common stock, shares			(107,364)	107,364
Conversion of redeemable common stock into common stock	733		$ (734)	$ 0	733
Exchange of common stock in connection with Merger, shares				2,440,582
Exchange of common stock in connection with Merger	45,541			$ 2	45,539
Fair value of replacement equity awards	817				817
Reclassification of warrant liability to permanent equity	5,235				5,235
Exercise of stock options, shares				16,483
Exercise of stock options	113				113
Stock-based compensation expense	262				262
Currency translation adjustments	23					23
Net loss	(10,873)						(10,873)
Ending balance, shares at Mar. 31, 2019				6,724,511
Ending balance at Mar. 31, 2019	29,440			$ 6	119,521	23	(90,110)
Convertible preferred shares, beginning balance at Dec. 31, 2018	$ 64,675	$ 64,675
Convertible preferred shares, beginning balance, shares at Dec. 31, 2018	40,079,567	40,079,567
Beginning balance at Dec. 31, 2018	$ (77,086)		$ 734	$ 0	2,151	0	(79,237)
Beginning balance, shares at Dec. 31, 2018			107,364	351,652
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of redeemable common stock into common stock	734
Repurchase of Series Seed convertible preferred stock, net of issuance costs of $11	$ 0
Exercise of stock options, shares	25,627
Net loss	$ (41,970)
Ending balance, shares at Sep. 30, 2019				12,436,205
Ending balance at Sep. 30, 2019	79,107			$ 12	200,421	(119)	(121,207)
Beginning balance at Mar. 31, 2019	29,440			$ 6	119,521	23	(90,110)
Beginning balance, shares at Mar. 31, 2019				6,724,511
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock and warrants for the purchase of common stock, net of issuance costs of $931, shares				5,670,000
Issuance of common stock and warrants for the purchase of common stock, net of issuance costs of $931	79,297			$ 6	79,291
Exercise of stock options, shares				700
Exercise of stock options	5				5
Exercise of warrants, shares				33,846
Exercise of warrants	440				440
Stock-based compensation expense	433				433
Currency translation adjustments	(65)					(65)
Net loss	(13,383)						(13,383)
Ending balance, shares at Jun. 30, 2019				12,429,057
Ending balance at Jun. 30, 2019	96,167			$ 12	199,690	(42)	(103,493)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of Series Seed convertible preferred stock, net of issuance costs of $11	0
Exercise of stock options, shares				7,148
Exercise of stock options	40				40
Stock-based compensation expense	691				691
Currency translation adjustments	(77)					(77)
Net loss	(17,714)						(17,714)
Ending balance, shares at Sep. 30, 2019				12,436,205
Ending balance at Sep. 30, 2019	$ 79,107			$ 12	$ 200,421	$ (119)	$ (121,207)