CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Series B‑1 Tranche A Redeemable Convertible Preferred Stock	Series B‑1 Tranche B Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-in Capital ASU 2018-07	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings ASU 2018-07	Retained Earnings	Total
Balance at Dec. 31, 2017	$ 23,866	$ 24,550	$ 24,423	$ 31,874
Balance (in shares) at Dec. 31, 2017	33,499,900	25,000,000	42,372,882	54,067,796
Balance at Jun. 30, 2018	$ 23,866	$ 24,550	$ 24,423	$ 31,874
Balance (in shares) at Jun. 30, 2018	33,499,900	25,000,000	42,372,882	54,067,796
Balance at Dec. 31, 2017					$ 3		$ 1,377			$ (57,170)	$ (55,790)
Balance (in shares) at Dec. 31, 2017					12,639
Increase (Decrease) in Temporary Equity and Stockholders' Equity (Deficit)
Stock-based compensation expense							473				473
Reorganization adjustment					$ (3)		3
Net loss										(16,400)	(16,400)
Balance at Jun. 30, 2018							1,840			(73,557)	(71,717)
Balance (in shares) at Jun. 30, 2018					12,639
Balance at Mar. 31, 2018	$ 23,866	$ 24,550	$ 24,423	$ 31,874
Balance (in shares) at Mar. 31, 2018	33,499,900	25,000,000	42,372,882	54,067,796
Balance at Jun. 30, 2018	$ 23,866	$ 24,550	$ 24,423	$ 31,874
Balance (in shares) at Jun. 30, 2018	33,499,900	25,000,000	42,372,882	54,067,796
Balance at Mar. 31, 2018					$ 3		1,564	$ (44)		(67,824)	(66,301)
Balance (in shares) at Mar. 31, 2018					12,639
Increase (Decrease) in Temporary Equity and Stockholders' Equity (Deficit)
Stock-based compensation expense							273				273
Reorganization adjustment					$ (3)		3
Unrealized loss on foreign currency translation								44			44
Net loss										(5,733)	(5,733)
Balance at Jun. 30, 2018							1,840			(73,557)	(71,717)
Balance (in shares) at Jun. 30, 2018					12,639
Increase (Decrease) in Temporary Equity and Stockholders' Equity (Deficit)
ASU 2018‑07 modified retrospective adjustment						$ (13)			$ 13
Balance at Dec. 31, 2018					$ 13		172,988	(9)		(90,109)	82,883
Balance (in shares) at Dec. 31, 2018					13,124,842
Increase (Decrease) in Temporary Equity and Stockholders' Equity (Deficit)
Stock-based compensation expense							1,177				1,177
Exercise of stock options							40				40
Exercise of stock options (in shares)					9,696
Unrealized gain on investments held								63			63
Net loss										(26,323)	(26,323)
Balance at Jun. 30, 2019					$ 13		174,205	54		(116,432)	57,840
Balance (in shares) at Jun. 30, 2019					13,134,538
Balance at Mar. 31, 2019					$ 13		173,577	34		(103,050)	70,574
Balance (in shares) at Mar. 31, 2019					13,127,128
Increase (Decrease) in Temporary Equity and Stockholders' Equity (Deficit)
Stock-based compensation expense							599				599
Exercise of stock options							29				29
Exercise of stock options (in shares)					7,410
Unrealized gain on investments held								20			20
Net loss										(13,382)	(13,382)
Balance at Jun. 30, 2019					$ 13		$ 174,205	$ 54		$ (116,432)	$ 57,840
Balance (in shares) at Jun. 30, 2019					13,134,538