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 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 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							200				200
Unrealized loss on foreign currency translation								$ (44)			(44)
Net loss										(10,667)	(10,667)
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)
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							578				578
Exercise of stock options							11				11
Exercise of stock options (in shares)					2,286
Unrealized gain on investments held								43			43
Net loss										(12,941)	(12,941)
Balance at Mar. 31, 2019					$ 13		$ 173,577	$ 34		$ (103,050)	$ 70,574
Balance (in shares) at Mar. 31, 2019					13,127,128