Consolidated Statements of Changes in Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Common stock	Additional paid-In Capital	Stock option receivable	Accumulated deficit	Accumulated other comprehensive loss
Beginning balance (in shares) at Dec. 31, 2016		18,438,742
Beginning balance at Dec. 31, 2016	$ 54,957	$ 1	$ 211,125	$ (75)	$ (151,576)	$ (4,518)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		16,263
Issuance of common stock under Employee Stock Purchase Plan	180		180
Issuance of common stock upon exercise of options (in shares)		269,842
Issuance of common stock upon exercise of options	705		630	75
Stock‑based compensation expense	4,080		4,080
Issuance of common stock, license agreement (in shares)		529,616
Issuance of common stock, license agreement	10,000		10,000
Issuance of common stock through private placement, net (in shares)		3,088,791
Issuance of common stock through private placement, net of issuance costs of $37	47,115	$ 2	47,113
Currency translation adjustment	78					78
Unrealized gains (losses) on securities	20					20
Net loss	(65,321)				(65,321)
Ending balance (in shares) at Dec. 31, 2017		22,343,254
Ending balance at Dec. 31, 2017	51,814	$ 3	273,128	0	(216,897)	(4,420)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		24,738
Issuance of common stock under Employee Stock Purchase Plan	196		196
Issuance of common stock upon exercise of options (in shares)		103,784
Issuance of common stock upon exercise of options	501		501
Stock‑based compensation expense	5,714		5,714
Currency translation adjustment	(153)					(153)
Unrealized gains (losses) on securities	16					16
Net loss	$ (65,336)				(65,336)
Ending balance (in shares) at Dec. 31, 2018	22,471,776	22,471,776
Ending balance at Dec. 31, 2018	$ (5,418)	$ 3	279,539	0	(280,403)	(4,557)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		17,205
Issuance of common stock under Employee Stock Purchase Plan	28		28
Issuance of common stock upon exercise of options (in shares)		125,600
Issuance of common stock upon exercise of options	150		150
Stock‑based compensation expense	5,161		5,161
Issuance of common stock through private placement, net (in shares)		3,178,174
Issuance of common stock through private placement, net of issuance costs of $37	5,715		5,715
Issuance of common stock, pre-funded warrants and warrants through private placement, net of issuance costs (in shares)		37,634,883
Issuance of common stock, pre-funded warrants and warrants through private placement, net of issuance costs	26,129	$ 4	26,125
Currency translation adjustment	34
Unrealized gains (losses) on securities	0
Net loss	(55,350)
Issuance of common stock, net of issuance costs (in shares)		22,188,706
Issuance of common stock, net of issuance costs	30,942	$ 2	30,940
Issuance of common stock through at-the-market offering, net (in shares)		615,453
Issuance of common stock through at-the-market offering, net	1,006		1,006
Issuance of restricted stock units (in shares)		93,750
Issuance of restricted stock units	$ 0
Ending balance (in shares) at Dec. 31, 2019	86,325,547	86,325,547
Ending balance at Dec. 31, 2019	$ 8,397	$ 9	$ 348,664	$ 0	$ (335,753)	$ (4,523)