Consolidated Statements of Changes in Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Common stock	Additional paid-In Capital	Accumulated deficit	Accumulated other comprehensive loss
Beginning balance (in shares) at Dec. 31, 2018		22,471,776
Beginning balance at Dec. 31, 2018	$ (5,418)	$ 3	$ 279,539	$ (280,403)	$ (4,557)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		11,943
Issuance of common stock under Employee Stock Purchase Plan	20		20
Issuance of common stock upon exercise of options (in shares)		115,600
Issuance of common stock upon exercise of options	145		145
Stock‑based compensation expense	1,180		1,180
Foreign currency translation adjustment	22				22
Net loss	(12,074)			(12,074)
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
Unrealized gain on securities	2				2
Ending balance (in shares) at Mar. 31, 2019		44,788,025
Ending balance at Mar. 31, 2019	14,819	$ 5	311,824	(292,477)	(4,533)
Beginning balance (in shares) at Dec. 31, 2018		22,471,776
Beginning balance at Dec. 31, 2018	(5,418)	$ 3	279,539	(280,403)	(4,557)
Increase (Decrease) in Stockholders' Equity
Foreign currency translation adjustment	29
Net loss	(28,468)
Unrealized gain on securities	3
Ending balance (in shares) at Jun. 30, 2019		44,952,951
Ending balance at Jun. 30, 2019	56	$ 5	313,447	(308,871)	(4,525)
Beginning balance (in shares) at Mar. 31, 2019		44,788,025
Beginning balance at Mar. 31, 2019	14,819	$ 5	311,824	(292,477)	(4,533)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock through at-the-market offering, net (in shares)		164,926
Issuance of common stock through at-the-market offering, net	372		372
Stock‑based compensation expense	1,251		1,251
Foreign currency translation adjustment	7				7
Net loss	(16,394)			(16,394)
Unrealized gain on securities	1				1
Ending balance (in shares) at Jun. 30, 2019		44,952,951
Ending balance at Jun. 30, 2019	$ 56	$ 5	313,447	(308,871)	(4,525)
Beginning balance (in shares) at Dec. 31, 2019	86,325,547	86,325,547
Beginning balance at Dec. 31, 2019	$ 8,397	$ 9	348,664	(335,753)	(4,523)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		78,583
Issuance of common stock under Employee Stock Purchase Plan	114		114
Issuance of common stock upon exercise of options (in shares)		5,128
Issuance of common stock upon exercise of options	3		3
Issuance of vested restricted stock units (in shares)		10,937
Issuance of common stock through at-the-market offering, net (in shares)		598,977
Issuance of common stock through at-the-market offering, net	1,141		1,141
Other financing fees	(147)		(147)
Stock‑based compensation expense	1,409		1,409
Foreign currency translation adjustment	(60)				(60)
Net loss	(19,620)			(19,620)
Ending balance (in shares) at Mar. 31, 2020		87,019,172
Ending balance at Mar. 31, 2020	$ (8,763)	$ 9	351,184	(355,373)	(4,583)
Beginning balance (in shares) at Dec. 31, 2019	86,325,547	86,325,547
Beginning balance at Dec. 31, 2019	$ 8,397	$ 9	348,664	(335,753)	(4,523)
Increase (Decrease) in Stockholders' Equity
Foreign currency translation adjustment	(29)
Net loss	$ (43,701)
Issuance of common stock upon exercise of common warrants (in shares)	16,585,148,000	4,934,723
Unrealized gain on securities	$ 0
Ending balance (in shares) at Jun. 30, 2020	100,847,810	100,847,810
Ending balance at Jun. 30, 2020	$ (13,052)	$ 10	370,944	(379,454)	(4,552)
Beginning balance (in shares) at Mar. 31, 2020		87,019,172
Beginning balance at Mar. 31, 2020	(8,763)	$ 9	351,184	(355,373)	(4,583)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)		37,500
Issuance of common stock upon exercise of options	98		98
Issuance of vested restricted stock units (in shares)		10,938
Issuance of common stock through at-the-market offering, net (in shares)		470,509
Issuance of common stock through at-the-market offering, net	967		967
Stock‑based compensation expense	1,481		1,481
Foreign currency translation adjustment	31				31
Net loss	(24,081)			(24,081)
Issuance of common stock upon exercise of pre-funded warrants (in shares)		8,342,128
Issuance of common stock upon exercise of pre-funded warrants	1	$ 1
Issuance of common stock upon exercise of common warrants (in shares)		4,967,563
Issuance of common stock upon exercise of common warrants	17,214	$ 0	17,214
Unrealized gain on securities	$ 0
Ending balance (in shares) at Jun. 30, 2020	100,847,810	100,847,810
Ending balance at Jun. 30, 2020	$ (13,052)	$ 10	$ 370,944	$ (379,454)	$ (4,552)