Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Common Stock Private Placement	Common Stock At-the-market sales	Common Stock	Additional Paid-in Capital Private Placement	Additional Paid-in Capital At-the-market sales	Additional Paid-in Capital	Accumulated Deficit	Private Placement	At-the-market sales	Total
Balance at Dec. 31, 2017			$ 3			$ 258,092	$ (221,772)			$ 36,323
Balance (in shares) at Dec. 31, 2017			34,186,342
Increase (decrease) in stockholders' equity
Share-based compensation - stock options and RSUs						1,119				1,119
Vesting of RSUs (in shares)			61,926
Net loss							(6,833)			(6,833)
Balance at Mar. 31, 2018			$ 3			259,211	(228,605)			30,609
Balance (in shares) at Mar. 31, 2018			34,248,268
Balance at Dec. 31, 2017			$ 3			258,092	(221,772)			36,323
Balance (in shares) at Dec. 31, 2017			34,186,342
Increase (decrease) in stockholders' equity
Net loss										(12,177)
Balance at Jun. 30, 2018			$ 3			260,199	(233,949)			26,253
Balance (in shares) at Jun. 30, 2018			34,377,329
Balance at Mar. 31, 2018			$ 3			259,211	(228,605)			30,609
Balance (in shares) at Mar. 31, 2018			34,248,268
Increase (decrease) in stockholders' equity
Share-based compensation - stock options and RSUs						988				988
Vesting of RSUs (in shares)			129,061
Net loss							(5,344)			(5,344)
Balance at Jun. 30, 2018			$ 3			260,199	(233,949)			26,253
Balance (in shares) at Jun. 30, 2018			34,377,329
Balance at Dec. 31, 2018			$ 3			261,722	(241,551)			20,174
Balance (in shares) at Dec. 31, 2018			34,377,329
Increase (decrease) in stockholders' equity
Adjustment to derivative liabilities upon adoption of ASU 2017-11						213	(213)
Share-based compensation - stock options and RSUs						490				490
Issuance of common stock	$ 1			$ 7,809	$ 860			$ 7,810	$ 860
Issuance of common stock (in shares)	8,426,750	665,974
Vesting of RSUs (in shares)			145,204
Net loss							(4,669)			(4,669)
Balance at Mar. 31, 2019			$ 4			271,094	(246,433)			24,665
Balance (in shares) at Mar. 31, 2019			43,615,257
Balance at Dec. 31, 2018			$ 3			261,722	(241,551)			20,174
Balance (in shares) at Dec. 31, 2018			34,377,329
Increase (decrease) in stockholders' equity
Issuance of common stock (in shares)		1,658,046
Net loss										(8,153)
Balance at Jun. 30, 2019			$ 4			272,977	(249,917)			23,064
Balance (in shares) at Jun. 30, 2019			44,632,329
Balance at Mar. 31, 2019			$ 4			271,094	(246,433)			24,665
Balance (in shares) at Mar. 31, 2019			43,615,257
Increase (decrease) in stockholders' equity
Share-based compensation - stock options and RSUs						479				479
Issuance of common stock						1,389				1,389
Issuance of common stock (in shares)		992,072	992,072
Issuance of common stock upon exercise of options						15				15
Issuance of common stock upon exercise of options (in shares)			25,000
Net loss							(3,484)			(3,484)
Balance at Jun. 30, 2019			$ 4			$ 272,977	$ (249,917)			$ 23,064
Balance (in shares) at Jun. 30, 2019			44,632,329