Consolidated Statements of Changes in Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock/Ordinary Shares	Additional paid in capital	Accumulated other comprehensive income	Accumulated deficit	Total
Stockholders' equity, beginning balance at Dec. 31, 2017	$ 367	$ 360,872	$ 27	$ (279,198)	$ 82,068
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2017	36,708
Consolidated Statements of Changes in Stockholders' Equity
Issuance of common stock	$ 35	19,353			19,388
Issuance of common stock (in shares)	3,526
Equity transaction costs		(916)			(916)
Stock-based compensation expense		1,244			1,244
Net loss				(13,342)	(13,342)
Stockholders' equity, ending balance at Mar. 31, 2018	$ 402	380,553	27	(292,540)	88,442
Stockholders' equity, ending balance (in shares) at Mar. 31, 2018	40,234
Stockholders' equity, beginning balance at Dec. 31, 2017	$ 367	360,872	27	(279,198)	82,068
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2017	36,708
Consolidated Statements of Changes in Stockholders' Equity
Net loss					(31,130)
Stockholders' equity, ending balance at Jun. 30, 2018	$ 410	384,557	27	(310,326)	74,668
Stockholders' equity, ending balance (in shares) at Jun. 30, 2018	40,959
Stockholders' equity, beginning balance at Mar. 31, 2018	$ 402	380,553	27	(292,540)	88,442
Stockholders' equity, beginning balance (in shares) at Mar. 31, 2018	40,234
Consolidated Statements of Changes in Stockholders' Equity
Issuance of common stock	$ 8	3,389			3,397
Issuance of common stock (in shares)	725
Equity transaction costs		(152)			(152)
Stock-based compensation expense		767			767
Net loss				(17,786)	(17,788)
Stockholders' equity, ending balance at Jun. 30, 2018	$ 410	384,557	27	(310,326)	74,668
Stockholders' equity, ending balance (in shares) at Jun. 30, 2018	40,959
Stockholders' equity, beginning balance at Dec. 31, 2018	$ 670	461,911	27	(393,978)	68,630
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2018	67,019
Consolidated Statements of Changes in Stockholders' Equity
Issuance of common stock	$ 43	10,014			10,057
Issuance of common stock (in shares)	4,317
Equity transaction costs		(270)			(270)
Stock-based compensation expense		1,907			1,907
Net loss				(20,217)	(20,217)
Stockholders' equity, ending balance at Mar. 31, 2019	$ 713	473,562	27	(414,195)	60,107
Stockholders' equity, ending balance (in shares) at Mar. 31, 2019	71,336
Stockholders' equity, beginning balance at Dec. 31, 2018	$ 670	461,911	27	(393,978)	68,630
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2018	67,019
Consolidated Statements of Changes in Stockholders' Equity
Net loss					(41,924)
Stockholders' equity, ending balance at Jun. 30, 2019	$ 729	478,551	27	(435,902)	43,405
Stockholders' equity, ending balance (in shares) at Jun. 30, 2019	72,906
Stockholders' equity, beginning balance at Mar. 31, 2019	$ 713	473,562	27	(414,195)	60,107
Stockholders' equity, beginning balance (in shares) at Mar. 31, 2019	71,336
Consolidated Statements of Changes in Stockholders' Equity
Issuance of common stock	$ 16	3,691			3,707
Issuance of common stock (in shares)	1,570
Equity transaction costs		(523)			(523)
Stock-based compensation expense		1,821			1,821
Net loss				(21,707)	(21,707)
Stockholders' equity, ending balance at Jun. 30, 2019	$ 729	$ 478,551	$ 27	$ (435,902)	$ 43,405
Stockholders' equity, ending balance (in shares) at Jun. 30, 2019	72,906