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 (loss)	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, 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