Organization and Basis of Presentation (Details) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended	6 Months Ended
	Jun. 21, 2019	Jun. 04, 2018	Mar. 23, 2018	Jun. 30, 2019	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash and cash equivalents				$ 64,931	$ 64,931		$ 50,422
Net working capital				59,600	59,600
Accumulated deficit				246,839	246,839		$ 186,024
Proceeds from sale of stock	$ 28,000	$ 41,900	$ 9,000
Sale of stock, shares issued (in shares)	20,410,000	25,555,556	7,968,128
Number of warrants (in shares)	20,410,000		7,968,128
Stock and warrants price per share (in usd per share)	$ 1.47
Exercise price per warrant (in usd per share)	$ 1.47		$ 1.20
Proceeds from issuance of common stock and the issuance and exercise of common stock warrants				$ 3,400	$ 31,243	$ 58,226
Exercise of common stock warrants (in shares)				3,400,000	3,361,115