Common Stock - Additional Information (Details) $ / shares in Units, $ in Thousands			12 Months Ended
	Nov. 14, 2017 USD ($)	Nov. 01, 2017 $ / shares shares	Dec. 31, 2017 USD ($) vote shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Nov. 30, 2017 $ / shares shares
Common Stock Capital Shares Reserved For Future Issuance [Line Items]
Proceeds from issuance of common stock | $			$ 6,908	$ 0	$ 12,650
Proceeds from sale of units | $	$ 7,000
Sale of stock, number of units issued (in shares)		5,525,000
Warrant price per unit (in dollars per unit) | $ / shares		$ 0.01
Common stock, shares issued (in shares)			34,702,565	24,531,964
Common Warrants
Common Stock Capital Shares Reserved For Future Issuance [Line Items]
Number of securities called by each warrant (in shares)		1
Expiration period		5 years
Pre-Funded Warrant
Common Stock Capital Shares Reserved For Future Issuance [Line Items]
Number of securities called by each warrant (in shares)		1
Shares of common stock into which warrants are exercisable (in shares)		4,475,000
Warrant price per unit (in dollars per unit) | $ / shares		$ 0.79
Common stock, shares issued (in shares)			4,475,000
Cowen and Company LLC
Common Stock Capital Shares Reserved For Future Issuance [Line Items]
Proceeds from issuance of common stock | $				$ 12,700
November 2017 Warrants | Common Stock
Common Stock Capital Shares Reserved For Future Issuance [Line Items]
Shares of common stock into which warrants are exercisable (in shares)						10,000,000
Warrant issued, exercise price (in dollars per share) | $ / shares						$ 0.80
Expiration period		5 years
Common Stock
Common Stock Capital Shares Reserved For Future Issuance [Line Items]
Number of votes per share | vote			1
Issuance of common stock (in shares)			5,525,000	1,446,781	1,446,781
Number of securities called by each warrant (in shares)		1
Sale of stock price per share (in dollars per share/unit) | $ / shares		$ 0.80