Common Stock - Additional Information (Details) $ / shares in Units, $ in Millions					12 Months Ended
	Jun. 04, 2018 USD ($) $ / shares shares	Mar. 23, 2018 USD ($) $ / shares shares	Nov. 14, 2017 USD ($)	Nov. 01, 2017 $ / shares shares	Dec. 31, 2018 USD ($) vote shares	Dec. 31, 2017 shares	Nov. 30, 2017 $ / shares shares
Common Stock Capital Shares Reserved For Future Issuance [Line Items]
Proceeds from sale of stock and units | $	$ 41.9	$ 9.0	$ 7.0
Sale of stock, number of units issued (in shares)	25,555,556	7,968,128		5,525,000
Shares of common stock into which warrants are exercisable (in shares)		7,968,128			8,765,496
Sale of stock price per share (in dollars per share/unit) | $ / shares	$ 1.80	$ 1.13
Warrant price per unit (in dollars per unit) | $ / shares		0.125		$ 0.01
Warrant issued, exercise price (in dollars per share) | $ / shares		$ 1.20
Expiration period		5 years
Common stock, shares issued (in shares)					77,456,180	34,702,565
Proceeds from warrant exercises | $					$ 7.3
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
Common Stock
Common Stock Capital Shares Reserved For Future Issuance [Line Items]
Number of votes per share | vote					1
Number of securities called by each warrant (in shares)				1
Sale of stock price per share (in dollars per share/unit) | $ / shares				$ 0.80
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