Organization and Operations - Liquidity and Going Concern (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	6 Months Ended
	Jun. 20, 2018	Oct. 31, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Common Stock
Accumulated deficit			$ (185,565)	$ (171,434)
Cash and cash equivalents			$ 49,931	$ 16,369	$ 29,369
Common Stock | Cowen
Common Stock
Percentage of commission paid to underwriter based on proceeds from common stock		0.03%
Common Stock | First ATM Program | Cowen
Common Stock
Aggregate maximum offering amount		$ 10,000
Common Stock | Second ATM Program
Common Stock
Number of shares issued			5,390,255
Share price (in dollars per share)			$ 1.59
Additional common stock issued			$ 8,600
Net proceeds			$ 8,300
Common Stock | June 2018 Financing | Oppenheimer & Co. Inc
Common Stock
Number of shares issued	42,000,000
Share price (in dollars per share)	$ 0.001
Additional common stock issued	$ 42,000
Number of warrants to purchase	42,000,000
Warrant purchase price per share	$ 1.00
Proceeds from public offering, net of issuance costs	$ 38,900