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Organization and Operations - Liquidity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended
	Feb. 06, 2019	Jul. 19, 2016	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Common Stock
Additional common stock issued			$ 2,045	$ 8,313
Accumulated deficit			(203,342)		$ (197,304)
Cash, cash equivalents and short-term investments			$ 51,700
Common Stock
Common Stock
Number of shares issued			353,746	539,026
Additional common stock issued				$ 1
Common Stock | Cowen
Common Stock
Percentage of commission paid to underwriter based on proceeds from common stock		3.00%
Common Stock | ATM
Common Stock
Number of shares issued			353,746
Share price (in dollars per share)			$ 5.96
Additional common stock issued			$ 2,100
Net proceeds			$ 2,000
Common Stock | First ATM Program | Cowen
Common Stock
Remaining current sales agreement		$ 22,600
Common Stock | February 2019 Financing | Oppenheimer & Co. Inc
Common Stock
Number of shares issued	4,000,000
Share price (in dollars per share)	$ 0.001
Additional common stock issued	$ 20,000
Number of warrants issued	2,000,000
Warrants price (in dollars per unit)	$ 5.00
Proceeds from public offering, net of issuance costs	$ 18,500

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