Organization and Operations - Liquidity (Details) $ / shares in Units, $ in Thousands					2 Months Ended	12 Months Ended
	Feb. 06, 2019 USD ($) $ / shares shares	Dec. 28, 2018	Jun. 19, 2018 USD ($) $ / shares shares	Jul. 16, 2016 USD ($)	Feb. 28, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 12, 2018 $ / shares
Common Stock
Additional common stock issued						$ 8,363	$ 8,036
Reverse stock split ratio (as a percent)		0.1
Minimum bid price requirement for continued listing on NASDAQ | $ / shares								$ 1.00
Accumulated deficit						(197,304)	$ (171,434)
Cash, cash equivalents and short-term investments						$ 37,600
Common Stock
Common Stock
Number of shares issued | shares						577,195	480,480
Additional common stock issued						$ 1
Common Stock | Subsequent Events
Common Stock
Number of shares issued | shares	4,000,000
Additional common stock issued	$ 20,000
Number of warrants issued | shares	2,000,000
Warrants price (in dollars per unit) | $ / shares	$ 5.00
Proceeds from public offering, net of issuance costs	$ 18,500				$ 20,500
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 | shares						577,195
Share price (in dollars per share) | $ / shares						$ 15.27
Additional common stock issued						$ 8,800
Proceeds from public offering, net of issuance costs						$ 8,400
Common Stock | ATM | Cowen | Subsequent Events
Common Stock
Number of shares issued | shares					353,746
Share price (in dollars per share) | $ / shares					$ 5.96
Additional common stock issued					$ 2,100
Proceeds from public offering, net of issuance costs					$ 2,000
Common Stock | First ATM Program | Cowen
Common Stock
Remaining current sales agreement				$ 22,600
Common Stock | June 2018 Financing | Oppenheimer & Co. Inc
Common Stock
Number of shares issued | shares			4,200,000
Share price (in dollars per share) | $ / shares			$ 0.001
Additional common stock issued			$ 42,000
Number of warrants issued | shares			4,200,000
Warrants price (in dollars per unit) | $ / shares			$ 10.00
Proceeds from public offering, net of issuance costs			$ 38,900