Common Stock - Private Placement of Common Stock (Details) $ / shares in Units, $ in Thousands			12 Months Ended
	Dec. 08, 2017 shares	Nov. 14, 2017 USD ($) item $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)
Private Placement of Common Stock
Gross proceeds		$ 18,000	$ 18,000
Fair value of warrant liability		12,335	11,862
Net proceeds from issuance of stock in private placement		$ 17,250
Additional proceeds from exercise of warrants			18,000
Additional shares issuable on exercise in cash of warrants			2,958,094
Issuance costs associated with private placement			2,172	$ 282
Private placement
Private Placement of Common Stock
Number of warrants issued per common stock | shares		1
Issuance costs associated with private placement		$ 750
Allocation of issuance cost to warrants (as a percentage)		68.00%
Allocation of issuance cost to common stock (as a percentage)		32.00%
Issuance costs associated with common stock		$ 236	$ 236
Number of days for registration		30 years
Number of days for declaration		60 years
Number of days for review		90 years
Number of purchasers not registered | item		2
HCV IX and Eli Lilly | Private placement
Private Placement of Common Stock
Number of affiliated directors and executive officers | item		3
Common Stock
Private Placement of Common Stock
Remaining proceeds allocated to common stock		$ 5,665
Common Stock | Private placement
Private Placement of Common Stock
Issuance of stock (in shares) | shares		2,958,094	2,958,094
Price per share (in dollars per share) | $ / shares		$ 6.085
Warrant Liability | Private placement
Private Placement of Common Stock
Exercise price of warrant | $ / shares		$ 6.085
Exercise expiring period		7 years
Other expense | Private placement
Private Placement of Common Stock
Issuance costs associated with warrants		$ 514
Minimum | HCV IX and Eli Lilly | Private placement
Private Placement of Common Stock
Direct Holding Common Stock Percentage		5.00%
Maximum | Private placement
Private Placement of Common Stock
Resale shares | shares	3,734,914