Common Stock - Private Placement of Common Stock (Details)		3 Months Ended		12 Months Ended
	Nov. 14, 2017 USD ($) director item $ / shares shares	Mar. 31, 2018 USD ($) shares	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Private Placement of Common Stock
Gross proceeds	$ 18,000,000
Fair value of warrant liability	14,344,000	$ 16,713,000		$ 11,862,000
Net proceeds from issuance of stock in private placement	$ 17,250,000
Additional proceeds from exercise of warrants		18,000,000
Additional shares issuable on exercise in cash of warrants		2,958,094
Issuance costs associated with private placement		28,000	$ 2,067,000
Issuance costs associated with common stock		$ 1,304,000
Private placement
Private Placement of Common Stock
Number of warrants issued per common stock | shares	1
Issuance costs associated with private placement	$ 750,000
Allocation of issuance cost to warrants (as a percentage)	80.00%
Allocation of issuance cost to common stock (as a percentage)	20.00%
Issuance costs associated with common stock	$ 152,000
Number of days for registration	30 days
Number of days for declaration	60 days
Number of days for review	90 days
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 | director	3
Common Stock
Private Placement of Common Stock
Issuance of stock (in shares) | shares		2,146,667
Remaining proceeds allocated to common stock	$ 3,656,000
Common Stock | Private placement
Private Placement of Common Stock
Issuance of stock (in shares) | shares	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				$ 598,000
Minimum | HCV IX and Eli Lilly | Private placement
Private Placement of Common Stock
Direct Holding Common Stock Percentage	5.00%