Stockholders' Equity - Equity Incentive Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Stockholders’ Equity
Stock-based compensation	$ 1,498	$ 1,795
Intrinsic value of options exercisable	$ 300
Weighted average exercise price (in dollars per share)	$ 5.49
Employee Stock Option
Stockholders’ Equity
Fair value of underlying instrument (in dollars per share)	$ 1.64
Unrecognized compensation expense	$ 13,700
Weighted average remaining period for recognition of unrecognized compensation expense (in years)	2 years 4 months 17 days
Per-share weighted-average grant date fair value of options granted (in dollars per share)	$ 1.19	$ 3.35
Research and Development Expense [Member]
Stockholders’ Equity
Stock-based compensation	$ 358	$ 707
General and Administrative Expense [Member]
Stockholders’ Equity
Stock-based compensation	$ 1,140	$ 1,088
Inducement Plan
Stockholders’ Equity
Shares available for future grant (in shares)	250,500		293,000
Maximum
Stockholders’ Equity
Term of award (in years)	10 years
Vesting period (in years)	4 years