Equity Incentive Plans - Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity Incentive Plans
Total stock-based compensation expense	$ 1,762	$ 3,630	$ 3,651
Assumptions used to compute employee stock based compensation under the Black-Scholes option pricing model
Risk-free interest rate (as a percent)	2.61%	2.57%	2.27%
Expective volatility (as a percent)	65.00%	70.00%	73.90%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Expected life (in years)	6 years 3 months	6 years 3 months	6 years 3 months
Minimum
Assumptions used to compute employee stock based compensation under the Black-Scholes option pricing model
Risk-free interest rate (as a percent)	1.74%
Common stock options
Equity Incentive Plans
Total stock-based compensation expense	$ 1,762	$ 3,630	$ 3,651
Unrecorded deferred stock-based compensation
Unrecorded deferred stock-based compensation balance related to stock options	$ 1,900
Weighted-average amortization period over which cost is expected to be recognized	1 year 4 months 24 days
Weighted average grant date fair value of options granted (in dollars per share)	$ 0.59
Common stock options | Research and development
Equity Incentive Plans
Total stock-based compensation expense	$ 522	1,274	1,184
Common stock options | General and administrative
Equity Incentive Plans
Total stock-based compensation expense	$ 1,240	$ 2,356	$ 2,467