Equity Plans - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,996	$ 2,013	$ 4,044	$ 3,992
Total unrecognized stock-based compensation costs related to stock options	14,700		$ 14,700
Period of unrecognized stock-based compensation costs to be recognized			2 years 10 months 24 days
Research and Development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	1,026	977	$ 2,092	2,100
General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 970	$ 1,036	$ 1,952	$ 1,892

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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