Equity Plans - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 2,048	$ 1,979
Total unrecognized stock-based compensation costs related to stock options	$ 16,200
Period of unrecognized stock-based compensation costs to be recognized	3 years 1 month 6 days
Research and Development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,066	1,122
General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 982	$ 857

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

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