Equity Plans - Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 2,201	$ 2,017	$ 6,193	$ 4,847
Total unrecognized stock-based compensation costs related to stock options	14,200		$ 14,200
Period of unrecognized stock-based compensation costs to be recognized			2 years 7 months 6 days
Research and Development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	1,137	1,046	$ 3,237	2,424
General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,064	$ 971	$ 2,956	$ 2,423

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

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