Equity Incentive Plan - Stock-based compensation expense (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 6,056	$ 1,419
Income tax benefits for stock-based compensation	0
Stock-based compensation costs capitalized	0
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	2,977	631
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 3,079	$ 788

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

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