Equity Incentive Plan - Stock-based compensation expense (Details) - USD ($)	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]
Stock-based compensation expense	$ 1,878,000	$ 342,000	$ 4,013,000	$ 832,000
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	920,000	147,000	2,091,000	357,000
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 958,000	$ 195,000	$ 1,922,000	$ 475,000

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

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