Stock options - Stock based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Stock options
Stock-based compensation expense included in total operating expenses	$ 1,609	$ 1,164
Unrecognized stock compensation cost, Options	11,341
Stock options.
Stock options
Stock-based compensation expense included in total operating expenses	$ 1,233	1,094
Weighted-average amortization period of unrecognized stock compensation cost	2 years 6 months
Restricted stock units
Stock options
Stock-based compensation expense included in total operating expenses	$ 310
Unrecognized stock compensation cost. Other than options	$ 3,022
Weighted-average amortization period of unrecognized stock compensation cost	2 years 10 months 24 days
Employee stock purchase plan
Stock options
Stock-based compensation expense included in total operating expenses	$ 66	70
Research and development
Stock options
Stock-based compensation expense included in total operating expenses	797	516
General and administrative
Stock options
Stock-based compensation expense included in total operating expenses	$ 812	$ 648

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

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