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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Stock options - Stock based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Stock options
Stock-based compensation expense included in total operating expenses	$ 1,285	$ 1,053	$ 3,610	$ 2,757
Unrecognized stock compensation cost	8,290		$ 8,290
Weighted-average amortization period of unrecognized stock compensation cost			2 years 2 months 12 days
Stock options.
Stock options
Stock-based compensation expense included in total operating expenses	1,055	988	$ 3,240	2,692
Restricted stock units
Stock options
Stock-based compensation expense included in total operating expenses	184		184
Unrecognized stock compensation cost	1,605		1,605
Employee stock purchase plan
Stock options
Stock-based compensation expense included in total operating expenses	46	65	186	65
Research and development
Stock options
Stock-based compensation expense included in total operating expenses	587	495	1,636	1,302
General and administrative
Stock options
Stock-based compensation expense included in total operating expenses	$ 698	$ 558	$ 1,974	$ 1,455