Summary of significant accounting policies - Going concern, Cash equivalents and Deferred offering costs (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Summary of significant accounting policies
Net losses	$ 10,926	$ 9,329	$ 43,000
Accumulated deficit	$ 113,945		$ 103,019