Nature of the business (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2020
Nature of the business
Net loss	$ (20,095)	$ (17,493)	$ (11,139)	$ (9,508)	$ (37,588)	$ (20,647)
Accumulated deficit	$ (149,886)				$ (149,886)		$ (112,298)