Commitments and contingencies - Components of lease expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Finance lease costs:
Amortization of right-of-use asset	$ 607	$ 0	$ 1,214	$ 0
Interest on lease liabilities	562	195	1,123	195
Operating lease costs	237	378	467	492
Total lease cost	$ 1,406	$ 378	$ 2,804	$ 492