Income taxes - Reconciliation of the U.S federal statutory income tax rate (Details)	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Reconciliation of the U.S federal statutory income tax rate
U.S. Federal statutory income tax rate	(21.00%)	(21.00%)	(21.00%)
State taxes, net of federal benefit	(1.60%)	(1.50%)	(2.60%)
Research and development expenses	2.20%	3.10%	3.80%
Remeasurement of deferred taxes as a result of tax reform			2.60%
Foreign tax rate differential	2.80%	2.40%	2.20%
Change in valuation allowance	16.00%	15.50%	14.00%
Return to provision	1.90%	0.40%
Other	(0.30%)	1.10%	1.00%
Effective income tax rate	0.00%	0.00%	0.00%