Income taxes - Components of deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Deferred tax assets:
Foreign net operating loss carryforwards	$ 9,092	$ 4,079
Federal net operating loss carryforwards	2,450	2,223
State net operating loss carryforwards	1,054	684
Property, plant and equipment	5,499
Capitalized start-up costs	1,748	1,886
Stock compensation	2,526	573
Accrued expenses	341	1,793
Lease liability	8,705
Total deferred tax assets	31,415	11,238
Deferred tax liabilities:
Right-to-use asset	(13,998)
Property, plant and equipment		(1,842)
Other	(88)
Total deferred tax liabilities	(14,086)	(1,842)
Valuation allowance	(17,329)	$ (9,396)	$ (4,724)
Deferred tax assets operating loss carryforwards
Federal net operating loss carryforwards having indefinite expiration period	11,667
Foreign net operating loss carryforwards having indefinite expiration period	53,483
State net operating loss carryforwards, expires between 2039 and 2040	$ 16,670