Income taxes - Components of deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2024
Deferred tax assets:
Net operating loss carryforwards	$ 267,196	$ 192,909
Research and development credits	15,144	2,676
Property, plant and equipment	4,735	4,596
Capitalized start-up costs	873	1,014
Stock compensation	20,297	16,478
Accrued expenses	5,595	6,397
Lease liability	6,931	7,318
Other	120	0
Total deferred tax assets	320,891	231,388
Valuation allowance	(312,035)	(221,629)	$ (161,890)
Net deferred tax assets	8,856	9,759
Deferred tax liabilities:
Right of use asset	(8,856)	(9,698)
Other	0	(61)
Total deferred tax liabilities	(8,856)	(9,759)
Net deferred tax assets (liabilities)	$ 0	$ 0

Amount before allocation of valuation allowances of deferred tax asset attributable to capitalized start-up cost.

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Amount before allocation of valuation allowances of deferred tax assets attributable to taxable temporary differences from lease liability.

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Amount of deferred tax liability attributable to taxable temporary differences.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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Amount, before allocation of valuation allowance, of deferred tax asset attributable to deductible temporary difference from in-process research and development cost acquired in business combination or from joint venture formation or both.

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Amount, after allocation of valuation allowances and deferred tax liability, of deferred tax asset attributable to deductible differences and carryforwards, without jurisdictional netting.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards.

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Amount, before allocation of valuation allowance, of deferred tax asset attributable to deductible temporary differences, classified as other.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from property, plant, and equipment.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from share-based compensation.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from accrued liabilities.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Amount, after deferred tax asset, of deferred tax liability attributable to taxable differences without jurisdictional netting.

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Amount of deferred tax liability attributable to taxable temporary differences from leasing arrangements.

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Amount of deferred tax liability attributable to taxable temporary differences classified as other.

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