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Income taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Income Tax Contingency [Line Items]
Income tax expense (benefit)	$ (10,411)	$ (1,930)	$ 382
Valuation allowance	240,660	190,428
Impact of unrecognized tax benefits on income tax rate if recognized	100	100
Domestic Tax Authority
Income Tax Contingency [Line Items]
Operating loss carry forwards	777,200	452,200
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Operating loss carry forwards	777,200	$ 452,200
Research Tax Credit Carryforward \| Domestic Tax Authority
Income Tax Contingency [Line Items]
Tax credit carry forwards	29,900
Research Tax Credit Carryforward \| State and Local Jurisdiction
Income Tax Contingency [Line Items]
Tax credit carry forwards	$ 20,800