Income taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Income Tax Contingency [Line Items]
Income tax expense (benefit)	$ 560	$ 1,152	$ (10,411)
Valuation allowance	360,594	302,381
Impact of unrecognized tax benefits on income tax rate if recognized	400	400
Domestic Tax Jurisdiction
Income Tax Contingency [Line Items]
Operating loss carry forwards	903,100	588,400
Domestic Tax Jurisdiction | Never Expire
Income Tax Contingency [Line Items]
Operating loss carry forwards	702,000
Domestic Tax Jurisdiction | 2032 through 2038
Income Tax Contingency [Line Items]
Operating loss carry forwards	201,100
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Operating loss carry forwards	903,100	$ 588,400
State and Local Jurisdiction | 2023
Income Tax Contingency [Line Items]
Operating loss carry forwards	29,700
State and Local Jurisdiction | 2024 and Beyond
Income Tax Contingency [Line Items]
Operating loss carry forwards	293,600
Research Tax Credit Carryforward | Domestic Tax Jurisdiction
Income Tax Contingency [Line Items]
Tax credit carry forwards	51,000
Research Tax Credit Carryforward | State and Local Jurisdiction
Income Tax Contingency [Line Items]
Tax credit carry forwards	$ 34,400