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Income taxes - Additional Information (Details) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Provision for income taxes $ (1,930) $ 382 $ 292
Valuation allowance $ 190,428 $ 127,336  
Expiration year 2033    
Impact of unrecognized tax benefits on income tax rate if recognized $ 100    
Domestic Tax Authority [Member]      
Operating loss carry forwards 664,500    
State and Local Jurisdiction [Member]      
Operating loss carry forwards 377,100    
Research Tax Credit Carryforward [Member] | Domestic Tax Authority [Member]      
Tax credit carry forwards 16,900    
Research Tax Credit Carryforward [Member] | State and Local Jurisdiction [Member]      
Tax credit carry forwards $ 12,500