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Summary of significant accounting policies - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2023
Mar. 31, 2023
Sep. 30, 2022
Accounting Policies [Abstract]      
Contract with customer, asset, after allowance for credit loss $ 3.9 $ 3.9 $ 3.4
Contract with customer, liability 1.9 1.9 $ 3.5
Contract with customer, liability, revenue recognized 1.4 2.7  
Revenue, remaining performance obligation, amount $ 7.7 $ 7.7