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Commitments and contingencies - Narrative (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2020
Feb. 29, 2020
Commitments and Contingencies Disclosure [Abstract]    
Performance bond commitments $ 36.5  
Percentage of employees covered by collective bargaining agreements 6.00%  
Loss contingency, maximum potential loss $ 31.0  
Loss contingency, disallowed amount $ 19.0 $ 12.0