Commitments and contingencies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Commitments and Contingencies Disclosure [Abstract]
Change in right-of-use asset			$ 2.9	$ 14.6
Change in operating lease liabilities			1.9	17.2
Operating lease expense	$ 3.9	$ 4.1	12.2	11.5
Operating lease payments	$ 3.6	$ 3.7	$ 11.1	$ 9.1
Lease weighted-average remaining lease term	15 years 7 months 6 days		15 years 7 months 6 days
Lease weighted average discount rate	6.50%		6.50%