Commitments and contingencies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Change in right-of-use asset	$ 1.2	$ 10.4
Change in operating lease liabilities	0.9	11.6
Operating lease expense	4.1	3.5
Operating lease payments	$ 3.6	$ 2.9
Lease weighted-average remaining lease term	15 years 10 months 24 days
Lease weighted average discount rate	6.40%