Commitments and contingencies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Commitments and Contingencies Disclosure [Abstract]
Change in right-of-use asset	$ 2.4	$ 1.2
Change in operating lease liabilities	2.2	0.9
Operating lease expense	4.0	4.1
Operating lease payments	$ 3.7	$ 3.6
Lease weighted-average remaining lease term	15 years 3 months 18 days
Lease weighted average discount rate	6.50%