Summary of significant accounting policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2023
Accounting Policies [Abstract]
Accounts receivable, allowance for credit loss	$ 0.9
Contract with customer, asset, after allowance for credit loss	2.5		$ 2.8
Contract with customer, liability	2.6		$ 3.0
Contract with customer, liability, revenue recognized	1.3	$ 2.3
Revenue, remaining performance obligation, amount	$ 7.3