Income Taxes - Current and Deferred Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Current taxes:
U.S. federal	$ 15.8	$ 8.2	$ 6.7
U.S. state and local	3.9	3.6	(0.1)
Total current taxes	86.9	70.4	34.6
Deferred taxes:
U.S. federal	(1.1)	3.4	1.4
U.S. state and local	0.0	(0.1)	2.7
Total deferred taxes	(2.4)	(0.3)	3.2
Income tax expense	84.5	70.1	37.8
Australia
Current taxes:
Foreign tax expense	2.2	2.8	1.8
Deferred taxes:
Other international	0.1	(0.2)	0.3
Brazil
Current taxes:
Foreign tax expense	16.0	12.6	8.0
Deferred taxes:
Other international	0.0	(0.8)	(1.3)
Germany
Current taxes:
Foreign tax expense	4.5	8.9	6.0
Singapore
Current taxes:
Foreign tax expense	4.9	2.0	1.9
Deferred taxes:
Other international	0.0	0.0	0.4
Switzerland
Current taxes:
Foreign tax expense	2.6	0.0	0.0
Deferred taxes:
Other international	0.4	0.0	0.0
United Kingdom
Current taxes:
Foreign tax expense	30.9	29.2	6.6
Deferred taxes:
Other international	(1.4)	(2.7)	0.1
Other international
Current taxes:
Foreign tax expense	6.1	3.1	3.7
Deferred taxes:
Other international	$ (0.4)	$ 0.1	$ (0.4)