Income Taxes - Unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Income Tax Disclosure [Abstract]
Unrecognized Tax Benefits	$ 12.8	$ 11.7	$ 12.5
Unrecognized Tax Benefits, Increases Resulting from Current Period Tax Positions	1.5	0.8	0.1
Unrecognized Tax Benefits, Decreases Resulting from Prior Period Tax Positions	0.7	0.4	0.0
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	(5.2)	(0.1)	(0.9)
Unrecognized Tax Benefits	9.8	12.8	11.7
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	9.8
Decrease in Unrecognized Tax Benefits is Reasonably Possible	0.3
Unrecognized Tax Expense (Benefits), Income Tax Penalties and Interest Expense	0.4	0.5	$ 0.1
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	$ 1.5	$ 1.1

Amount of decrease reasonably possible in the next twelve months for the unrecognized tax benefit.

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Amount of unrecognized tax benefits.

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Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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