Fair Value, Not Measured at Fair Value (Detail) - USD ($) $ in Millions	Mar. 31, 2025	Sep. 30, 2024
Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 174.3	$ 481.2
Long-term Debt, Fair Value	863.1	693.5
Cash and Cash Equivalent	174.3	481.2
Debt	872.5	706.4
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	174.3	481.2
Long-term Debt, Fair Value	0.0	0.0
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Long-term Debt, Fair Value	853.2	683.6
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Long-term Debt, Fair Value	$ 9.9	$ 9.9