Fair Value, Not Measured at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2025	Sep. 30, 2025
Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 211.7	$ 379.2
Long-term Debt, Fair Value	810.0	818.8
Cash and Cash Equivalent	211.7	379.2
Debt	793.2	802.7
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	211.7	379.2
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	810.0	808.9
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	$ 0.0	$ 9.9