Fair Value, Not Measured at Fair Value (Detail) - USD ($) $ in Millions	Jun. 30, 2023	Sep. 30, 2022
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-Term Debt	$ 707.2	$ 706.0
Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	401.0	264.8
Long-term Debt, Fair Value	654.6	583.2
Cash and cash equivalents	401.0	264.8
Long-Term Debt	707.2	706.0
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	401.0	264.8
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	642.1	570.7
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	$ 12.5	$ 12.5