Fair Value Measurements, Not Measured at Fair Value (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 223.0	$ 318.8
Restricted cash	0.2	16.2
Debt, net	458.9	111.7
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	223.0	318.8
Other Assets, Fair Value Disclosure	0.2	16.2
Long-term Debt, Fair Value	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Other Assets, Fair Value Disclosure	0.0	0.0
Long-term Debt, Fair Value	485.8	113.2
Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Other Assets, Fair Value Disclosure	0.0	0.0
Long-term Debt, Fair Value	0.0	0.0
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	223.0	318.8
Other Assets, Fair Value Disclosure	0.2	16.2
Long-term Debt, Fair Value	485.8	113.2
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, net	$ 465.8	$ 112.4