Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2020	Sep. 30, 2019
Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 3,620.6	$ 4,033.9
Estimate of Fair Value Measurement | Level 2 inputs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	3,985.3	4,158.4
Money market | Estimate of Fair Value Measurement | Level 1 inputs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 1,778.0	$ 1,552.0