Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018
Estimate of Fair Value Measurement | Level 2 inputs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 3,957.1	$ 4,000.1
Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	4,165.4	4,158.5
Money market | Estimate of Fair Value Measurement | Level 1 inputs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 1,200.0	$ 1,050.0