Fair Value of Financial Instruments (Details) - USD ($)	Dec. 31, 2017	Sep. 30, 2017
Estimate of Fair Value Measurement | Level 2 inputs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 4,334,900,000	$ 3,522,500,000
Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	4,266,800,000	3,429,900,000
Money market | Estimate of Fair Value Measurement | Level 1 inputs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 0	$ 800,000,000