Fair Value Measurements - Fair Value of Financial Instruments (Details) - USD ($)	Mar. 31, 2020	Sep. 30, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt face amount	$ 2,974,000,000	$ 3,224,000,000
Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	2,443,000,000	3,037,000,000
Term Loan Credit Agreement due December 15, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt face amount	2,624,000,000	2,874,000,000
Term Loan Credit Agreement due December 15, 2024 | Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	2,162,000,000	2,739,000,000
Convertible 2.25% senior notes due June 15, 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt face amount	350,000,000	350,000,000
Convertible 2.25% senior notes due June 15, 2023 | Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	$ 281,000,000	$ 298,000,000