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Long-Term Debt - Schedule of Long-term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2020	Jun. 08, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Financing lease obligation	$ 6
Financing lease obligation			$ 7
Subtotal	1,227		943
Less: current maturities	(34)		(41)
Long-term debt	1,193		902
Convertible Notes
Debt Instrument [Line Items]
Unamortized discount on convertible notes	$ (30)		(34)
3.25 percent convertible notes due 2037 \| Convertible Notes
Debt Instrument [Line Items]
Interest rate (percent)	3.25%
Debt	$ 320		319
7.875 percent convertible notes due 2026 \| Convertible Notes
Debt Instrument [Line Items]
Interest rate (percent)	7.875%
Debt	$ 23		23
6.25 percent notes due 2025 \| Notes
Debt Instrument [Line Items]
Interest rate (percent)	6.25%
Debt	$ 295		0
Term loan due 2024 \| Term Loan
Debt Instrument [Line Items]
Debt	$ 168		175
6.25 percent notes due 2024
Debt Instrument [Line Items]
Interest rate (percent)		6.25%
6.25 percent notes due 2024 \| Notes
Debt Instrument [Line Items]
Interest rate (percent)	6.25%
Debt	$ 445		444
Borrowings and securitization
Debt Instrument [Line Items]
Borrowings and securitization	$ 0		$ 9