Debt - Summary of Net Interest and Other Expense (Income) (Parenthetical) (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Accelerated amortization	[1]	$ 6	$ 1	$ 101
Notes due 2018 and 2022 [Member]
Debt Instrument [Line Items]
Loss on early retirement of debt				16
Gain (loss) on repurchase of debt instrument				(3)
Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Gain (loss) on repurchase of debt instrument				$ 5

[1] Fiscal year 2017 includes $92 million of accelerated accretion of the recorded debt discount for the 2029 Notes, while the remaining amounts in each year related to the accelerated amortization of debt issuance costs.