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Debt - Summary of Debt Issuance Cost Amortization (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Debt Disclosure [Abstract]      
Normal amortization $ 7 $ 7 $ 8
Accelerated amortization [1] 6 1 101
Total $ 13 $ 8 $ 109
[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.