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Debt and Other Obligations Debt and Other Obligations - Short-Term Debt (Details) - USD ($) $ in Millions | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
--- | --- | --- | ---
Short-Term Debt, Gross [Line Items] |  |  |
Short-term debt, net of discounts | $ 922 | $ 1,216 | $ 1,999
Short-term Borrowings Gross | $ 922 |  |
Weighted average interest rate - discount notes | 2.152% | 2.045% | 1.00%
Short-Term Debt, Gross [Domain] |  |  |
Short-Term Debt, Gross [Line Items] |  |  |
Short-term debt, net of discounts |  | $ 1,217 |

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