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Debt - Summary of Debt Issuance Cost Amortization (Details) - USD ($) $ in Millions | 12 Months Ended
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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.

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- Definition The portion of the component of interest expense representing the noncash expenses charged against earnings in the period to allocate debt discount and premium, and the costs to issue debt and obtain financing over the related debt instruments for amount utilizing accelerated amortization periods generally due to termination, prepayment or refinancing. + References No definition available. + Details
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Balance Type: | debit
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- Definition The portion of the component of interest expense representing the noncash expenses charged against earnings in the period to allocate debt discount and premium, and the costs to issue debt and obtain financing over the related debt instruments for amount utilizing standard amortization periods. + References No definition available. + Details
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- Definition Amount of amortization expense attributable to debt discount (premium) and debt issuance costs. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28555-108399 + Details
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- References No definition available. + Details
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