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Summary of Significant Accounting Policies Allowance for Uncollectible Accounts (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2020 | Mar. 31, 2019 | Mar. 31, 2020 | Mar. 31, 2019 | Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Defined Benefit Plan, Other Cost (Credit) | $ 62 | $ 65 | $ 127 | $ 129 |
Financing Receivable, after Allowance for Credit Loss | 128 |  | 128 |  | $ 120
Other Deferred Costs, Net | 10 |  | 10 |  |
Carrying Value |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Financing Receivable, after Allowance for Credit Loss | $ 137 |  | $ 137 |  | $ 131

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