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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Energy Prepayment Obligations and Discounts on Sales (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2004
Energy Prepayment Obligations and Discounts on Sales |  |  |  |
Allowance for Doubtful Accounts Receivable, Current | $ 0 | $ 1 |  |
MLGW prepayment |  |  |  | $ 1,500
MLGW prepayment period |  |  |  | 180 months
Recognition of Deferred Revenue | 100 | 100 | $ 100 |
MLGW |  |  |  |
Energy Prepayment Obligations and Discounts on Sales |  |  |  |
Recognition of Deferred Revenue | $ 1,490 |  |  |
Deferred revenue expected recognition each year | 100 |  |  |
Deferred revenue, revenue recognized | $ 10 | 100 |  |
Total |  |  |  |
Energy Prepayment Obligations and Discounts on Sales |  |  |  |
Discounts reducing electricity sales | $ 46 | $ 46 |  |

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- Definition A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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