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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, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2004
Energy Prepayment Obligations and Discounts on Sales |  |  |  |
Accounts Receivable, Allowance for Credit Loss, Current |  | $ 1 |  |
MLGW prepayment |  |  |  | $ 1,500
MLGW prepayment period |  |  |  | 180 months
Recognition of Deferred Revenue | $ 0 | $ 10 | $ 100 |
MLGW |  |  |  |
Energy Prepayment Obligations and Discounts on Sales |  |  |  |
Recognition of Deferred Revenue | $ 1,500 |  |  |
Deferred revenue expected recognition each year | 10 million | 100 million |  | 100 million
Deferred revenue, revenue recognized |  | $ 100 |  |
Total |  |  |  |
Energy Prepayment Obligations and Discounts on Sales |  |  |  |
Discounts reducing electricity sales | $ 4 | $ 46 |  |

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- Definition Amount of allowance for credit loss on accounts receivable, classified as current. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -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=120391452&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5074-111524 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446 + Details
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