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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
--- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2004
Energy Prepayment Obligations and Discounts on Sales |  |  |  |
MLGW prepayment |  |  |  | $ 1,500
MLGW prepayment period |  |  |  | 180 months
MLGW |  |  |  |
Energy Prepayment Obligations and Discounts on Sales |  |  |  |
Deferred revenue expected recognition each year | $ 100 |  |  |
Deferred revenue, revenue recognized | 100 | $ 100 | $ 100 |
Total |  |  |  |
Energy Prepayment Obligations and Discounts on Sales |  |  |  |
Discounts reducing electricity sales | $ 46 | $ 46 | $ 46 |

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- Definition Amount of revenue recognized that was previously reported as deferred or unearned revenue. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 605 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 13.A.4(a).Q1 Response)) -URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780 + Details
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- Definition The increase (decrease) during the reporting period, excluding the portion taken into income, in the liability reflecting revenue yet to be earned for which cash or other forms of consideration was received or recorded as a receivable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 + Details
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- Definition The amount of consideration recognized during the period for the milestone or milestones. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 605 -SubTopic 28 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=68058744&loc=SL6892177-166501 + Details
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- Definition Aggregate cash discounts given by the entity for services rendered to customers for early payments of accounts due. For example, selling terms of "2/10, net 30" entitle a customer to a 2% discount for prompt payment within 10 days, otherwise full payment is expected in 30 days. Discounts are a deduction from gross revenue in arriving at net revenue. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1(a)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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