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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Energy Prepayment Obligations and Discounts on Sales (Details) (USD $) | 12 Months Ended | 24 Months Ended | 108 Months Ended | 12 Months Ended | 108 Months Ended | 12 Months Ended
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Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2004 | Sep. 30, 2004 | Sep. 30, 2013 DEU [Member] | Sep. 30, 2013 MLGW | Sep. 30, 2012 MLGW | Sep. 30, 2011 MLGW | Sep. 30, 2013 MLGW | Sep. 30, 2013 Total | Sep. 30, 2012 Total | Sep. 30, 2011 Total
Energy Prepayment Obligations and Discounts on Sales |  |  |  |  |  |  |  |  |  |  |  |  |
Incremental amount of discounted energy units | $ 1,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Discount per kilowatt-hour | 0.025 |  |  |  |  |  |  |  |  |  |  |  |
Discounted energy units contract period 1 | 5 years |  |  |  |  |  |  |  |  |  |  |  |
Discounted energy units contract period 2 | 10 years |  |  |  |  |  |  |  |  |  |  |  |
Discounted energy units contract period 3 | 15 years |  |  |  |  |  |  |  |  |  |  |  |
Discounted energy units contract period 4 | 20 years |  |  |  |  |  |  |  |  |  |  |  |
Total sales through DEU program since inception |  |  |  |  | 55,000,000 |  |  |  |  |  |  |  |
Deferred revenue, revenue recognized | 2,000,000 | 5,000,000 | 5,000,000 |  |  | 54,000,000 | 100,000,000 | 100,000,000 | 100,000,000 | 990,000,000 | 47,000,000 | 47,000,000 | 47,000,000
MLGW prepayment |  |  |  | 1,500,000,000 |  |  |  |  |  |  |  |  |
MLGW prepayment period |  |  |  | 180 months |  |  |  |  |  |  |  |  |
Deferred revenue expected recognition each year |  |  |  |  |  |  | $ 100,000,000 |  |  |  |  |  |

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