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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) (USD $) | 12 Months Ended |  |  |  |  |
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Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2014 Nuclear | Sep. 30, 2013 Nuclear | Sep. 30, 2012 Nuclear | Sep. 30, 2014 Coal-Fired | Sep. 30, 2013 Coal-Fired | Sep. 30, 2012 Coal-Fired | Sep. 30, 2014 Hydroelectric | Sep. 30, 2013 Hydroelectric | Sep. 30, 2012 Hydroelectric | Sep. 30, 2014 Gas and Oil-Fired | Sep. 30, 2013 Gas and Oil-Fired | Sep. 30, 2012 Gas and Oil-Fired | Sep. 30, 2014 Transmission | Sep. 30, 2013 Transmission | Sep. 30, 2012 Transmission | Sep. 30, 2014 Other | Sep. 30, 2013 Other | Sep. 30, 2012 Other | Sep. 30, 2014 Electricity Generation Plant, Non-Nuclear | Sep. 30, 2013 Electricity Generation Plant, Non-Nuclear | Sep. 30, 2014 Fuel Fabrication and Blending Facilities | Sep. 30, 2013 Fuel Fabrication and Blending Facilities | Jun. 30, 2011 Environmental Agreements Units
Property, Plant, and Equipment, and Depreciation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Depreciation | $ 1,600,000,000 | $ 1,400,000,000 | $ 1,700,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Composite depreciation rate for completed plant | 3.42% | 3.12% | 3.78% | 2.90% | 2.86% | 2.71% | 4.37% | 3.47% | 5.65% | 1.44% | 1.30% | 1.35% | 3.23% | 3.21% | 3.67% | 2.76% | 2.76% | 2.99% | 8.40% | 8.14% | 8.10% |  |  |  |  |
Number of units to be idled |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 18
Accelerated depreciation | 206,000,000 | 49,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital leases | 102,000,000 | 47,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 102,000,000 | 42,000,000 | 0 | 5,000,000 |
Allowance for funds used during construction and nuclear fuel expenditures | 175,000,000 | 168,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
AFUDC minimum total project cost | 1,000,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum construction period. | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accumulated balance of qualifying projects | 3,300,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capitalized software amortization period | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unamortized computer software costs | 22,000,000 | 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization expense of capitalized computer software costs | $ 31,000,000 | $ 31,000,000 | $ 31,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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