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Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Sep. 30, 2016
Accounting Policies [Abstract] |
Property, Plant, and Equipment Depreciation Rates | Average depreciation rates by asset class are as follows:
Property, Plant, and Equipment Depreciation Rates At September 30 (percent)
| 2016 |  | 2015 |  | 2014
Asset Class |  |  |  |  |
Nuclear | 2.37 |  |  | 2.81 |  |  | 2.90 |
Coal-fired | 3.50 |  |  | 5.50 |  |  | 4.37 |
Hydroelectric | 1.29 |  |  | 1.30 |  |  | 1.44 |
Gas and oil-fired | 3.09 |  |  | 3.18 |  |  | 3.23 |
Transmission | 2.80 |  |  | 2.78 |  |  | 2.76 |
Other | 8.97 |  |  | 8.65 |  |  | 8.40 |

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