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Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Sep. 30, 2018
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)
| 2018 |  | 2017 |  | 2016
Asset Class |  |  |  |  |
Nuclear | 2.64 |  |  | 2.66 |  |  | 2.37 |
Coal-fired | 2.32 |  |  | 2.33 |  |  | 3.50 |
Hydroelectric | 1.57 |  |  | 1.58 |  |  | 1.29 |
Gas and oil-fired | 2.93 |  |  | 3.27 |  |  | 3.09 |
Transmission | 1.32 |  |  | 1.34 |  |  | 2.80 |
Other | 5.90 |  |  | 6.12 |  |  | 8.97 |

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