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Revenue Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2004
Electric revenue |  |  |  |  |  |  |  |  | $ 11,159 | $ 11,075 | $ 10,586 |
Percentage of total operating revenues [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Other revenue |  |  |  |  |  |  |  |  | 159 | 158 | 153 |
Revenues | $ 3,239 | $ 2,604 | $ 2,750 | $ 2,725 | $ 3,185 | $ 2,707 | $ 2,792 | $ 2,549 | $ 11,318 | $ 11,233 | 10,739 |
MLGW's % of operating revenues |  |  |  |  |  |  |  |  | 9.00% |  |  |
NES's % of operating revenues |  |  |  |  |  |  |  |  | 8.00% |  |  |
MLGW prepayment |  |  |  |  |  |  |  |  |  |  |  | $ 1,500
MLGW |  |  |  |  |  |  |  |  |  |  |  |
Percentage of total operating revenues [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Deferred Revenue, Description |  |  |  |  |  |  |  |  | 10 | 100 |  | 100
Federal agencies and other [Member] |  |  |  |  |  |  |  |  |  |  |  |
Electric revenue |  |  |  |  |  |  |  |  | $ 122 | $ 129 | 132 |
20-year contract arrangement [Member] |  |  |  |  |  |  |  |  |  |  |  |
Electric revenue |  |  |  |  |  |  |  |  | 6,350 |  |  |
10-year contract arrangement [Member] |  |  |  |  |  |  |  |  |  |  |  |
Electric revenue |  |  |  |  |  |  |  |  | 938 |  |  |
5-year contract arrangement [Member] |  |  |  |  |  |  |  |  |  |  |  |
Electric revenue |  |  |  |  |  |  |  |  | 3,063 |  |  |
Local Power Company [Member] |  |  |  |  |  |  |  |  |  |  |  |
Electric revenue |  |  |  |  |  |  |  |  | 10,351 | 10,262 | 9,741 |
Industries Directly Served [Member] |  |  |  |  |  |  |  |  |  |  |  |
Electric revenue |  |  |  |  |  |  |  |  | 686 | 695 | 735 |
Capitalized revenue during pre-commercial plant operations [Member] |  |  |  |  |  |  |  |  |  |  |  |
Electric revenue |  |  |  |  |  |  |  |  | $ 0 | $ 11 | $ 22 |

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