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Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($) $ in Millions | 12 Months Ended | 57 Months Ended
--- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023
Property, Plant, and Equipment, and Depreciation |  |  |  |
Depreciation | $ 1,800 | $ 1,900 | $ 1,800 |
Composite depreciation rate for completed plant | 2.92% | 3.14% | 2.98% |
Reacquired Rights | $ 312 | $ 324 |  | $ 324
Amortization of Reacquired Rights | $ 11 | 10 | $ 6 |
Capitalized software amortization period | 7 years |  |  |
Accelerated depreciation |  | 177 | $ 140 | 659
Accumulated Amortization of Reacquired Rights | $ 63 | $ 52 |  | $ 52
Amortization Reacquired Rights, Year 1 | 12 |  |  |
Amortization Reacquired Rights, Year 2 | 11 |  |  |
Amortization Reacquired Rights, Year 3 | 12 |  |  |
Amortization Reacquired Rights, Year 4 | 11 |  |  |
Amortization Reacquired Rights, Year 5 | 11 |  |  |
Amortization Reacquired Rights, Year Thereafter | 255 |  |  |
Reimbursements from DOE | 284 |  |  |
Accounts receivable from DOE | $ 13 |  |  |
Nuclear |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 2.71% | 2.73% | 2.72% |
Coal-fired |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 4.13% | 4.98% | 4.27% |
Hydroelectric |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 1.83% | 1.82% | 1.85% |
Gas and oil-fired |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 3.31% | 3.17% | 3.38% |
Transmission |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 1.52% | 1.52% | 1.51% |
Other |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 4.98% | 4.43% | 3.64% |

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