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Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Property, Plant, and Equipment, and Depreciation |  |  |
Depreciation | $ 1,900 | $ 1,800 | $ 1,400
Composite depreciation rate for completed plant | 3.14% | 2.98% | 2.28%
Reacquired Rights | $ 324 | $ 178 |
Amortization of Reacquired Rights | $ 10 | 6 | $ 8
Capitalized software amortization period | 7 years |  |
Accelerated depreciation | $ 177 | 140 | $ 136
Accumulated Amortization of Reacquired Rights | 52 | $ 42 |
Amortization Reacquired Rights, Year 1 | 11 |  |
Amortization Reacquired Rights, Year 2 | 12 |  |
Amortization Reacquired Rights, Year 3 | 11 |  |
Amortization Reacquired Rights, Year 4 | 12 |  |
Amortization Reacquired Rights, Year 5 | 11 |  |
Amortization Reacquired Rights, Year Thereafter | 267 |  |
Reimbursements from DOE | 219 |  |
Accounts receivable from DOE | $ 11 |  |
Nuclear |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 2.73% | 2.72% | 2.38%
Coal-fired |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 4.98% | 4.27% | 1.95%
Hydroelectric |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.82% | 1.85% | 1.60%
Gas and oil-fired |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 3.17% | 3.38% | 2.98%
Transmission |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.52% | 1.51% | 1.34%
Other |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 4.43% | 3.64% | 7.12%

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