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Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Property, Plant, and Equipment, and Depreciation |  |  |
Depreciation | $ 1,900 | $ 1,800 | $ 1,900
Composite depreciation rate for completed plant | 3.04% | 2.92% | 3.14%
Reacquired Rights | $ 301 | $ 312 |
Amortization of Reacquired Rights | $ 11 | 11 | $ 10
Capitalized software amortization period | 7 years |  |
Accumulated Amortization of Reacquired Rights | $ 74 | $ 63 |
Amortization Reacquired Rights, Year 1 | 11 |  |
Amortization Reacquired Rights, Year 2 | 12 |  |
Amortization Reacquired Rights, Year 3 | 11 |  |
Amortization Reacquired Rights, Year 4 | 11 |  |
Amortization Reacquired Rights, Year 5 | 12 |  |
Amortization Reacquired Rights, Year Thereafter | 244 |  |
Reimbursements from DOE | 334 |  |
Accounts receivable from DOE | $ 6 |  |
Nuclear |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 2.71% | 2.71% | 2.73%
Coal-fired |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 4.82% | 4.13% | 4.98%
Hydroelectric |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.82% | 1.83% | 1.82%
Gas and oil-fired |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 3.16% | 3.31% | 3.17%
Transmission |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.55% | 1.52% | 1.52%
Other |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 5.22% | 4.98% | 4.43%

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