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Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2019
Property, Plant, and Equipment, and Depreciation |  |  |
Depreciation | $ 1,400 | $ 1,600 | $ 1,800
Composite depreciation rate for completed plant | 2.28% | 2.74% | 3.09%
Reacquired Rights | $ 184 | $ 192 |
Amortization of Reacquired Rights | $ 8 | 8 | $ 8
Capitalized software amortization period | 7 years |  |
Unamortized computer software costs | $ 27 | 54 |
Amortization expense of capitalized computer software costs | 38 | 42 | 38
Accelerated depreciation | 136 | $ 387 | $ 566
Increase (decrease) in Year 1 due to new depreciation study | $ 369 |  |
Nuclear |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 2.38% | 2.38% | 2.38%
Coal-fired |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.95% | 3.62% | 4.96%
Hydroelectric |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.60% | 1.60% | 1.61%
Gas and oil-fired |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 2.98% | 3.04% | 3.00%
Transmission |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.34% | 1.34% | 1.34%
Other |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 7.12% | 7.26% | 7.16%

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