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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($) | 12 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Property, Plant, and Equipment, and Depreciation |  |  |
Loans and other long-term receivables, net (including current portion) | $ 120,000,000 | $ 123,000,000 |
Depreciation | 1,800,000,000 | $ 1,300,000,000 | $ 1,300,000,000
Composite depreciation rate for completed plant |  | 2.45% | 2.49%
Accelerated depreciation | 566,000,000 | $ 48,000,000 | $ 104,000,000
Reacquired Rights | 200,000,000 | 208,000,000 |
Amortization of Reacquired Rights | $ 8,000,000 | 8,000,000 | 4,000,000
Capitalized software amortization period | 7 years |  |
Unamortized computer software costs | $ 63,000,000 | 53,000,000 |
Amortization expense of capitalized computer software costs | $ 38,000,000 | $ 32,000,000 | $ 26,000,000
Nuclear |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 2.38% | 2.64% | 2.66%
Coal-fired |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 4.96% | 2.32% | 2.33%
Hydroelectric |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.61% | 1.57% | 1.58%
Gas and oil-fired |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 3.00% | 2.93% | 3.27%
Transmission |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.34% | 1.32% | 1.34%
Other |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 7.16% | 5.90% | 6.12%
Electricity Generation Plant, Non-Nuclear |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Capital Leases, Net | $ 146,000,000 | $ 149,000,000 |

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