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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, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Property, Plant, and Equipment, and Depreciation |  |  |
Loans and other long-term receivables, net (including current portion) | $ 123,000,000 | $ 107,000,000 |
Depreciation | $ 1,300,000,000 | $ 1,300,000,000 | $ 1,400,000,000
Composite depreciation rate for completed plant | 2.45% | 2.49% | 2.97%
Accelerated depreciation | $ 48,000,000 | $ 104,000,000 | $ 139,000,000
CGLP Capital lease liability | 76,000,000 |  |
AFUDC minimum total project cost | 1,000,000,000 |  |
Allowance for funds used during construction |  |  | 235,000,000
Reacquired Rights | 208,000,000 | 215,000,000 |
Amortization of Reacquired Rights | $ 8,000,000 | 4,000,000 | 1,000,000
Capitalized software amortization period | 7 years |  |
Unamortized computer software costs | $ 53,000,000 | 42,000,000 |
Amortization expense of capitalized computer software costs | $ 32,000,000 | $ 26,000,000 | $ 43,000,000
Nuclear |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 2.64% | 2.66% | 2.37%
Coal-fired |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 2.32% | 2.33% | 3.50%
Hydroelectric |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.57% | 1.58% | 1.29%
Gas and oil-fired |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 2.93% | 3.27% | 3.09%
Transmission |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.32% | 1.34% | 2.80%
Other |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 5.90% | 6.12% | 8.97%
Electricity Generation Plant, Non-Nuclear |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Capital Leases, Net | $ 149,000,000 | $ 161,000,000 |

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