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Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Property, Plant, and Equipment, and Depreciation |  |  |
Depreciation | $ 1,600 | $ 1,800 | $ 1,300
Composite depreciation rate for completed plant | 2.74% | 3.09% | 2.45%
Accelerated depreciation |  | $ 566 | $ 48
Reacquired Rights | $ 192 | 200 |
Amortization of Reacquired Rights | $ 8 | 8 | 8
Capitalized software amortization period | seven years |  |
Unamortized computer software costs | $ 54 | 63 |
Amortization expense of capitalized computer software costs | $ 42 | $ 38 | $ 32
Nuclear |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 2.38% | 2.38% | 2.64%
Coal-fired |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 3.62% | 4.96% | 2.32%
Hydroelectric |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.60% | 1.61% | 1.57%
Gas and oil-fired |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 3.04% | 3.00% | 2.93%
Transmission |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 1.34% | 1.34% | 1.32%
Other |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |
Composite depreciation rate for completed plant | 7.26% | 7.16% | 5.90%

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- Definition Amount of expense for amortization of capitalized computer software costs. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 985 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 985 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128487-111756 + Details
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