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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Sep. 30, 2015 USD ($) | Sep. 30, 2014 USD ($) | Sep. 30, 2013 USD ($) | Jun. 30, 2011 Units
Property, Plant, and Equipment, and Depreciation |  |  |  |
Depreciation | $ 1,700 | $ 1,600 | $ 1,400 |
Composite depreciation rate for completed plant | 3.71% | 3.42% | 3.12% |
Accelerated depreciation | $ 383 | $ 206 | $ 49 |
Capital leases | 94 | 102 |  |
Allowance for funds used during construction | 214 | 175 |  |
AFUDC minimum total project cost | $ 1,000 |  |  |
Minimum construction period | 3 years |  |  |
Accumulated balance of qualifying projects | $ 4,100 |  |  |
Capitalized software amortization period | 5 years |  |  |
Unamortized computer software costs | $ 18 | 22 |  |
Amortization expense of capitalized computer software costs | 38 | 31 | $ 31 |
Electricity Generation Plant, Non-Nuclear [Member] |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Capital leases | $ 94 | $ 102 |  |
Nuclear |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 2.81% | 2.90% | 2.86% |
Coal-fired |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 5.50% | 4.37% | 3.47% |
Hydroelectric |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 1.30% | 1.44% | 1.30% |
Gas and oil-fired |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 3.18% | 3.23% | 3.21% |
Transmission |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 2.78% | 2.76% | 2.76% |
Other |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 8.65% | 8.40% | 8.14% |
Environmental Agreements |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Number of units to be idled | Units |  |  |  | 18

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- Definition Interest on funds invested in capital projects has been capitalized only for projects with (1) an expected total project cost of $1.0 billion or more, and (2) an estimated construction period of at least three years in duration. + References No definition available. + Details
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- Definition Amount before accumulated depreciation of leased physical assets used in the normal conduct of business to produce goods and services. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6455314&loc=d3e45014-112735 + Details
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- Definition For each income statement presented, the amount charged to expense for amortization of capitalized computer software costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 985 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756 + Details
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