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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, 2016 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2014 USD ($) | Jun. 30, 2011 Units
Property, Plant, and Equipment, and Depreciation |  |  |  |
Depreciation | $ 1,400 | $ 1,700 | $ 1,600 |
Composite depreciation rate for completed plant | 2.97% | 3.71% | 3.42% |
Change in Depreciation Due to License Change | $ 79 |  |  |
Accelerated depreciation | 139 | $ 383 | $ 206 |
CGLP Capital lease liability | 76 |  |  |
Allowance for funds used during construction | 235 | 214 |  |
AFUDC minimum total project cost | $ 1,000 |  |  |
Minimum construction period | 3 years |  |  |
Accumulated balance of qualifying projects | $ 4,500 |  |  |
Capitalized software amortization period | 5 years |  |  |
Unamortized computer software costs | $ 27 | 18 |  |
Amortization expense of capitalized computer software costs | 43 | 38 | $ 31 |
Electricity Generation Plant, Non-Nuclear |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Capital Leases, Net | $ 163 | $ 94 |  |
Nuclear |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 2.37% | 2.81% | 2.90% |
Coal-fired |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 3.50% | 5.50% | 4.37% |
Hydroelectric |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 1.29% | 1.30% | 1.44% |
Gas and oil-fired |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 3.09% | 3.18% | 3.23% |
Transmission |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 2.80% | 2.78% | 2.76% |
Other |  |  |  |
Property, Plant, and Equipment, and Depreciation |  |  |  |
Composite depreciation rate for completed plant | 8.97% | 8.65% | 8.40% |
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 The total gross amount less the charge for the use of the long-lived depreciable assets subject to a lease meeting the criteria for capitalization. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737 + Details
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