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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
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2014 Nuclear	Sep. 30, 2013 Nuclear	Sep. 30, 2012 Nuclear	Sep. 30, 2014 Coal-Fired	Sep. 30, 2013 Coal-Fired	Sep. 30, 2012 Coal-Fired	Sep. 30, 2014 Hydroelectric	Sep. 30, 2013 Hydroelectric	Sep. 30, 2012 Hydroelectric	Sep. 30, 2014 Gas and Oil-Fired	Sep. 30, 2013 Gas and Oil-Fired	Sep. 30, 2012 Gas and Oil-Fired	Sep. 30, 2014 Transmission	Sep. 30, 2013 Transmission	Sep. 30, 2012 Transmission	Sep. 30, 2014 Other	Sep. 30, 2013 Other	Sep. 30, 2012 Other	Sep. 30, 2014 Electricity Generation Plant, Non-Nuclear	Sep. 30, 2013 Electricity Generation Plant, Non-Nuclear	Sep. 30, 2014 Fuel Fabrication and Blending Facilities	Sep. 30, 2013 Fuel Fabrication and Blending Facilities	Jun. 30, 2011 Environmental Agreements Units
Property, Plant, and Equipment, and Depreciation
Depreciation	$ 1,600,000,000	$ 1,400,000,000	$ 1,700,000,000
Composite depreciation rate for completed plant	3.42%	3.12%	3.78%	2.90%	2.86%	2.71%	4.37%	3.47%	5.65%	1.44%	1.30%	1.35%	3.23%	3.21%	3.67%	2.76%	2.76%	2.99%	8.40%	8.14%	8.10%
Number of units to be idled																										18
Accelerated depreciation	206,000,000	49,000,000
Capital leases	102,000,000	47,000,000																				102,000,000	42,000,000	0	5,000,000
Allowance for funds used during construction and nuclear fuel expenditures	175,000,000	168,000,000
AFUDC minimum total project cost	1,000,000,000
Minimum construction period.	3 years
Accumulated balance of qualifying projects	3,300,000,000
Capitalized software amortization period	5 years
Unamortized computer software costs	22,000,000	5,000,000
Amortization expense of capitalized computer software costs	$ 31,000,000	$ 31,000,000	$ 31,000,000

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The accumulated balance of costs for qualifying projects used to calculate AFUDC.

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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.

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The amortization period.

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The minimum construction period necessary in order to capitalize allowance for funds used during construction.

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For each income statement presented, the amount charged to expense for amortization of capitalized computer software costs.

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The carrying amount of capitalized computer software costs net of accumulated amortization as of the balance sheet date.

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Amount before accumulated depreciation of leased physical assets used in the normal conduct of business to produce goods and services.

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Amount capitalized of allowance for funds used during construction.

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