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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																									12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2013 Nuclear	Sep. 30, 2012 Nuclear	Sep. 30, 2011 Nuclear	Sep. 30, 2013 Coal-Fired	Sep. 30, 2012 Coal-Fired	Sep. 30, 2011 Coal-Fired	Sep. 30, 2013 Hydroelectric	Sep. 30, 2012 Hydroelectric	Sep. 30, 2011 Hydroelectric	Sep. 30, 2013 Gas and Oil-Fired	Sep. 30, 2012 Gas and Oil-Fired	Sep. 30, 2011 Gas and Oil-Fired	Sep. 30, 2013 Transmission	Sep. 30, 2012 Transmission	Sep. 30, 2011 Transmission	Sep. 30, 2013 Other	Sep. 30, 2012 Other	Sep. 30, 2011 Other	Sep. 30, 2013 Electricity Generation Plant, Non-Nuclear	Sep. 30, 2012 Electricity Generation Plant, Non-Nuclear	Sep. 30, 2013 Fuel Fabrication and Blending Facilities	Sep. 30, 2012 Fuel Fabrication and Blending Facilities	Sep. 30, 2013 Regulatory assets [Member]	Jun. 30, 2011 Environmental Agreements Units
Property, Plant, and Equipment, and Depreciation
Depreciation	$ 1,400,000,000	$ 1,700,000,000	$ 1,400,000,000
Composite depreciation rate for completed plant	3.12%	3.78%	3.21%	2.86%	2.71%	2.58%	3.47%	5.65%	3.80%	1.30%	1.35%	1.43%	3.21%	3.67%	3.70%	2.76%	2.99%	3.39%	8.14%	8.10%	7.39%
Number of units to be idled																											18
Accelerated depreciation	49,000,000	308,000,000
Capital leases	47,000,000	35,000,000																				42,000,000	24,000,000	5,000,000	11,000,000
Allowance for funds used during construction and nuclear fuel expenditures	168,000,000	171,000,000	126,000,000																							23,000,000
AFUDC minimum total project cost	1,000,000,000
Minimum construction period.	3 years
Accumulated balance of qualifying projects	3,100,000,000
Capitalized software amortization period	5 years
Unamortized computer software costs	5,000,000	26,000,000
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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For each income statement presented, the amount charged to expense for amortization of capitalized computer software costs.

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Amount capitalized of allowance for funds used during construction.

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