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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, 2018	Sep. 30, 2017	Sep. 30, 2016
Property, Plant, and Equipment, and Depreciation
Loans and other long-term receivables, net (including current portion)	$ 123,000,000	$ 107,000,000
Depreciation	$ 1,300,000,000	$ 1,300,000,000	$ 1,400,000,000
Composite depreciation rate for completed plant	2.45%	2.49%	2.97%
Accelerated depreciation	$ 48,000,000	$ 104,000,000	$ 139,000,000
CGLP Capital lease liability	76,000,000
AFUDC minimum total project cost	1,000,000,000
Allowance for funds used during construction			235,000,000
Reacquired Rights	208,000,000	215,000,000
Amortization of Reacquired Rights	$ 8,000,000	4,000,000	1,000,000
Capitalized software amortization period	7 years
Unamortized computer software costs	$ 53,000,000	42,000,000
Amortization expense of capitalized computer software costs	$ 32,000,000	$ 26,000,000	$ 43,000,000
Nuclear
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	2.64%	2.66%	2.37%
Coal-fired
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	2.32%	2.33%	3.50%
Hydroelectric
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	1.57%	1.58%	1.29%
Gas and oil-fired
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	2.93%	3.27%	3.09%
Transmission
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	1.32%	1.34%	2.80%
Other
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	5.90%	6.12%	8.97%
Electricity Generation Plant, Non-Nuclear
Property, Plant, and Equipment, and Depreciation
Capital Leases, Net	$ 149,000,000	$ 161,000,000

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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 total gross amount less the charge for the use of the long-lived depreciable assets subject to a lease meeting the criteria for capitalization.

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Amount of expense for amortization of capitalized computer software costs.

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The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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Composite depreciation rate for public utility plants in service.

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