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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, 2019	Sep. 30, 2018	Sep. 30, 2017
Property, Plant, and Equipment, and Depreciation
Loans and other long-term receivables, net (including current portion)	$ 120,000,000	$ 123,000,000
Depreciation	1,800,000,000	$ 1,300,000,000	$ 1,300,000,000
Composite depreciation rate for completed plant		2.45%	2.49%
Accelerated depreciation	566,000,000	$ 48,000,000	$ 104,000,000
Reacquired Rights	200,000,000	208,000,000
Amortization of Reacquired Rights	$ 8,000,000	8,000,000	4,000,000
Capitalized software amortization period	7 years
Unamortized computer software costs	$ 63,000,000	53,000,000
Amortization expense of capitalized computer software costs	$ 38,000,000	$ 32,000,000	$ 26,000,000
Nuclear
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	2.38%	2.64%	2.66%
Coal-fired
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	4.96%	2.32%	2.33%
Hydroelectric
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	1.61%	1.57%	1.58%
Gas and oil-fired
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	3.00%	2.93%	3.27%
Transmission
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	1.34%	1.32%	1.34%
Other
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	7.16%	5.90%	6.12%
Electricity Generation Plant, Non-Nuclear
Property, Plant, and Equipment, and Depreciation
Capital Leases, Net	$ 146,000,000	$ 149,000,000

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