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Related Parties Related Parties (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Related Parties
Remaining appropriation investment	$ 258								$ 258
Current borrowing capacity	150								150
Revenue from sales of electricity	3,239	$ 2,604	$ 2,750	$ 2,725	$ 3,185	$ 2,707	$ 2,792	$ 2,549	11,318	$ 11,233	$ 10,739
Other income										0	0
Return on power program appropriation investment									(6)	(5)	(5)
Related Party Transactions
Related Parties
Revenue from sales of electricity									118	122	126
Other income									258	240	136
Operating expenses									222	220	216
Additions to property, plant, and equipment									10	8	16
Cash and cash equivalents	45				46				45	46	46
Accounts receivable, net	76				60				76	60	84
Receivables, Long-term Contracts or Programs	53				46				53	46	35
Accounts payable and accrued liabilities	69				69				69	69	71
Long-term power bonds, net	$ 0				$ 0				0	0	1
Return on power program appropriation investment									$ (6)	$ (5)	$ (5)