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Revenue Revenue (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	Sep. 30, 2004
Electric revenue									$ 11,159	$ 11,075	$ 10,586
Percentage of total operating revenues [Abstract]
Other revenue									159	158	153
Revenues	$ 3,239	$ 2,604	$ 2,750	$ 2,725	$ 3,185	$ 2,707	$ 2,792	$ 2,549	$ 11,318	$ 11,233	10,739
MLGW's % of operating revenues									9.00%
NES's % of operating revenues									8.00%
MLGW prepayment												$ 1,500
MLGW
Percentage of total operating revenues [Abstract]
Deferred Revenue, Description									10	100		100
Federal agencies and other [Member]
Electric revenue									$ 122	$ 129	132
20-year contract arrangement [Member]
Electric revenue									6,350
10-year contract arrangement [Member]
Electric revenue									938
5-year contract arrangement [Member]
Electric revenue									3,063
Local Power Company [Member]
Electric revenue									10,351	10,262	9,741
Industries Directly Served [Member]
Electric revenue									686	695	735
Capitalized revenue during pre-commercial plant operations [Member]
Electric revenue									$ 0	$ 11	$ 22

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- Definition

Revenue from sales of electricity

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MLGW's % of operating revenues

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NES's % of operating revenues

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Other revenue

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Percentage of total operating revenues [Abstract]

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Description of the transaction or nature of business giving rise to deferred revenue or unearned revenue, other than annual membership fees. Deferred revenue is a liability related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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