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Revenue Customer Type (Details) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020 USD ($) Units	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2020 USD ($) Units	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2004 USD ($)
Electric revenue									$ 10,104	$ 11,159	$ 11,075
Other revenue									145	159	158
Revenues	$ 2,899	$ 2,251	$ 2,521	$ 2,578	$ 3,239	$ 2,604	$ 2,750	$ 2,725	$ 10,249	11,318	11,233
Percent of sales of electricity to LPCs									93.00%
Bill credits for LTA									$ (163)	(14)	0
Number of LPCs signed LTA | Units	142								142
Percentage of total operating revenues									91.80%
Total number of LPCs | Units	153								153
Number of LPCs signed Flexibility Agreement | Units	64								64
Revenue Capitalized During Pre-Commercial Operations									$ 0	$ 0	(11)
MLGW's % of operating revenues									9.00%
NES's % of operating revenues									8.00%
MLGW prepayment												$ 1,500
20-year contract arrangement [Member]
Percentage of total operating revenues									74.80%
5-year contract arrangement [Member]
Number of LPCs signed LTA | Units	11								11
Percentage of total operating revenues									17.00%
MLGW
Deferred Revenue, Description									10 million	100 million		100 million
TENNESSEE
Electric revenue									$ 6,740	$ 7,419	7,350
VIRGINIA
Electric revenue									42	45	48
NORTH CAROLINA
Electric revenue									65	74	66
MISSISSIPPI
Electric revenue									941	1,063	1,052
KENTUCKY
Electric revenue									624	691	696
GEORGIA
Electric revenue									249	270	267
ALABAMA
Electric revenue									1,439	1,593	1,600
Federal agencies and other [Member]
Electric revenue									110	122	129
20-year contract arrangement [Member]
Electric revenue									7,666
5-year contract arrangement [Member]
Electric revenue									1,740
Local Power Company [Member]
Electric revenue									9,406	10,351	10,262
Industries Directly Served [Member]
Electric revenue									588	686	695
Capitalized revenue during pre-commercial plant operations [Member]
Electric revenue									$ 0	$ 0	$ 11

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