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ACQUISITIONS AND DISPOSITIONS - Southern Power Narrative (Details) $ in Millions		9 Months Ended
	Jan. 17, 2020 USD ($)	Sep. 30, 2020 USD ($) agreement	Sep. 30, 2019 USD ($)	Jun. 30, 2020 MW	Dec. 31, 2019 USD ($)	May 31, 2019 MW
Business Acquisition [Line Items]
Construction work in progress		$ 8,861			$ 7,880
Preliminary gain on sale		$ 36	$ 2,517
SOUTHERN POWER CO
Business Acquisition [Line Items]
Approximate nameplate capacity (in MWs) \| MW				300
Business acquisition, number of Power Purchase Agreements \| agreement		2
Construction work in progress		$ 335			$ 515
Preliminary gain on sale		39	$ 24
SOUTHERN POWER CO \| Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Mankato expansion
Business Acquisition [Line Items]
Approximate nameplate capacity (in MWs) \| MW						385
Nacogdoches biomass-fueled facility	$ 663
Preliminary gain on sale	39
Preliminary gain on sale, net of tax	$ 23
SOUTHERN POWER CO \| Series of Construction Projects
Business Acquisition [Line Items]
Construction work in progress		244
SOUTHERN POWER CO \| Series of Construction Projects \| Minimum
Business Acquisition [Line Items]
Estimated future construction payments		475
SOUTHERN POWER CO \| Series of Construction Projects \| Maximum
Business Acquisition [Line Items]
Estimated future construction payments		$ 545