Acquisitions and Dispositions - Southern Power Narrative (Details) $ in Millions	1 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($) MW	Sep. 30, 2021 USD ($) MW	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Business Acquisition [Line Items]
Construction work in progress	$ 9,611	$ 9,611		$ 8,697
Gain on sale		171	$ 36
SOUTHERN POWER CO
Business Acquisition [Line Items]
Construction work in progress	$ 274	274		$ 127
Gain on sale		39	$ 39
SOUTHERN POWER CO | Wind Generating Facility
Business Acquisition [Line Items]
Gain on sale		$ 37
SOUTHERN POWER CO | Garland Solar Storage
Business Acquisition [Line Items]
Nameplate capacity placed in service (in MWs) | MW	45	45
SOUTHERN POWER CO | Series of Construction Projects
Business Acquisition [Line Items]
Construction costs incurred to date	$ 341	$ 341
Construction work in progress	228	228
SOUTHERN POWER CO | Series of Construction Projects | Minimum
Business Acquisition [Line Items]
Estimated future construction payments	400	400
SOUTHERN POWER CO | Series of Construction Projects | Maximum
Business Acquisition [Line Items]
Estimated future construction payments	$ 460	$ 460