Acquisitions and Dispositions - Southern Power Narrative (Details) $ in Millions			9 Months Ended
	Jan. 20, 2020 USD ($)	Jun. 13, 2019 USD ($) MW	Sep. 30, 2019 USD ($) MW	May 31, 2019 MW	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Construction work in progress			$ 7,804		$ 7,254
Property, plant, and equipment, subject to lien			117,591		116,914
SOUTHERN POWER CO
Business Acquisition [Line Items]
Construction work in progress			541		430
Property, plant, and equipment, subject to lien			14,397		$ 14,883
SOUTHERN POWER CO | Lien
Business Acquisition [Line Items]
Property, plant, and equipment, subject to lien			545
SOUTHERN POWER CO | Disposal Group, Held-for-sale, Not Discontinued Operations | Mankato expansion | Scenario, Forecast
Business Acquisition [Line Items]
Nacogdoches biomass-fueled facility	$ 650
Gain on termination fee	$ 25
SOUTHERN POWER CO | Disposal Group, Held-for-sale, Not Discontinued Operations | Nacogdoches Biomass-Fueled Facility
Business Acquisition [Line Items]
Approximate nameplate capacity (MW) | MW		115
Nacogdoches biomass-fueled facility		$ 461
After tax gain		$ 88
SOUTHERN POWER CO | Wind Generating Facility
Business Acquisition [Line Items]
Gain on sale			$ 17
SOUTHERN POWER CO | Mankato expansion
Business Acquisition [Line Items]
Approximate nameplate capacity (MW) | MW			385	385
SOUTHERN POWER CO | Series of Construction Projects
Business Acquisition [Line Items]
Construction work in progress			$ 337
SOUTHERN POWER CO | Series of Construction Projects | Minimum
Business Acquisition [Line Items]
Estimated future construction payments			405
SOUTHERN POWER CO | Series of Construction Projects | Maximum
Business Acquisition [Line Items]
Estimated future construction payments			$ 450