ACQUISITIONS AND DISPOSITIONS - Southern Power Narrative (Details) $ in Millions		3 Months Ended
	Jan. 17, 2020 USD ($)	Mar. 31, 2020 USD ($) agreement facility MW	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	May 31, 2019 MW
Business Acquisition [Line Items]
Construction work in progress		$ 8,360		$ 7,880
Preliminary gain		$ 38	$ 2,503
SOUTHERN POWER CO
Business Acquisition [Line Items]
Approximate nameplate capacity (MW) | MW		300
Business acquisition, number of Power Purchase Agreements | agreement		2
Construction work in progress		$ 534		$ 515
Preliminary gain		39	$ 0
SOUTHERN POWER CO | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Mankato expansion
Business Acquisition [Line Items]
Approximate nameplate capacity (MW) | MW					385
Gain on sale		$ 39
Nacogdoches biomass-fueled facility	$ 663
Preliminary gain	39
After tax gain	$ 23
SOUTHERN POWER CO | Series of Business Acquisitions [Member]
Business Acquisition [Line Items]
Number of facilities under construction | facility		2
SOUTHERN POWER CO | Series of Construction Projects
Business Acquisition [Line Items]
Construction work in progress		$ 447
SOUTHERN POWER CO | Series of Construction Projects | Minimum
Business Acquisition [Line Items]
Estimated future construction payments		490
SOUTHERN POWER CO | Series of Construction Projects | Maximum
Business Acquisition [Line Items]
Estimated future construction payments		$ 535