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Introduction - Goodwill (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Goodwill [Line Items]    
Goodwill $ 6,226 $ 6,268
SOUTHERN POWER CO    
Goodwill [Line Items]    
Goodwill 2 2
SOUTHERN Co GAS    
Goodwill [Line Items]    
Goodwill 5,925 5,967
SOUTHERN Co GAS | Gas distribution operations    
Goodwill [Line Items]    
Goodwill 4,702 4,702
SOUTHERN Co GAS | Gas marketing services    
Goodwill [Line Items]    
Goodwill $ 1,223 $ 1,265