v3.10.0.1
Derivatives - Pre-Tax Effects of Derivatives Not Designated as Hedging (Details) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Energy-related derivatives | Not Designated as Hedging Instrument          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative instruments not designated as hedging instruments, gain (loss), net     $ (126) $ (86) $ 38
Energy-related derivatives | Natural Gas Revenues | Not Designated as Hedging Instrument          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative instruments not designated as hedging instruments, gain (loss), net     (122) (80) 33
Energy-related derivatives | Cost of Natural Gas | Not Designated as Hedging Instrument          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative instruments not designated as hedging instruments, gain (loss), net     (6) (2) 3
Energy-related derivatives | Wholesale Electric Revenues | Not Designated as Hedging Instrument          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative instruments not designated as hedging instruments, gain (loss), net     2 (4) $ 2
Weather Derivatives          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative, gain (loss) on derivative, net $ 6   5 23  
Southern Company Gas          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative, gain (loss) on derivative, net 3   19 24  
Southern Company Gas | Energy-related derivatives | Not Designated as Hedging Instrument          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative instruments not designated as hedging instruments, gain (loss), net 36   (128) (82)  
Southern Company Gas | Energy-related derivatives | Natural Gas Revenues | Not Designated as Hedging Instrument          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative instruments not designated as hedging instruments, gain (loss), net 33   (122) (80)  
Southern Company Gas | Energy-related derivatives | Cost of Natural Gas | Not Designated as Hedging Instrument          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative instruments not designated as hedging instruments, gain (loss), net $ 3   $ (6) $ (2)  
Predecessor | Southern Company Gas          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative, gain (loss) on derivative, net   $ (162)      
Predecessor | Southern Company Gas | Energy-related derivatives | Not Designated as Hedging Instrument          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative instruments not designated as hedging instruments, gain (loss), net   (63)      
Predecessor | Southern Company Gas | Energy-related derivatives | Natural Gas Revenues | Not Designated as Hedging Instrument          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative instruments not designated as hedging instruments, gain (loss), net   (1)      
Predecessor | Southern Company Gas | Energy-related derivatives | Cost of Natural Gas | Not Designated as Hedging Instrument          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative instruments not designated as hedging instruments, gain (loss), net   (62)      
Predecessor | Southern Company Gas | Weather Derivatives          
Derivative Instruments, Gain (Loss) [Line Items]          
Derivative, gain (loss) on derivative, net   $ 3