v3.10.0.1
Summary of Significant Accounting Policies - Schedule of Total AFUDC and Interest Capitalized (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
Capitalized Contract Cost [Line Items]          
Total AFUDC and interest capitalized     $ 210 $ 249 $ 327
Alabama Power          
Capitalized Contract Cost [Line Items]          
Total AFUDC and interest capitalized     84 54 39
Georgia Power          
Capitalized Contract Cost [Line Items]          
Total AFUDC and interest capitalized     94 63 68
Mississippi Power          
Capitalized Contract Cost [Line Items]          
Total AFUDC and interest capitalized     0 72 124
Southern Power          
Capitalized Contract Cost [Line Items]          
Total AFUDC and interest capitalized     17 11 $ 44
Southern Company Gas          
Capitalized Contract Cost [Line Items]          
Total AFUDC and interest capitalized $ 6   $ 14 $ 19  
Southern Company Gas | Predecessor          
Capitalized Contract Cost [Line Items]          
Total AFUDC and interest capitalized   $ 4