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Income Taxes - Reconciliation of Federal Statutory Income Tax Rate (Details)
 6 Months Ended 12 Months Ended
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Reconciliation of federal statutory income tax rate to effective income tax rate          
Federal statutory rate     21.00% 35.00% 35.00%
State income tax, net of federal deduction     1.80% 12.50% 2.00%
Employee stock plans' dividend deduction     (1.00%) (4.00%) (1.20%)
Non-deductible book depreciation     0.80% 3.10% 0.90%
Flowback of excess deferred income taxes     (4.00%) (0.30%) (0.10%)
AFUDC-Equity     (1.00%) (2.60%) (2.00%)
ITC basis difference     (0.60%) (1.70%) (5.00%)
AFUDC-Equity portion of Kemper IGCC charge       15.70%  
Federal PTCs     (4.70%) (12.10%) (1.20%)
Amortization of ITC     (2.00%) (4.20%) (0.90%)
Tax impact on sale of subsidiaries     8.60%    
Tax Reform Legislation     (1.40%) (25.60%)  
Noncontrolling interests     (0.40%) (1.40%) (0.30%)
Other     (0.80%) (1.10%) 0.10%
Effective income tax (benefit) rate     16.30% 13.30% 27.30%
Alabama Power          
Reconciliation of federal statutory income tax rate to effective income tax rate          
Federal statutory rate     21.00% 35.00% 35.00%
State income tax, net of federal deduction     5.00% 4.50% 4.20%
Employee stock plans' dividend deduction     (0.00%) (0.00%) (0.00%)
Non-deductible book depreciation     0.60% 0.90% 1.00%
Flowback of excess deferred income taxes     (1.80%) 0.00% 0.00%
AFUDC-Equity     (1.00%) (1.00%) (0.70%)
ITC basis difference     (0.00%) (0.00%) (0.00%)
AFUDC-Equity portion of Kemper IGCC charge       (0.00%)  
Federal PTCs     (0.00%) (0.00%) (0.00%)
Amortization of ITC     (0.10%) (0.20%) (0.20%)
Tax impact on sale of subsidiaries     0.00%    
Tax Reform Legislation     0.00% 0.30%  
Noncontrolling interests     (0.00%) (0.00%) (0.00%)
Other     (0.10%) 0.10% (0.50%)
Effective income tax (benefit) rate     23.60% 39.60% 38.80%
Georgia Power          
Reconciliation of federal statutory income tax rate to effective income tax rate          
Federal statutory rate     21.00% 35.00% 35.00%
State income tax, net of federal deduction     5.50% 2.00% 2.10%
Employee stock plans' dividend deduction     (0.00%) (0.00%) (0.00%)
Non-deductible book depreciation     1.20% 0.70% 0.80%
Flowback of excess deferred income taxes     0.00% (0.10%) (0.10%)
AFUDC-Equity     (1.40%) (0.60%) (0.80%)
ITC basis difference     (0.00%) (0.00%) (0.00%)
AFUDC-Equity portion of Kemper IGCC charge       (0.00%)  
Federal PTCs     (0.00%) (0.00%) (0.00%)
Amortization of ITC     (0.20%) (0.10%) (0.20%)
Tax impact on sale of subsidiaries     0.00%    
Tax Reform Legislation     (4.90%) (0.40%)  
Noncontrolling interests     (0.00%) (0.00%) (0.00%)
Other     0.10% 0.20% (0.10%)
Effective income tax (benefit) rate     21.30% 36.70% 36.70%
Mississippi Power          
Reconciliation of federal statutory income tax rate to effective income tax rate          
Federal statutory rate     21.00% 35.00% 35.00%
State income tax, net of federal deduction     (65.10%) (0.60%) 5.70%
Employee stock plans' dividend deduction     (0.00%) (0.00%) (0.00%)
Non-deductible book depreciation     0.70% (0.10%) (0.70%)
Flowback of excess deferred income taxes     (4.10%) 0.00% 0.30%
AFUDC-Equity     (0.00%) (0.00%) 28.50%
ITC basis difference     (0.00%) (0.00%) (0.00%)
AFUDC-Equity portion of Kemper IGCC charge       (5.30%)  
Federal PTCs     (0.00%) (0.00%) (0.00%)
Amortization of ITC     (0.20%) (0.00%) 0.10%
Tax impact on sale of subsidiaries     0.00%    
Tax Reform Legislation     (26.30%) (11.90%)  
Noncontrolling interests     (0.00%) (0.00%) (0.00%)
Other     (1.40%) 0.00% (0.40%)
Effective income tax (benefit) rate     (75.40%) 17.10% 68.50%
Southern Power          
Reconciliation of federal statutory income tax rate to effective income tax rate          
Federal statutory rate     21.00% 35.00% 35.00%
State income tax, net of federal deduction     (90.80%) (22.20%) (9.10%)
Employee stock plans' dividend deduction     (0.00%) (0.00%) (0.00%)
Non-deductible book depreciation     0.00% 0.00% 0.00%
Flowback of excess deferred income taxes     0.00% 0.00% 0.00%
AFUDC-Equity     (0.00%) (0.00%) (0.00%)
ITC basis difference     (0.20%) (10.00%) (96.30%)
AFUDC-Equity portion of Kemper IGCC charge       (0.00%)  
Federal PTCs     (156.60%) (72.50%) (23.30%)
Amortization of ITC     (55.40%) (20.60%) (13.40%)
Tax impact on sale of subsidiaries     0.00%    
Tax Reform Legislation     96.10% (416.10%)  
Noncontrolling interests     (14.90%) (8.60%) (6.20%)
Other     2.00% (10.70%) 4.70%
Effective income tax (benefit) rate     (198.80%) (525.70%) (108.60%)
Southern Company Gas          
Reconciliation of federal statutory income tax rate to effective income tax rate          
Federal statutory rate 35.00%   21.00% 35.00%  
State income tax, net of federal deduction 3.60%   9.20% 10.00%  
Flowback of excess deferred income taxes 0.00%   (3.00%) (0.20%)  
Amortization of ITC (0.40%)   (0.10%) (0.20%)  
Tax impact on sale of subsidiaries 0.00%   28.50% 0.00%  
Tax Reform Legislation 0.00%   (0.40%) 15.00%  
Other 1.80%   0.30% 0.60%  
Effective income tax (benefit) rate 40.00%   55.50% 60.20%  
Predecessor | Southern Company Gas          
Reconciliation of federal statutory income tax rate to effective income tax rate          
Federal statutory rate   35.00%      
State income tax, net of federal deduction   3.50%      
Flowback of excess deferred income taxes   0.00%      
Amortization of ITC   (0.00%)      
Tax impact on sale of subsidiaries   0.00%      
Tax Reform Legislation   0.00%      
Other   (0.90%)      
Effective income tax (benefit) rate   37.60%