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Retirement Benefits - Components of Net Periodic Benefit Cost and Estimated Future Benefit Payments (Details) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     100.00%    
Actual plan asset allocations (as percent)       100.00%  
Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     100.00% 100.00%  
Actual plan asset allocations (as percent) 100.00%   100.00% 100.00%  
Components of net periodic          
Service cost     $ 22 $ 23 $ 21
Interest cost     76 78 79
Expected return on plan assets     (60) (58) (59)
Net amortization     21 21 6
Net periodic benefit cost     59 $ 64 47
Benefit Payments          
Benefit Payments, 2017 $ 145   145    
Benefit Payments, 2018 150   150    
Benefit Payments, 2019 155   155    
Benefit Payments, 2020 159   159    
Benefit Payments, 2021 162   162    
Benefit Payments, 2022 to 2026 823   823    
Subsidy Receipts          
Subsidy Receipts, 2017 (10)   (10)    
Subsidy Receipts, 2018 (11)   (11)    
Subsidy Receipts, 2019 (12)   (12)    
Subsidy Receipts, 2020 (13)   (13)    
Subsidy Receipts, 2021 (14)   (14)    
Subsidy Receipts, 2022 to 2026 (73)   (73)    
Benefit Payments and Subsidy Receipts, Total          
Benefit Payments and Subsidy Receipts, 2017 135   135    
Benefit Payments and Subsidy Receipts, 2018 139   139    
Benefit Payments and Subsidy Receipts, 2019 143   143    
Benefit Payments and Subsidy Receipts, 2020 146   146    
Benefit Payments and Subsidy Receipts, 2021 148   148    
Benefit Payments and Subsidy Receipts, 2022 to 2026 750   750    
Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)       100.00%  
Actual plan asset allocations (as percent)       100.00%  
Components of net periodic          
Service cost     262 $ 257 213
Interest cost     422 445 435
Expected return on plan assets     (782) (724) (645)
Amortization prior service costs     12    
Recognized net (gain) loss     150 215 110
Net amortization     14 25 26
Net periodic benefit cost     66 $ 218 139
Benefit Payments          
Benefit Payments, 2017 571   571    
Benefit Payments, 2018 593   593    
Benefit Payments, 2019 620   620    
Benefit Payments, 2020 646   646    
Benefit Payments, 2021 666   666    
Benefit Payments, 2022 to 2026 $ 3,673   $ 3,673    
Alabama Power [Member]          
Components of net periodic          
Amortization of regulatory assets         123
Alabama Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     100.00%    
Actual plan asset allocations (as percent) 100.00%   100.00% 100.00%  
Components of net periodic          
Service cost     $ 5 $ 6 5
Interest cost     18 20 20
Expected return on plan assets     (25) (26) (25)
Net amortization     6 5 4
Net periodic benefit cost     4 $ 5 4
Benefit Payments          
Benefit Payments, 2017 $ 32   32    
Benefit Payments, 2018 33   33    
Benefit Payments, 2019 34   34    
Benefit Payments, 2020 35   35    
Benefit Payments, 2021 36   36    
Benefit Payments, 2022 to 2026 183   183    
Subsidy Receipts          
Subsidy Receipts, 2017 (3)   (3)    
Subsidy Receipts, 2018 (3)   (3)    
Subsidy Receipts, 2019 (4)   (4)    
Subsidy Receipts, 2020 (4)   (4)    
Subsidy Receipts, 2021 (4)   (4)    
Subsidy Receipts, 2022 to 2026 (22)   (22)    
Benefit Payments and Subsidy Receipts, Total          
Benefit Payments and Subsidy Receipts, 2017 29   29    
Benefit Payments and Subsidy Receipts, 2018 30   30    
Benefit Payments and Subsidy Receipts, 2019 30   30    
Benefit Payments and Subsidy Receipts, 2020 31   31    
Benefit Payments and Subsidy Receipts, 2021 32   32    
Benefit Payments and Subsidy Receipts, 2022 to 2026 $ 161   $ 161    
Alabama Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     100.00%    
Actual plan asset allocations (as percent) 100.00%   100.00% 100.00%  
Components of net periodic          
Service cost     $ 57 $ 59 48
Interest cost     95 106 103
Expected return on plan assets     (184) (178) (168)
Recognized net (gain) loss     40 55 31
Net amortization     3 6 7
Net periodic benefit cost     11 $ 48 21
Benefit Payments          
Benefit Payments, 2017 $ 122   122    
Benefit Payments, 2018 127   127    
Benefit Payments, 2019 132   132    
Benefit Payments, 2020 137   137    
Benefit Payments, 2021 142   142    
Benefit Payments, 2022 to 2026 $ 777   $ 777    
Georgia Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     100.00%    
Actual plan asset allocations (as percent) 100.00%   100.00% 100.00%  
Components of net periodic          
Service cost     $ 6 $ 7 6
Interest cost     30 34 34
Expected return on plan assets     (22) (24) (25)
Amortization prior service costs     1    
Net amortization     10 11 2
Net periodic benefit cost     24 $ 28 17
Benefit Payments          
Benefit Payments, 2017 $ 54   54    
Benefit Payments, 2018 56   56    
Benefit Payments, 2019 58   58    
Benefit Payments, 2020 59   59    
Benefit Payments, 2021 60   60    
Benefit Payments, 2022 to 2026 303   303    
Subsidy Receipts          
Subsidy Receipts, 2017 (4)   (4)    
Subsidy Receipts, 2018 (5)   (5)    
Subsidy Receipts, 2019 (5)   (5)    
Subsidy Receipts, 2020 (5)   (5)    
Subsidy Receipts, 2021 (6)   (6)    
Subsidy Receipts, 2022 to 2026 (32)   (32)    
Benefit Payments and Subsidy Receipts, Total          
Benefit Payments and Subsidy Receipts, 2017 50   50    
Benefit Payments and Subsidy Receipts, 2018 51   51    
Benefit Payments and Subsidy Receipts, 2019 53   53    
Benefit Payments and Subsidy Receipts, 2020 54   54    
Benefit Payments and Subsidy Receipts, 2021 54   54    
Benefit Payments and Subsidy Receipts, 2022 to 2026 $ 271   $ 271    
Georgia Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     100.00%    
Actual plan asset allocations (as percent) 100.00%   100.00% 100.00%  
Components of net periodic          
Service cost     $ 70 $ 73 62
Interest cost     136 154 153
Expected return on plan assets     (258) (251) (228)
Amortization prior service costs     3    
Recognized net (gain) loss     55 76 41
Net amortization     5 9 10
Net periodic benefit cost     8 61 38
Benefit Payments          
Benefit Payments, 2017 $ 184   184    
Benefit Payments, 2018 190   190    
Benefit Payments, 2019 196   196    
Benefit Payments, 2020 202   202    
Benefit Payments, 2021 206   206    
Benefit Payments, 2022 to 2026 $ 1,126   1,126    
Gulf Power [Member]          
Components of net periodic          
Service cost     12 12 10
Interest cost     19 20 19
Expected return on plan assets     (34) (32) (28)
Recognized net (gain) loss     6 9 5
Net amortization     1 1 1
Net periodic benefit cost     $ 4 $ 10 7
Gulf Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     100.00%    
Actual plan asset allocations (as percent) 100.00%   100.00% 100.00%  
Components of net periodic          
Service cost     $ 1 $ 1 1
Interest cost     3 3 3
Expected return on plan assets     (1) (1) (1)
Net periodic benefit cost     3 $ 3 3
Benefit Payments          
Benefit Payments, 2017 $ 5   5    
Benefit Payments, 2018 5   5    
Benefit Payments, 2019 6   6    
Benefit Payments, 2020 6   6    
Benefit Payments, 2021 6   6    
Benefit Payments, 2022 to 2026 30   30    
Subsidy Receipts          
Subsidy Receipts, 2017 0   0    
Subsidy Receipts, 2018 0   0    
Subsidy Receipts, 2019 (1)   (1)    
Subsidy Receipts, 2020 (1)   (1)    
Subsidy Receipts, 2021 (1)   (1)    
Subsidy Receipts, 2022 to 2026 (3)   (3)    
Benefit Payments and Subsidy Receipts, Total          
Benefit Payments and Subsidy Receipts, 2017 5   5    
Benefit Payments and Subsidy Receipts, 2018 5   5    
Benefit Payments and Subsidy Receipts, 2019 5   5    
Benefit Payments and Subsidy Receipts, 2020 5   5    
Benefit Payments and Subsidy Receipts, 2021 5   5    
Benefit Payments and Subsidy Receipts, 2022 to 2026 $ 27   $ 27    
Gulf Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     100.00%    
Actual plan asset allocations (as percent) 100.00%   100.00% 100.00%  
Components of net periodic          
Service cost     $ 12 $ 12  
Interest cost     19 20  
Amortization prior service costs     1    
Benefit Payments          
Benefit Payments, 2017 $ 20   20    
Benefit Payments, 2018 22   22    
Benefit Payments, 2019 23   23    
Benefit Payments, 2020 24   24    
Benefit Payments, 2021 26   26    
Benefit Payments, 2022 to 2026 $ 149   149    
Mississippi Power [Member]          
Components of net periodic          
Service cost     13 13 10
Interest cost     19 21 20
Expected return on plan assets     (35) (33) (29)
Recognized net (gain) loss     7 10 5
Net amortization     1 1 1
Net periodic benefit cost     $ 5 $ 12 7
Mississippi Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     100.00%    
Actual plan asset allocations (as percent) 100.00%   100.00% 100.00%  
Components of net periodic          
Service cost     $ 1 $ 1 1
Interest cost     3 4 4
Expected return on plan assets     (1) (2) (2)
Net amortization     1 1 0
Net periodic benefit cost     4 $ 4 3
Benefit Payments          
Benefit Payments, 2017 $ 6   6    
Benefit Payments, 2018 6   6    
Benefit Payments, 2019 7   7    
Benefit Payments, 2020 7   7    
Benefit Payments, 2021 7   7    
Benefit Payments, 2022 to 2026 36   36    
Subsidy Receipts          
Subsidy Receipts, 2017 (1)   (1)    
Subsidy Receipts, 2018 (1)   (1)    
Subsidy Receipts, 2019 (1)   (1)    
Subsidy Receipts, 2020 (1)   (1)    
Subsidy Receipts, 2021 (1)   (1)    
Subsidy Receipts, 2022 to 2026 (1)   (1)    
Benefit Payments and Subsidy Receipts, Total          
Benefit Payments and Subsidy Receipts, 2017 5   5    
Benefit Payments and Subsidy Receipts, 2018 5   5    
Benefit Payments and Subsidy Receipts, 2019 6   6    
Benefit Payments and Subsidy Receipts, 2020 6   6    
Benefit Payments and Subsidy Receipts, 2021 6   6    
Benefit Payments and Subsidy Receipts, 2022 to 2026 $ 35   $ 35    
Mississippi Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     100.00%    
Actual plan asset allocations (as percent) 100.00%   100.00% 100.00%  
Components of net periodic          
Service cost     $ 13 $ 13  
Interest cost     19 $ 21  
Amortization prior service costs     1    
Benefit Payments          
Benefit Payments, 2017 $ 22   22    
Benefit Payments, 2018 23   23    
Benefit Payments, 2019 24   24    
Benefit Payments, 2020 26   26    
Benefit Payments, 2021 27   27    
Benefit Payments, 2022 to 2026 154   154    
Successor [Member] | Southern Company Gas [Member] | Other Postretirement Benefit Plan [Member]          
Components of net periodic          
Service cost 1        
Interest cost 5        
Expected return on plan assets (3)        
Amortization of regulatory assets 2        
Amortization prior service costs 0        
Amortization net gain (loss) 0        
Net periodic benefit cost 5        
Benefit Payments          
Benefit Payments, 2017 20   20    
Benefit Payments, 2018 20   20    
Benefit Payments, 2019 21   21    
Benefit Payments, 2020 22   22    
Benefit Payments, 2021 22   22    
Benefit Payments, 2022 to 2026 111   111    
Successor [Member] | Southern Company Gas [Member] | Pension plans          
Components of net periodic          
Service cost 15        
Interest cost 20        
Expected return on plan assets (35)        
Amortization of regulatory assets 13        
Amortization prior service costs 0        
Amortization net gain (loss) 0        
Net periodic benefit cost 13        
Benefit Payments          
Benefit Payments, 2017 71   71    
Benefit Payments, 2018 72   72    
Benefit Payments, 2019 73   73    
Benefit Payments, 2020 74   74    
Benefit Payments, 2021 74   74    
Benefit Payments, 2022 to 2026 $ 363   $ 363    
Predecessor [Member] | Southern Company Gas [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Actual plan asset allocations (as percent)       100.00%  
Components of net periodic          
Service cost   $ 1   $ 2 2
Interest cost   5   13 15
Expected return on plan assets   (3)   (7) (7)
Amortization of regulatory assets   0   0 0
Amortization prior service costs   1   3 3
Amortization net gain (loss)   2   6 6
Net periodic benefit cost   4   $ 11 13
Predecessor [Member] | Southern Company Gas [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Actual plan asset allocations (as percent)       100.00%  
Components of net periodic          
Service cost   13   $ 28 24
Interest cost   21   45 47
Expected return on plan assets   (33)   (65) (65)
Amortization of regulatory assets   0   0 0
Amortization prior service costs   (1)   (2) (2)
Amortization net gain (loss)   13   31 22
Net periodic benefit cost   $ 13   $ 37 $ 26
Fixed income | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     29.00% 28.00%  
Actual plan asset allocations (as percent) 31.00%   31.00% 30.00%  
Fixed income | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     23.00% 23.00%  
Actual plan asset allocations (as percent) 29.00%   29.00% 23.00%  
Fixed income | Alabama Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     23.00%    
Actual plan asset allocations (as percent) 29.00%   29.00% 23.00%  
Fixed income | Georgia Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     23.00%    
Actual plan asset allocations (as percent) 29.00%   29.00% 23.00%  
Fixed income | Gulf Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     23.00%    
Actual plan asset allocations (as percent) 29.00%   29.00% 23.00%  
Fixed income | Mississippi Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     38.00%    
Actual plan asset allocations (as percent) 43.00%   43.00% 38.00%  
Fixed income | Mississippi Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     23.00%    
Actual plan asset allocations (as percent) 29.00%   29.00% 23.00%  
Fixed income | Successor [Member] | Southern Company Gas [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     24.00%    
Actual plan asset allocations (as percent) 23.00%   23.00%    
Fixed income | Predecessor [Member] | Southern Company Gas [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Actual plan asset allocations (as percent)       24.00%  
Fixed income | Predecessor [Member] | Southern Company Gas [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Actual plan asset allocations (as percent)       29.00%  
Domestic equity | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     39.00% 42.00%  
Actual plan asset allocations (as percent) 40.00%   40.00% 38.00%  
Domestic equity | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     26.00% 26.00%  
Actual plan asset allocations (as percent) 29.00%   29.00% 30.00%  
Domestic equity | Alabama Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     46.00%    
Actual plan asset allocations (as percent) 44.00%   44.00% 45.00%  
Domestic equity | Alabama Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     26.00%    
Actual plan asset allocations (as percent) 29.00%   29.00% 30.00%  
Domestic equity | Georgia Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     36.00%    
Actual plan asset allocations (as percent) 35.00%   35.00% 34.00%  
Domestic equity | Georgia Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     26.00%    
Actual plan asset allocations (as percent) 29.00%   29.00% 30.00%  
Domestic equity | Gulf Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     25.00%    
Actual plan asset allocations (as percent) 28.00%   28.00% 29.00%  
Domestic equity | Gulf Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     26.00%    
Actual plan asset allocations (as percent) 29.00%   29.00% 30.00%  
Domestic equity | Mississippi Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     21.00%    
Actual plan asset allocations (as percent) 23.00%   23.00% 24.00%  
Domestic equity | Mississippi Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     26.00%    
Actual plan asset allocations (as percent) 29.00%   29.00% 30.00%  
Special situations | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     1.00% 1.00%  
Actual plan asset allocations (as percent) 1.00%   1.00% 1.00%  
Special situations | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     3.00% 3.00%  
Actual plan asset allocations (as percent) 2.00%   2.00% 2.00%  
Special situations | Alabama Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     1.00%    
Actual plan asset allocations (as percent) 1.00%   1.00% 1.00%  
Special situations | Alabama Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     3.00%    
Actual plan asset allocations (as percent) 2.00%   2.00% 2.00%  
Special situations | Georgia Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     1.00%    
Actual plan asset allocations (as percent) 1.00%   1.00% 0.00%  
Special situations | Georgia Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     3.00%    
Actual plan asset allocations (as percent) 2.00%   2.00% 2.00%  
Special situations | Gulf Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     3.00%    
Actual plan asset allocations (as percent) 2.00%   2.00% 2.00%  
Special situations | Gulf Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     3.00%    
Actual plan asset allocations (as percent) 2.00%   2.00% 2.00%  
Special situations | Mississippi Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     3.00%    
Actual plan asset allocations (as percent) 2.00%   2.00% 2.00%  
Special situations | Mississippi Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     3.00%    
Actual plan asset allocations (as percent) 2.00%   2.00% 2.00%  
International equity | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     23.00% 21.00%  
Actual plan asset allocations (as percent) 21.00%   21.00% 23.00%  
International equity | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     25.00% 25.00%  
Actual plan asset allocations (as percent) 22.00%   22.00% 23.00%  
International equity | Alabama Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     22.00%    
Actual plan asset allocations (as percent) 20.00%   20.00% 20.00%  
International equity | Alabama Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     25.00%    
Actual plan asset allocations (as percent) 22.00%   22.00% 23.00%  
International equity | Georgia Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     24.00%    
Actual plan asset allocations (as percent) 24.00%   24.00% 27.00%  
International equity | Georgia Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     25.00%    
Actual plan asset allocations (as percent) 22.00%   22.00% 23.00%  
International equity | Gulf Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     24.00%    
Actual plan asset allocations (as percent) 21.00%   21.00% 22.00%  
International equity | Gulf Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     25.00%    
Actual plan asset allocations (as percent) 22.00%   22.00% 23.00%  
International equity | Mississippi Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     20.00%    
Actual plan asset allocations (as percent) 18.00%   18.00% 18.00%  
International equity | Mississippi Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     25.00%    
Actual plan asset allocations (as percent) 22.00%   22.00% 23.00%  
Domestic Fixed Income Investments [Member] | Alabama Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     24.00%    
Actual plan asset allocations (as percent) 29.00%   29.00% 27.00%  
Domestic Fixed Income Investments [Member] | Georgia Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     33.00%    
Actual plan asset allocations (as percent) 35.00%   35.00% 25.00%  
Domestic Fixed Income Investments [Member] | Gulf Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     25.00%    
Actual plan asset allocations (as percent) 31.00%   31.00% 25.00%  
Global Fixed Income Investments [Member] | Georgia Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)        
Actual plan asset allocations (as percent)   8.00%  
Real Estate Investment [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     5.00% 5.00%  
Actual plan asset allocations (as percent) 5.00%   5.00% 6.00%  
Real Estate Investment [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     14.00% 14.00%  
Actual plan asset allocations (as percent) 13.00%   13.00% 16.00%  
Real Estate Investment [Member] | Alabama Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     4.00%    
Actual plan asset allocations (as percent) 4.00%   4.00% 5.00%  
Real Estate Investment [Member] | Alabama Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     14.00%    
Actual plan asset allocations (as percent) 13.00%   13.00% 16.00%  
Real Estate Investment [Member] | Georgia Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     4.00%    
Actual plan asset allocations (as percent) 4.00%   4.00% 4.00%  
Real Estate Investment [Member] | Georgia Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     14.00%    
Actual plan asset allocations (as percent) 13.00%   13.00% 16.00%  
Real Estate Investment [Member] | Gulf Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     14.00%    
Actual plan asset allocations (as percent) 13.00%   13.00% 16.00%  
Real Estate Investment [Member] | Gulf Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     14.00%    
Actual plan asset allocations (as percent) 13.00%   13.00% 16.00%  
Real Estate Investment [Member] | Mississippi Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     11.00%    
Actual plan asset allocations (as percent) 10.00%   10.00% 13.00%  
Real Estate Investment [Member] | Mississippi Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     14.00%    
Actual plan asset allocations (as percent) 13.00%   13.00% 16.00%  
Private Equity Funds [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     3.00% 3.00%  
Actual plan asset allocations (as percent) 2.00%   2.00% 2.00%  
Private Equity Funds [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     9.00% 9.00%  
Actual plan asset allocations (as percent) 5.00%   5.00% 6.00%  
Private Equity Funds [Member] | Alabama Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     3.00%    
Actual plan asset allocations (as percent) 2.00%   2.00% 2.00%  
Private Equity Funds [Member] | Alabama Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     9.00%    
Actual plan asset allocations (as percent) 5.00%   5.00% 6.00%  
Private Equity Funds [Member] | Georgia Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     2.00%    
Actual plan asset allocations (as percent) 1.00%   1.00% 2.00%  
Private Equity Funds [Member] | Georgia Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     9.00%    
Actual plan asset allocations (as percent) 5.00%   5.00% 6.00%  
Private Equity Funds [Member] | Gulf Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     9.00%    
Actual plan asset allocations (as percent) 5.00%   5.00% 6.00%  
Private Equity Funds [Member] | Gulf Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     9.00%    
Actual plan asset allocations (as percent) 5.00%   5.00% 6.00%  
Private Equity Funds [Member] | Mississippi Power [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     7.00%    
Actual plan asset allocations (as percent) 4.00%   4.00% 5.00%  
Private Equity Funds [Member] | Mississippi Power [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Target plan asset allocations (as percent)     9.00%    
Actual plan asset allocations (as percent) 5.00%   5.00% 6.00%  
Private Equity Funds [Member] | Predecessor [Member] | Southern Company Gas [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Actual plan asset allocations (as percent)       0.00%  
Private Equity Funds [Member] | Predecessor [Member] | Southern Company Gas [Member] | Pension plans          
Defined Benefit Plan Disclosure [Line Items]          
Actual plan asset allocations (as percent)       2.00%