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Retirement Benefits - Fair Values of Pension Plan and Other Postretirement Benefit Plan Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2015
Assets Fair Value        
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00%      
Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets $ 12,992 $ 11,583   $ 9,234
Assets Fair Value        
Fair value, plan assets 11,956 10,583    
Alternative investments 2,037 $ 1,881    
Target plan asset allocations (as percent)   100.00%    
Actual plan asset allocations (as percent)   100.00%    
Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets $ 3,564 $ 2,937    
Target plan asset allocations (as percent) 26.00% 26.00%    
Actual plan asset allocations (as percent) 31.00% 29.00%    
Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets $ 2,958 $ 2,341    
Target plan asset allocations (as percent) 25.00% 25.00%    
Actual plan asset allocations (as percent) 25.00% 22.00%    
Pension plans | Fixed income        
Assets Fair Value        
Target plan asset allocations (as percent) 23.00% 23.00%    
Actual plan asset allocations (as percent) 24.00% 29.00%    
Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets $ 841 $ 588    
Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets 8 13    
Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 1,201 991    
Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 650 524    
Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 228 998    
Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 1,657 1,462    
Alternative investments $ 1,188 $ 1,152    
Target plan asset allocations (as percent) 14.00% 14.00%    
Actual plan asset allocations (as percent) 13.00% 13.00%    
Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets $ 180 $ 180    
Alternative investments $ 180 $ 180    
Target plan asset allocations (as percent) 3.00% 3.00%    
Actual plan asset allocations (as percent) 1.00% 2.00%    
Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets $ 669 $ 549    
Alternative investments $ 669 $ 549    
Target plan asset allocations (as percent) 9.00% 9.00%    
Actual plan asset allocations (as percent) 6.00% 5.00%    
Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets $ 1,053 $ 944   833
Assets Fair Value        
Fair value, plan assets 927 838    
Alternative investments $ 61 $ 57    
Target plan asset allocations (as percent) 100.00% 100.00%    
Actual plan asset allocations (as percent) 100.00% 100.00%    
Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets $ 167 $ 146    
Target plan asset allocations (as percent) 37.00% 39.00%    
Actual plan asset allocations (as percent) 40.00% 40.00%    
Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets $ 123 $ 98    
Target plan asset allocations (as percent) 23.00% 23.00%    
Actual plan asset allocations (as percent) 23.00% 21.00%    
Other postretirement benefit plans | Fixed income        
Assets Fair Value        
Target plan asset allocations (as percent) 30.00% 29.00%    
Actual plan asset allocations (as percent) 29.00% 31.00%    
Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets $ 32 $ 24    
Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 37 30    
Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 55 49    
Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 10 41    
Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair value, plan assets 426 382    
Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 52 46    
Alternative investments $ 36 $ 35    
Target plan asset allocations (as percent) 5.00% 5.00%    
Actual plan asset allocations (as percent) 5.00% 5.00%    
Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair value, plan assets $ 5 $ 5    
Alternative investments $ 5 $ 5    
Target plan asset allocations (as percent) 1.00% 1.00%    
Actual plan asset allocations (as percent) 1.00% 1.00%    
Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets $ 20 $ 17    
Alternative investments $ 20 $ 17    
Target plan asset allocations (as percent) 4.00% 3.00%    
Actual plan asset allocations (as percent) 2.00% 2.00%    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Assets Fair Value        
Fair value, plan assets $ 4,646 $ 4,547    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 2,405 2,010    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 1,555 1,231    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets 0 0    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 217 996    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 469 310    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 205 207    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 132 118    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 47 37    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 10 41    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair value, plan assets 0 0    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 16 11    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Other Observable Inputs (Level 2) | Pension plans        
Assets Fair Value        
Fair value, plan assets 5,273 4,155    
Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 1,159 927    
Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 1,403 1,110    
Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 841 588    
Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets 8 13    
Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 1,201 991    
Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 650 524    
Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 11 2    
Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Other Observable Inputs (Level 2) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 661 574    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 35 28    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 76 61    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 32 24    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 37 30    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 55 49    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair value, plan assets 426 382    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Pension plans        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0    
Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets 2,836 2,517   2,279
Assets Fair Value        
Fair value, plan assets 2,844 2,513    
Alternative investments $ 485 $ 447    
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
ALABAMA POWER CO | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets $ 848 $ 697    
Target plan asset allocations (as percent) 26.00%      
Actual plan asset allocations (as percent) 31.00% 29.00%    
ALABAMA POWER CO | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets $ 703 $ 556    
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 25.00% 22.00%    
ALABAMA POWER CO | Pension plans | Fixed income        
Assets Fair Value        
Target plan asset allocations (as percent) 23.00%      
Actual plan asset allocations (as percent) 24.00% 29.00%    
ALABAMA POWER CO | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets $ 200 $ 140    
ALABAMA POWER CO | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets 2 3    
ALABAMA POWER CO | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 286 235    
ALABAMA POWER CO | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 155 124    
ALABAMA POWER CO | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 54 237    
ALABAMA POWER CO | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 394 348    
Alternative investments $ 283 $ 274    
Target plan asset allocations (as percent) 14.00%      
Actual plan asset allocations (as percent) 13.00% 13.00%    
ALABAMA POWER CO | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets $ 43 $ 43    
Alternative investments $ 43 $ 43    
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 1.00% 2.00%    
ALABAMA POWER CO | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets $ 159 $ 130    
Alternative investments $ 159 $ 130    
Target plan asset allocations (as percent) 9.00%      
Actual plan asset allocations (as percent) 6.00% 5.00%    
ALABAMA POWER CO | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets $ 406 $ 367   363
Assets Fair Value        
Fair value, plan assets 405 366    
Alternative investments $ 21 $ 20    
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
ALABAMA POWER CO | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets $ 64 $ 61    
Target plan asset allocations (as percent) 42.00%      
Actual plan asset allocations (as percent) 44.00% 44.00%    
ALABAMA POWER CO | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets $ 30 $ 25    
Target plan asset allocations (as percent) 22.00%      
Actual plan asset allocations (as percent) 22.00% 20.00%    
ALABAMA POWER CO | Other postretirement benefit plans | Domestic Fixed Income Investments        
Assets Fair Value        
Target plan asset allocations (as percent) 28.00%      
Actual plan asset allocations (as percent) 28.00% 29.00%    
ALABAMA POWER CO | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets $ 11 $ 7    
ALABAMA POWER CO | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 12 10    
ALABAMA POWER CO | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 7 5    
ALABAMA POWER CO | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 2 14    
ALABAMA POWER CO | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair value, plan assets 253 220    
ALABAMA POWER CO | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 17 16    
Alternative investments $ 12 $ 12    
Target plan asset allocations (as percent) 4.00%      
Actual plan asset allocations (as percent) 4.00% 4.00%    
ALABAMA POWER CO | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair value, plan assets $ 2 $ 2    
Alternative investments $ 2 $ 2    
Target plan asset allocations (as percent) 1.00%      
Actual plan asset allocations (as percent) 0.00% 1.00%    
ALABAMA POWER CO | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets $ 7 $ 6    
Alternative investments $ 7 $ 6    
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 2.00% 2.00%    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Assets Fair Value        
Fair value, plan assets $ 1,104 $ 1,079    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 572 477    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 370 292    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 51 236    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 111 74    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 75 82    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 52 51    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 16 13    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 2 14    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 5 4    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans        
Assets Fair Value        
Fair value, plan assets 1,255 987    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 276 220    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 333 264    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 200 140    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets 2 3    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 286 235    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 155 124    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 3 1    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 309 264    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 12 10    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 14 12    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 11 7    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 12 10    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 7 5    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair value, plan assets 253 220    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
ALABAMA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets 563 499   430
Assets Fair Value        
Fair value, plan assets 564 498    
Alternative investments $ 97 $ 88    
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
MISSISSIPPI POWER CO | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets $ 168 $ 139    
Target plan asset allocations (as percent) 26.00%      
Actual plan asset allocations (as percent) 31.00% 29.00%    
MISSISSIPPI POWER CO | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets $ 139 $ 109    
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 25.00% 22.00%    
MISSISSIPPI POWER CO | Pension plans | Fixed income        
Assets Fair Value        
Target plan asset allocations (as percent) 23.00%      
Actual plan asset allocations (as percent) 24.00% 29.00%    
MISSISSIPPI POWER CO | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets $ 40 $ 28    
MISSISSIPPI POWER CO | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets   1    
MISSISSIPPI POWER CO | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 56 46    
MISSISSIPPI POWER CO | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 31 25    
MISSISSIPPI POWER CO | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 11 47    
MISSISSIPPI POWER CO | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 78 69    
Alternative investments $ 56 $ 54    
Target plan asset allocations (as percent) 14.00%      
Actual plan asset allocations (as percent) 13.00% 13.00%    
MISSISSIPPI POWER CO | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets $ 9 $ 8    
Alternative investments $ 9 $ 8    
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 1.00% 2.00%    
MISSISSIPPI POWER CO | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets $ 32 $ 26    
Alternative investments $ 32 $ 26    
Target plan asset allocations (as percent) 9.00%      
Actual plan asset allocations (as percent) 6.00% 5.00%    
MISSISSIPPI POWER CO | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets $ 25 $ 23   23
Assets Fair Value        
Fair value, plan assets 24 24    
Alternative investments $ 3 $ 3    
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
MISSISSIPPI POWER CO | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets $ 6 $ 6    
Target plan asset allocations (as percent) 21.00%      
Actual plan asset allocations (as percent) 25.00% 23.00%    
MISSISSIPPI POWER CO | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets $ 5 $ 4    
Target plan asset allocations (as percent) 21.00%      
Actual plan asset allocations (as percent) 20.00% 18.00%    
MISSISSIPPI POWER CO | Other postretirement benefit plans | Fixed income        
Assets Fair Value        
Target plan asset allocations (as percent) 37.00%      
Actual plan asset allocations (as percent) 38.00% 43.00%    
MISSISSIPPI POWER CO | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets $ 5 $ 5    
MISSISSIPPI POWER CO | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 2 2    
MISSISSIPPI POWER CO | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 1 1    
MISSISSIPPI POWER CO | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 1 2    
MISSISSIPPI POWER CO | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 3 3    
Alternative investments $ 2 $ 2    
Target plan asset allocations (as percent) 12.00%      
Actual plan asset allocations (as percent) 11.00% 10.00%    
MISSISSIPPI POWER CO | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Target plan asset allocations (as percent) 2.00%      
Actual plan asset allocations (as percent) 1.00% 2.00%    
MISSISSIPPI POWER CO | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets $ 1 $ 1    
Alternative investments $ 1 $ 1    
Target plan asset allocations (as percent) 7.00%      
Actual plan asset allocations (as percent) 5.00% 4.00%    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Assets Fair Value        
Fair value, plan assets $ 218 $ 215    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 113 95    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 73 58    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets   0    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 10 47    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 22 15    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 9 9    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 4 4    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 3 2    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 1 2    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 1 1    
MISSISSIPPI POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans        
Assets Fair Value        
Fair value, plan assets 249 195    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 55 44    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 66 51    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 40 28    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets   1    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 56 46    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 31 25    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 1 0    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 12 12    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 2 2    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 2 2    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 5 5    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 2 2    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 1 1    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets   0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
MISSISSIPPI POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets 4,058 3,621   3,196
Assets Fair Value        
Fair value, plan assets 4,069 3,615    
Alternative investments $ 693 $ 643    
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
GEORGIA POWER CO | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets $ 1,213 $ 1,003    
Target plan asset allocations (as percent) 26.00%      
Actual plan asset allocations (as percent) 31.00% 29.00%    
GEORGIA POWER CO | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets $ 1,006 $ 800    
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 25.00% 22.00%    
GEORGIA POWER CO | Pension plans | Fixed income        
Assets Fair Value        
Target plan asset allocations (as percent) 23.00%      
Actual plan asset allocations (as percent) 24.00% 29.00%    
GEORGIA POWER CO | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets $ 286 $ 201    
GEORGIA POWER CO | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets 3 4    
GEORGIA POWER CO | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 409 338    
GEORGIA POWER CO | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 221 179    
GEORGIA POWER CO | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 78 341    
GEORGIA POWER CO | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 564 500    
Alternative investments $ 404 $ 394    
Target plan asset allocations (as percent) 14.00%      
Actual plan asset allocations (as percent) 13.00% 13.00%    
GEORGIA POWER CO | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets $ 61 $ 61    
Alternative investments $ 61 $ 61    
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 1.00% 2.00%    
GEORGIA POWER CO | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets $ 228 $ 188    
Alternative investments $ 228 $ 188    
Target plan asset allocations (as percent) 9.00%      
Actual plan asset allocations (as percent) 6.00% 5.00%    
GEORGIA POWER CO | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets $ 386 $ 354   358
Assets Fair Value        
Fair value, plan assets 384 352    
Alternative investments $ 19 $ 18    
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
GEORGIA POWER CO | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets $ 64 $ 54    
Target plan asset allocations (as percent) 36.00%      
Actual plan asset allocations (as percent) 38.00% 35.00%    
GEORGIA POWER CO | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets $ 60 $ 48    
Target plan asset allocations (as percent) 24.00%      
Actual plan asset allocations (as percent) 24.00% 24.00%    
GEORGIA POWER CO | Other postretirement benefit plans | Domestic Fixed Income Investments        
Assets Fair Value        
Target plan asset allocations (as percent) 33.00%      
Actual plan asset allocations (as percent) 31.00% 35.00%    
GEORGIA POWER CO | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets $ 6 $ 5    
GEORGIA POWER CO | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 11 9    
GEORGIA POWER CO | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 41 38    
GEORGIA POWER CO | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 4 15    
GEORGIA POWER CO | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair value, plan assets 173 162    
GEORGIA POWER CO | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 17 14    
Alternative investments $ 11 $ 11    
Target plan asset allocations (as percent) 4.00%      
Actual plan asset allocations (as percent) 4.00% 4.00%    
GEORGIA POWER CO | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair value, plan assets $ 2 $ 2    
Alternative investments $ 2 $ 2    
Target plan asset allocations (as percent) 1.00%      
Actual plan asset allocations (as percent) 1.00% 1.00%    
GEORGIA POWER CO | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets $ 6 $ 5    
Alternative investments $ 6 $ 5    
Target plan asset allocations (as percent) 2.00%      
Actual plan asset allocations (as percent) 2.00% 1.00%    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Assets Fair Value        
Fair value, plan assets $ 1,582 $ 1,552    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 819 686    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 529 420    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 74 340    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 160 106    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 77 74    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 53 45    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 14 11    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 4 15    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 6 3    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans        
Assets Fair Value        
Fair value, plan assets 1,794 1,420    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 394 317    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 477 380    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 286 201    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets 3 4    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 409 338    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 221 179    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 4 1    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 288 260    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 11 9    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 46 37    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 6 5    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 11 9    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 41 38    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair value, plan assets 173 162    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
GEORGIA POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets 553 491   420
Assets Fair Value        
Fair value, plan assets 555 490    
Alternative investments $ 94 $ 86    
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
GULF POWER CO | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets $ 166 $ 136    
Target plan asset allocations (as percent) 26.00%      
Actual plan asset allocations (as percent) 31.00% 29.00%    
GULF POWER CO | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets $ 137 $ 109    
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 25.00% 22.00%    
GULF POWER CO | Pension plans | Fixed income        
Assets Fair Value        
Target plan asset allocations (as percent) 23.00%      
Actual plan asset allocations (as percent) 24.00% 29.00%    
GULF POWER CO | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets $ 39 $ 27    
GULF POWER CO | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets   1    
GULF POWER CO | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 57 47    
GULF POWER CO | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 30 24    
GULF POWER CO | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 10 46    
GULF POWER CO | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 77 67    
Alternative investments $ 55 $ 53    
Target plan asset allocations (as percent) 14.00%      
Actual plan asset allocations (as percent) 13.00% 13.00%    
GULF POWER CO | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets $ 8 $ 8    
Alternative investments $ 8 $ 8    
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 1.00% 2.00%    
GULF POWER CO | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets $ 31 $ 25    
Alternative investments $ 31 $ 25    
Target plan asset allocations (as percent) 9.00%      
Actual plan asset allocations (as percent) 6.00% 5.00%    
GULF POWER CO | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets $ 20 $ 18   17
Assets Fair Value        
Fair value, plan assets 19 19    
Alternative investments $ 3 $ 3    
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
GULF POWER CO | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets $ 6 $ 5    
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 30.00% 28.00%    
GULF POWER CO | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets $ 4 $ 4    
Target plan asset allocations (as percent) 24.00%      
Actual plan asset allocations (as percent) 24.00% 21.00%    
GULF POWER CO | Other postretirement benefit plans | Domestic Fixed Income Investments        
Assets Fair Value        
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 26.00% 31.00%    
GULF POWER CO | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets $ 1 $ 1    
GULF POWER CO | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 2 2    
GULF POWER CO | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 1 1    
GULF POWER CO | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 1 2    
GULF POWER CO | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 3 3    
Alternative investments $ 2 $ 2    
Target plan asset allocations (as percent) 14.00%      
Actual plan asset allocations (as percent) 13.00% 13.00%    
GULF POWER CO | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 1.00% 2.00%    
GULF POWER CO | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets $ 1 $ 1    
Alternative investments $ 1 $ 1    
Target plan asset allocations (as percent) 9.00%      
Actual plan asset allocations (as percent) 6.00% 5.00%    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Assets Fair Value        
Fair value, plan assets $ 216 $ 210    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 112 93    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 72 57    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets   0    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 10 46    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 22 14    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 8 8    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 4 3    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 2 2    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 1 2    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 1 1    
GULF POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans        
Assets Fair Value        
Fair value, plan assets 245 194    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 54 43    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 65 52    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 39 27    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets   1    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 57 47    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 30 24    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 8 8    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 2 2    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 2 2    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 1 1    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 2 2    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 1 1    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair value, plan assets   0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
GULF POWER CO | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN POWER CO | Pension plans        
Assets Fair Value        
Fair value, plan assets 138      
Alternative investments $ 24      
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00%      
SOUTHERN POWER CO | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets $ 41      
Target plan asset allocations (as percent) 26.00%      
Actual plan asset allocations (as percent) 31.00%      
SOUTHERN POWER CO | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets $ 34      
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 25.00%      
SOUTHERN POWER CO | Pension plans | Fixed income        
Assets Fair Value        
Target plan asset allocations (as percent) 23.00%      
Actual plan asset allocations (as percent) 24.00%      
SOUTHERN POWER CO | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets $ 10      
SOUTHERN POWER CO | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 14      
SOUTHERN POWER CO | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 8      
SOUTHERN POWER CO | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 2      
SOUTHERN POWER CO | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 19      
Alternative investments $ 14      
Target plan asset allocations (as percent) 14.00%      
Actual plan asset allocations (as percent) 13.00%      
SOUTHERN POWER CO | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets $ 2      
Alternative investments $ 2      
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 1.00%      
SOUTHERN POWER CO | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets $ 8      
Alternative investments $ 8      
Target plan asset allocations (as percent) 9.00%      
Actual plan asset allocations (as percent) 6.00%      
SOUTHERN POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Assets Fair Value        
Fair value, plan assets $ 53      
SOUTHERN POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 28      
SOUTHERN POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 18      
SOUTHERN POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 2      
SOUTHERN POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 5      
SOUTHERN POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans        
Assets Fair Value        
Fair value, plan assets 61      
SOUTHERN POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 13      
SOUTHERN POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 16      
SOUTHERN POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 10      
SOUTHERN POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 14      
SOUTHERN POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 8      
SOUTHERN POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN POWER CO | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0      
SOUTHERN Co GAS | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets 1,068 983 $ 837  
Assets Fair Value        
Fair value, plan assets 945 983    
Alternative investments 65 100    
SOUTHERN Co GAS | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 478 485    
SOUTHERN Co GAS | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 166 185    
SOUTHERN Co GAS | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 85 85    
SOUTHERN Co GAS | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 39 41    
SOUTHERN Co GAS | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets   66    
SOUTHERN Co GAS | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 157 100    
Alternative investments 48 83    
SOUTHERN Co GAS | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 19 19    
Alternative investments 16 15    
SOUTHERN Co GAS | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 1 2    
Alternative investments 1 2    
SOUTHERN Co GAS | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets 125 105 100  
Assets Fair Value        
Fair value, plan assets 121 105    
Alternative investments $ 1 $ 2    
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
SOUTHERN Co GAS | Other postretirement benefit plans | Cash and Cash Equivalents        
Assets Fair Value        
Target plan asset allocations (as percent) 1.00%      
Actual plan asset allocations (as percent) 2.00% 1.00%    
SOUTHERN Co GAS | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets $ 72 $ 61    
SOUTHERN Co GAS | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets $ 22 $ 18    
SOUTHERN Co GAS | Other postretirement benefit plans | Fixed income        
Assets Fair Value        
Target plan asset allocations (as percent) 24.00%      
Actual plan asset allocations (as percent) 20.00% 23.00%    
SOUTHERN Co GAS | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets $ 24 $ 23    
SOUTHERN Co GAS | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 3 3    
Alternative investments $ 1 $ 2    
SOUTHERN Co GAS | Other postretirement benefit plans | Equity Securities        
Assets Fair Value        
Target plan asset allocations (as percent) 72.00%      
Actual plan asset allocations (as percent) 76.00% 74.00%    
SOUTHERN Co GAS | Other postretirement benefit plans | Other Types Of Investments        
Assets Fair Value        
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 2.00% 2.00%    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Assets Fair Value        
Fair value, plan assets $ 242 $ 158    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 155 142    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets   0    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 84 12    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 3 4    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 5 4    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 3 3    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 2 1    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Pension plans        
Assets Fair Value        
Fair value, plan assets 638 725    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 323 343    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 166 185    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 85 85    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 39 41    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets   66    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 25 5    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 115 99    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 69 58    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 22 18    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 24 23    
SOUTHERN Co GAS | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Pension plans        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Pension plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets   0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair value, plan assets 0 0    
SOUTHERN Co GAS | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair value, plan assets $ 0 0    
Predecessor | SOUTHERN Co GAS | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets     837 $ 847
Predecessor | SOUTHERN Co GAS | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Fair value of plan assets   $ 99 $ 100  
AGL Resources Inc. Retirement Plan | SOUTHERN Co GAS        
Assets Fair Value        
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
AGL Resources Inc. Retirement Plan | SOUTHERN Co GAS | Cash and Cash Equivalents        
Assets Fair Value        
Target plan asset allocations (as percent) 2.00%      
Actual plan asset allocations (as percent) 6.00% 1.00%    
AGL Resources Inc. Retirement Plan | SOUTHERN Co GAS | Fixed income        
Assets Fair Value        
Target plan asset allocations (as percent) 15.00%      
Actual plan asset allocations (as percent) 19.00% 20.00%    
AGL Resources Inc. Retirement Plan | SOUTHERN Co GAS | Equity Securities        
Assets Fair Value        
Target plan asset allocations (as percent) 53.00%      
Actual plan asset allocations (as percent) 65.00% 69.00%    
AGL Resources Inc. Retirement Plan | SOUTHERN Co GAS | Other Types Of Investments        
Assets Fair Value        
Target plan asset allocations (as percent) 30.00%      
Actual plan asset allocations (as percent) 10.00% 10.00%