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Retirement Benefits - Fair Values of Pension Plan and Other Postretirement Benefit Plan Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Jun. 30, 2016
Dec. 31, 2014
Liabilities 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]        
Defined Benefit Plan, Fair Value of Plan Assets $ 11,583 $ 9,234   $ 9,690
Assets Fair Value        
Fair Value, Plan Assets 10,583 9,083    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets 1,881 1,881    
Liabilities Fair Value        
Fair Value, Plan Liabilities   (1)    
Fair Value, Plan Assets and Liabilities   $ 9,082    
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 $ 2,937 $ 2,313    
Liabilities Fair Value        
Target plan asset allocations (as percent) 26.00% 26.00%    
Actual plan asset allocations (as percent) 29.00% 30.00%    
Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets $ 2,341 $ 2,152    
Liabilities Fair Value        
Target plan asset allocations (as percent) 25.00% 25.00%    
Actual plan asset allocations (as percent) 22.00% 23.00%    
Pension plans | Fixed income        
Liabilities Fair Value        
Target plan asset allocations (as percent) 23.00% 23.00%    
Actual plan asset allocations (as percent) 29.00% 23.00%    
Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets $ 588 $ 454    
Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 13 199    
Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 991 1,140    
Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 524 500    
Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 998 145    
Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 1,462 1,484    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 1,152 $ 1,185    
Liabilities Fair Value        
Target plan asset allocations (as percent) 14.00% 14.00%    
Actual plan asset allocations (as percent) 13.00% 16.00%    
Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets $ 180 $ 160    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 180 $ 160    
Liabilities Fair Value        
Target plan asset allocations (as percent) 3.00% 3.00%    
Actual plan asset allocations (as percent) 2.00% 2.00%    
Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets $ 549 $ 536    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 549 $ 536    
Liabilities Fair Value        
Target plan asset allocations (as percent) 9.00% 9.00%    
Actual plan asset allocations (as percent) 5.00% 6.00%    
Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets $ 944 $ 833   900
Assets Fair Value        
Fair Value, Plan Assets 838 834    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 57 $ 63    
Liabilities Fair Value        
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 $ 146 $ 158    
Liabilities Fair Value        
Target plan asset allocations (as percent) 39.00% 42.00%    
Actual plan asset allocations (as percent) 40.00% 38.00%    
Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets $ 98 $ 103    
Liabilities Fair Value        
Target plan asset allocations (as percent) 23.00% 21.00%    
Actual plan asset allocations (as percent) 21.00% 23.00%    
Other postretirement benefit plans | Fixed income        
Liabilities Fair Value        
Target plan asset allocations (as percent) 29.00% 28.00%    
Actual plan asset allocations (as percent) 31.00% 30.00%    
Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets $ 24 $ 22    
Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets   7    
Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 30 38    
Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 49 42    
Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 41 20    
Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair Value, Plan Assets 382 370    
Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 46 51    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 35 $ 40    
Liabilities Fair Value        
Target plan asset allocations (as percent) 5.00% 5.00%    
Actual plan asset allocations (as percent) 5.00% 6.00%    
Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets $ 5 $ 5    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 5 $ 5    
Liabilities Fair Value        
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 $ 17 $ 18    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 17 $ 18    
Liabilities Fair Value        
Target plan asset allocations (as percent) 3.00% 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 $ 4,547 $ 3,121    
Liabilities Fair Value        
Fair Value, Plan Liabilities   (1)    
Fair Value, Plan Assets and Liabilities   3,120    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 2,010 1,632    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 1,231 1,190    
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 996 0    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 310 299    
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 207 168    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 118 106    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 37 40    
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 | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets   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 41 11    
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 11 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 4,155 4,081    
Liabilities Fair Value        
Fair Value, Plan Liabilities   0    
Fair Value, Plan Assets and Liabilities   4,081    
Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 927 681    
Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 1,110 962    
Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets 588 454    
Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 13 199    
Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 991 1,140    
Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 524 500    
Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 2 145    
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 574 603    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 28 52    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 61 63    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets 24 22    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets   7    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 30 38    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 49 42    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 9    
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair Value, Plan Assets 382 370    
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    
Liabilities Fair Value        
Fair Value, Plan Liabilities   0    
Fair Value, Plan Assets and Liabilities   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 | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets   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 [Member] | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets 2,517 2,279   2,396
Assets Fair Value        
Fair Value, Plan Assets 2,513 2,241    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 447 $ 458    
Liabilities Fair Value        
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
Alabama Power [Member] | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets $ 697 $ 571    
Liabilities Fair Value        
Target plan asset allocations (as percent) 26.00%      
Actual plan asset allocations (as percent) 29.00% 30.00%    
Alabama Power [Member] | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets $ 556 $ 538    
Liabilities Fair Value        
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 22.00% 23.00%    
Alabama Power [Member] | Pension plans | Fixed income        
Liabilities Fair Value        
Target plan asset allocations (as percent) 23.00%      
Actual plan asset allocations (as percent) 29.00% 23.00%    
Alabama Power [Member] | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets $ 140 $ 112    
Alabama Power [Member] | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 3 49    
Alabama Power [Member] | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 235 280    
Alabama Power [Member] | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 124 123    
Alabama Power [Member] | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 237 36    
Alabama Power [Member] | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 348 375    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 274 $ 301    
Liabilities Fair Value        
Target plan asset allocations (as percent) 14.00%      
Actual plan asset allocations (as percent) 13.00% 16.00%    
Alabama Power [Member] | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets $ 43      
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 43      
Liabilities Fair Value        
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 2.00% 2.00%    
Alabama Power [Member] | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets $ 130 $ 157    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 130 $ 157    
Liabilities Fair Value        
Target plan asset allocations (as percent) 9.00%      
Actual plan asset allocations (as percent) 5.00% 6.00%    
Alabama Power [Member] | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets $ 367 $ 363   392
Assets Fair Value        
Fair Value, Plan Assets 366 361    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 20 $ 21    
Liabilities Fair Value        
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
Alabama Power [Member] | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets $ 61 $ 65    
Liabilities Fair Value        
Target plan asset allocations (as percent) 46.00%      
Actual plan asset allocations (as percent) 44.00% 45.00%    
Alabama Power [Member] | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets $ 25 $ 26    
Liabilities Fair Value        
Target plan asset allocations (as percent) 22.00%      
Actual plan asset allocations (as percent) 20.00% 20.00%    
Alabama Power [Member] | Other postretirement benefit plans | Domestic Fixed Income Investments [Member]        
Liabilities Fair Value        
Target plan asset allocations (as percent) 24.00%      
Actual plan asset allocations (as percent) 29.00% 27.00%    
Alabama Power [Member] | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets $ 7 $ 8    
Alabama Power [Member] | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 2    
Alabama Power [Member] | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 10 13    
Alabama Power [Member] | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 5 6    
Alabama Power [Member] | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 14 3    
Alabama Power [Member] | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair Value, Plan Assets 220 212    
Alabama Power [Member] | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 16 19    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 12 $ 14    
Liabilities Fair Value        
Target plan asset allocations (as percent) 4.00%      
Actual plan asset allocations (as percent) 4.00% 5.00%    
Alabama Power [Member] | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets $ 2      
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 2      
Liabilities Fair Value        
Target plan asset allocations (as percent) 1.00%      
Actual plan asset allocations (as percent) 1.00% 1.00%    
Alabama Power [Member] | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets $ 6 $ 7    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 6 $ 7    
Liabilities Fair Value        
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 2.00% 2.00%    
Alabama Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets $ 1,079 $ 771    
Alabama Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 477 403    
Alabama Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 292 294    
Alabama Power [Member] | 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 [Member] | 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 [Member] | 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 [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 236 0    
Alabama Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 74 74    
Alabama Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Alabama Power [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 82 77    
Alabama Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 51 57    
Alabama Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 13 14    
Alabama Power [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | 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 [Member] | 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 [Member] | 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 14 1    
Alabama Power [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 4 5    
Alabama Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Alabama Power [Member] | 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 [Member] | Significant Other Observable Inputs (Level 2) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets 987 1,012    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 220 168    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 264 244    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets 140 112    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 3 49    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 235 280    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 124 123    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 1 36    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 264 263    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 10 8    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 12 12    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets 7 8    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 2    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 10 13    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 5 6    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 2    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair Value, Plan Assets 220 212    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Alabama Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | 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 [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | 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 [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Alabama Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets 3,621 3,196   3,383
Assets Fair Value        
Fair Value, Plan Assets 3,615 3,143    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 643 $ 641    
Liabilities Fair Value        
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
Georgia Power [Member] | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets $ 1,003 $ 801    
Liabilities Fair Value        
Target plan asset allocations (as percent) 26.00%      
Actual plan asset allocations (as percent) 29.00% 30.00%    
Georgia Power [Member] | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets $ 800 $ 755    
Liabilities Fair Value        
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 22.00% 23.00%    
Georgia Power [Member] | Pension plans | Fixed income        
Liabilities Fair Value        
Target plan asset allocations (as percent) 23.00%      
Actual plan asset allocations (as percent) 29.00% 23.00%    
Georgia Power [Member] | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets $ 201 $ 157    
Georgia Power [Member] | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 4 69    
Georgia Power [Member] | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 338 394    
Georgia Power [Member] | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 179 173    
Georgia Power [Member] | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 341 50    
Georgia Power [Member] | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 500 524    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 394 $ 421    
Liabilities Fair Value        
Target plan asset allocations (as percent) 14.00%      
Actual plan asset allocations (as percent) 13.00% 16.00%    
Georgia Power [Member] | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets $ 61      
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 61      
Liabilities Fair Value        
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 2.00% 2.00%    
Georgia Power [Member] | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets $ 188 $ 220    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 188 $ 220    
Liabilities Fair Value        
Target plan asset allocations (as percent) 9.00%      
Actual plan asset allocations (as percent) 5.00% 6.00%    
Georgia Power [Member] | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets $ 354 $ 358   395
Assets Fair Value        
Fair Value, Plan Assets 352 364    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 18 $ 19    
Liabilities Fair Value        
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
Georgia Power [Member] | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets $ 54 $ 66    
Liabilities Fair Value        
Target plan asset allocations (as percent) 36.00%      
Actual plan asset allocations (as percent) 35.00% 34.00%    
Georgia Power [Member] | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets $ 48 $ 53    
Liabilities Fair Value        
Target plan asset allocations (as percent) 24.00%      
Actual plan asset allocations (as percent) 24.00% 27.00%    
Georgia Power [Member] | Other postretirement benefit plans | Domestic Fixed Income Investments [Member]        
Liabilities Fair Value        
Target plan asset allocations (as percent) 33.00%      
Actual plan asset allocations (as percent) 35.00% 25.00%    
Georgia Power [Member] | Other postretirement benefit plans | Global Fixed Income Investments [Member]        
Liabilities Fair Value        
Target plan asset allocations (as percent)      
Actual plan asset allocations (as percent) 8.00%    
Georgia Power [Member] | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets $ 5 $ 5    
Georgia Power [Member] | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 2    
Georgia Power [Member] | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 9 12    
Georgia Power [Member] | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 38 30    
Georgia Power [Member] | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 15 16    
Georgia Power [Member] | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair Value, Plan Assets 162 158    
Georgia Power [Member] | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 14 15    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 11 $ 12    
Liabilities Fair Value        
Target plan asset allocations (as percent) 4.00%      
Actual plan asset allocations (as percent) 4.00% 4.00%    
Georgia Power [Member] | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets $ 2      
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 2      
Liabilities Fair Value        
Target plan asset allocations (as percent) 1.00%      
Actual plan asset allocations (as percent) 1.00% 0.00%    
Georgia Power [Member] | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets $ 5 $ 7    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 5 $ 7    
Liabilities Fair Value        
Target plan asset allocations (as percent) 2.00%      
Actual plan asset allocations (as percent) 1.00% 2.00%    
Georgia Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets $ 1,552 $ 1,080    
Georgia Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 686 565    
Georgia Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 420 412    
Georgia Power [Member] | 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 [Member] | 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 [Member] | 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 [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 340 0    
Georgia Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 106 103    
Georgia Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Georgia Power [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 74 55    
Georgia Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 45 30    
Georgia Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 11 12    
Georgia Power [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | 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 [Member] | 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 [Member] | 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 15 10    
Georgia Power [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 3 3    
Georgia Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Georgia Power [Member] | 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 [Member] | Significant Other Observable Inputs (Level 2) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets 1,420 1,422    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 317 236    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 380 343    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets 201 157    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 4 69    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 338 394    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 179 173    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 1 50    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 260 290    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 9 36    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 37 41    
Georgia Power [Member] | 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    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 2    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 9 12    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 38 30    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 6    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair Value, Plan Assets 162 158    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Georgia Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | 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 [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | 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 [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Trust-owned life insurance        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Georgia Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets 491 420   435
Assets Fair Value        
Fair Value, Plan Assets 490 412    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 86 $ 84    
Liabilities Fair Value        
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
Gulf Power [Member] | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets $ 136 $ 104    
Liabilities Fair Value        
Target plan asset allocations (as percent) 26.00%      
Actual plan asset allocations (as percent) 29.00% 30.00%    
Gulf Power [Member] | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets $ 109 $ 99    
Liabilities Fair Value        
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 22.00% 23.00%    
Gulf Power [Member] | Pension plans | Fixed income        
Liabilities Fair Value        
Target plan asset allocations (as percent) 23.00%      
Actual plan asset allocations (as percent) 29.00% 23.00%    
Gulf Power [Member] | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets $ 27 $ 21    
Gulf Power [Member] | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 1 9    
Gulf Power [Member] | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 47 51    
Gulf Power [Member] | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 24 23    
Gulf Power [Member] | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 46 7    
Gulf Power [Member] | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 67 69    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 53 $ 55    
Liabilities Fair Value        
Target plan asset allocations (as percent) 14.00%      
Actual plan asset allocations (as percent) 13.00% 16.00%    
Gulf Power [Member] | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets $ 8      
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 8      
Liabilities Fair Value        
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 2.00% 2.00%    
Gulf Power [Member] | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets $ 25 $ 29    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 25 $ 29    
Liabilities Fair Value        
Target plan asset allocations (as percent) 9.00%      
Actual plan asset allocations (as percent) 5.00% 6.00%    
Gulf Power [Member] | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets $ 18 $ 17   18
Assets Fair Value        
Fair Value, Plan Assets 19 17    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 3 $ 3    
Liabilities Fair Value        
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
Gulf Power [Member] | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets $ 5 $ 4    
Liabilities Fair Value        
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 28.00% 29.00%    
Gulf Power [Member] | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets $ 4 $ 4    
Liabilities Fair Value        
Target plan asset allocations (as percent) 24.00%      
Actual plan asset allocations (as percent) 21.00% 22.00%    
Gulf Power [Member] | Other postretirement benefit plans | Domestic Fixed Income Investments [Member]        
Liabilities Fair Value        
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 31.00% 25.00%    
Gulf Power [Member] | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets $ 1 $ 1    
Gulf Power [Member] | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 2 2    
Gulf Power [Member] | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 1 1    
Gulf Power [Member] | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 2 1    
Gulf Power [Member] | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 3 3    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 2 $ 2    
Liabilities Fair Value        
Target plan asset allocations (as percent) 14.00%      
Actual plan asset allocations (as percent) 13.00% 16.00%    
Gulf Power [Member] | Other postretirement benefit plans | Special situations        
Liabilities Fair Value        
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 2.00% 2.00%    
Gulf Power [Member] | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets $ 1 $ 1    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 1 $ 1    
Liabilities Fair Value        
Target plan asset allocations (as percent) 9.00%      
Actual plan asset allocations (as percent) 5.00% 6.00%    
Gulf Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets $ 210 $ 141    
Gulf Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 93 73    
Gulf Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 57 54    
Gulf Power [Member] | 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 [Member] | 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    
Gulf Power [Member] | 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 [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 46 0    
Gulf Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 14 14    
Gulf Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Gulf Power [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 8 7    
Gulf Power [Member] | 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    
Gulf Power [Member] | 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 [Member] | 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 [Member] | 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 [Member] | 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 [Member] | 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    
Gulf Power [Member] | 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 [Member] | 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 [Member] | Significant Other Observable Inputs (Level 2) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets 194 187    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 43 31    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 52 45    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets 27 21    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 1 9    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 47 51    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 24 23    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 7    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 8 7    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 2 1    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 2 2    
Gulf Power [Member] | 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 [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 2 2    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 1 1    
Gulf Power [Member] | 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 [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | 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 [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | 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 [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Gulf Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets 499 430   446
Assets Fair Value        
Fair Value, Plan Assets 498 423    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 88 $ 87    
Liabilities Fair Value        
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
Mississippi Power [Member] | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets $ 139 $ 108    
Liabilities Fair Value        
Target plan asset allocations (as percent) 26.00%      
Actual plan asset allocations (as percent) 29.00% 30.00%    
Mississippi Power [Member] | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets $ 109 $ 101    
Liabilities Fair Value        
Target plan asset allocations (as percent) 25.00%      
Actual plan asset allocations (as percent) 22.00% 23.00%    
Mississippi Power [Member] | Pension plans | Fixed income        
Liabilities Fair Value        
Target plan asset allocations (as percent) 23.00%      
Actual plan asset allocations (as percent) 29.00% 23.00%    
Mississippi Power [Member] | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets $ 28 $ 21    
Mississippi Power [Member] | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 1 9    
Mississippi Power [Member] | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 46 53    
Mississippi Power [Member] | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 25 23    
Mississippi Power [Member] | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 47 7    
Mississippi Power [Member] | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 69 71    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 54 $ 57    
Liabilities Fair Value        
Target plan asset allocations (as percent) 14.00%      
Actual plan asset allocations (as percent) 13.00% 16.00%    
Mississippi Power [Member] | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets $ 8      
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 8      
Liabilities Fair Value        
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 2.00% 2.00%    
Mississippi Power [Member] | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets $ 26 $ 30    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 26 $ 30    
Liabilities Fair Value        
Target plan asset allocations (as percent) 9.00%      
Actual plan asset allocations (as percent) 5.00% 6.00%    
Mississippi Power [Member] | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets $ 23 $ 23   24
Assets Fair Value        
Fair Value, Plan Assets 24 23    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 3 $ 4    
Liabilities Fair Value        
Target plan asset allocations (as percent) 100.00%      
Actual plan asset allocations (as percent) 100.00% 100.00%    
Mississippi Power [Member] | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets $ 6 $ 4    
Liabilities Fair Value        
Target plan asset allocations (as percent) 21.00%      
Actual plan asset allocations (as percent) 23.00% 24.00%    
Mississippi Power [Member] | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets $ 4 $ 4    
Liabilities Fair Value        
Target plan asset allocations (as percent) 20.00%      
Actual plan asset allocations (as percent) 18.00% 18.00%    
Mississippi Power [Member] | Other postretirement benefit plans | Fixed income        
Liabilities Fair Value        
Target plan asset allocations (as percent) 38.00%      
Actual plan asset allocations (as percent) 43.00% 38.00%    
Mississippi Power [Member] | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets $ 5 $ 6    
Mississippi Power [Member] | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 2 2    
Mississippi Power [Member] | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 1 1    
Mississippi Power [Member] | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 2 1    
Mississippi Power [Member] | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 3 4    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 2 $ 3    
Liabilities Fair Value        
Target plan asset allocations (as percent) 11.00%      
Actual plan asset allocations (as percent) 10.00% 13.00%    
Mississippi Power [Member] | Other postretirement benefit plans | Special situations        
Liabilities Fair Value        
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 2.00% 2.00%    
Mississippi Power [Member] | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets $ 1 $ 1    
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 1 $ 1    
Liabilities Fair Value        
Target plan asset allocations (as percent) 7.00%      
Actual plan asset allocations (as percent) 4.00% 5.00%    
Mississippi Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets $ 215 $ 145    
Mississippi Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 95 76    
Mississippi Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 58 55    
Mississippi Power [Member] | 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 [Member] | 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    
Mississippi Power [Member] | 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 [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 47 0    
Mississippi Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 15 14    
Mississippi Power [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Mississippi Power [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 9 7    
Mississippi Power [Member] | 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    
Mississippi Power [Member] | 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    
Mississippi Power [Member] | 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 [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | 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 [Member] | 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 [Member] | 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    
Mississippi Power [Member] | 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 [Member] | 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 [Member] | Significant Other Observable Inputs (Level 2) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets 195 191    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 44 32    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 51 46    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets 28 21    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 1 9    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 46 53    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 25 23    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 7    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 12 12    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 2 1    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 2 2    
Mississippi Power [Member] | 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 6    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 2 2    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 1 1    
Mississippi Power [Member] | 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 [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | 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 [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations        
Assets Fair Value        
Fair Value, Plan Assets 0      
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | 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 [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Mortgage- and asset-backed securities        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Mississippi Power [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0 0    
Successor [Member] | Southern Company Gas [Member] | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets 983   $ 837  
Assets Fair Value        
Fair Value, Plan Assets 983      
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets 100      
Successor [Member] | Southern Company Gas [Member] | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 485      
Successor [Member] | Southern Company Gas [Member] | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 185      
Successor [Member] | Southern Company Gas [Member] | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets 85      
Successor [Member] | Southern Company Gas [Member] | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 41      
Successor [Member] | Southern Company Gas [Member] | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 66      
Successor [Member] | Southern Company Gas [Member] | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 100      
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets 83      
Successor [Member] | Southern Company Gas [Member] | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 19      
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets 15      
Successor [Member] | Southern Company Gas [Member] | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 2      
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets 2      
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets 105   100  
Assets Fair Value        
Fair Value, Plan Assets 105      
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 2      
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Cash and Cash Equivalents [Member]        
Liabilities Fair Value        
Target plan asset allocations (as percent) 1.00%      
Actual plan asset allocations (as percent) 1.00%      
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets $ 61      
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets $ 18      
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Fixed income        
Liabilities Fair Value        
Target plan asset allocations (as percent) 24.00%      
Actual plan asset allocations (as percent) 23.00%      
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets $ 23      
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 3      
Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets $ 2      
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Equity Securities        
Liabilities Fair Value        
Target plan asset allocations (as percent) 72.00%      
Actual plan asset allocations (as percent) 74.00%      
Successor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Other Types Of Investments [Member]        
Liabilities Fair Value        
Target plan asset allocations (as percent) 3.00%      
Actual plan asset allocations (as percent) 2.00%      
Successor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets $ 158      
Successor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 142      
Successor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | 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      
Successor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 12      
Successor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 4      
Successor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 4      
Successor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 3      
Successor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | 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      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets 725      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 343      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 185      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets 85      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 41      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 66      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 5      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 99      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 58      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 18      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 23      
Successor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Domestic equity        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | International equity        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Pooled funds        
Assets Fair Value        
Fair Value, Plan Assets 0      
Successor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Cash equivalents and other        
Assets Fair Value        
Fair Value, Plan Assets $ 0      
Predecessor [Member] | Southern Company Gas [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Other Postretirement Benefits Payable   9    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 847 837 906
Liabilities Fair Value        
Actual plan asset allocations (as percent)   100.00%    
% of Fair Value of Plan Assets   100.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | Cash and Cash Equivalents [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 4    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   0.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | Fixed income        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 242    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   29.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | U.S. Treasury, government, and agency bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 151    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   18.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | Corporate bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 91    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   11.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | Private equity        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 20    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   2.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | US Large Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 274    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   32.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | US Small Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 81    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   9.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | International Companies [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 125    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   15.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | Emerging Markets [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 28    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   3.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | Equity Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 508    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   59.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | Hedge Funds, Global Opportunity [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 40    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   5.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | Absolute Return [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 42    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   5.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension plans | Other Types Of Investments [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 102    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   12.00%    
Predecessor [Member] | Southern Company Gas [Member] | Pension Plans, Including 401H Portion of Other Retirement Benefit [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 856    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 99 $ 100 $ 99
Liabilities Fair Value        
Actual plan asset allocations (as percent)   100.00%    
% of Fair Value of Plan Assets   100.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Cash and Cash Equivalents [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 1    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   1.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Fixed income        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 22    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   24.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   0.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Corporate bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 22    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   24.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Private equity        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   0.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | US Large Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 52    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   58.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | US Small Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   0.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | International Companies [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 15    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   17.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Emerging Markets [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   0.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Equity Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 67    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   75.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Hedge Funds, Global Opportunity [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   0.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Absolute Return [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   0.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other postretirement benefit plans | Other Types Of Investments [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0    
Liabilities Fair Value        
Actual plan asset allocations (as percent)   0.00%    
Predecessor [Member] | Southern Company Gas [Member] | Other Postretirement Benefits, Excluding 401H Accounts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 90    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 136    
Liabilities Fair Value        
% of Fair Value of Plan Assets   16.00%    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash and Cash Equivalents [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 4    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Fixed income        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | US Large Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   75    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | US Small Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   57    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International Companies [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Emerging Markets [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Equity Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   132    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Hedge Funds, Global Opportunity [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Absolute Return [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Other Types Of Investments [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 1    
Liabilities Fair Value        
% of Fair Value of Plan Assets   1.00%    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash and Cash Equivalents [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 1    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Fixed income        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | US Large Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | US Small Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International Companies [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Emerging Markets [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Equity Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Hedge Funds, Global Opportunity [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Absolute Return [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Other Types Of Investments [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 618    
Liabilities Fair Value        
% of Fair Value of Plan Assets   72.00%    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Cash and Cash Equivalents [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Fixed income        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   242    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   151    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   91    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | US Large Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   199    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | US Small Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   24    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | International Companies [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   125    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Emerging Markets [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   28    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Equity Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   376    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Hedge Funds, Global Opportunity [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Absolute Return [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Pension plans | Other Types Of Investments [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 89    
Liabilities Fair Value        
% of Fair Value of Plan Assets   99.00%    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash and Cash Equivalents [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Fixed income        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   22    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   22    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | US Large Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   52    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | US Small Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International Companies [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   15    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Emerging Markets [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Equity Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   67    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Hedge Funds, Global Opportunity [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Absolute Return [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Other Types Of Investments [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 102    
Liabilities Fair Value        
% of Fair Value of Plan Assets   12.00%    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Cash and Cash Equivalents [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Fixed income        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   20    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | US Large Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | US Small Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | International Companies [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Emerging Markets [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Equity Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Hedge Funds, Global Opportunity [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   40    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Absolute Return [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   42    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Pension plans | Other Types Of Investments [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   102    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0    
Liabilities Fair Value        
% of Fair Value of Plan Assets   0.00%    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Cash and Cash Equivalents [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Fixed income        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Corporate bonds        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Private equity        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | US Large Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | US Small Cap [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | International Companies [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Emerging Markets [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Equity Securities        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Hedge Funds, Global Opportunity [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Absolute Return [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   0    
Predecessor [Member] | Southern Company Gas [Member] | Significant Unobservable Inputs (Level 3) | Other postretirement benefit plans | Other Types Of Investments [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0