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RETIREMENT BENEFITS - Fair Values of Pension Plan and Other Postretirement Benefit Plan Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Assets:    
Target Allocation 100.00%  
Actual Allocation 100.00%  
Pension plans    
Assets:    
Plan assets $ 17,225 $ 15,430
Target Allocation   100.00%
Actual Allocation   100.00%
Pension plans | Equity    
Assets:    
Target Allocation 51.00% 51.00%
Actual Allocation 53.00% 56.00%
Pension plans | Domestic equity    
Assets:    
Plan assets $ 4,421 $ 4,099
Pension plans | International equity    
Assets:    
Plan assets $ 4,145 $ 4,157
Pension plans | Fixed income    
Assets:    
Target Allocation 23.00% 23.00%
Actual Allocation 22.00% 23.00%
Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 1,209 $ 951
Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 10 9
Pension plans | Corporate bonds    
Assets:    
Plan assets 1,752 1,673
Pension plans | Pooled funds    
Assets:    
Plan assets 771 772
Pension plans | Cash equivalents and other    
Assets:    
Plan assets 412 361
Pension plans | Real estate investments    
Assets:    
Plan assets $ 2,744 $ 2,138
Target Allocation 14.00% 14.00%
Actual Allocation 15.00% 13.00%
Pension plans | Special situations    
Assets:    
Plan assets $ 171 $ 166
Target Allocation 3.00% 3.00%
Actual Allocation 1.00% 1.00%
Pension plans | Private equity    
Assets:    
Plan assets $ 1,590 $ 1,104
Target Allocation 9.00% 9.00%
Actual Allocation 9.00% 7.00%
Other postretirement benefit plans    
Assets:    
Plan assets $ 1,249 $ 1,159
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Other postretirement benefit plans | Equity    
Assets:    
Target Allocation 64.00% 63.00%
Actual Allocation 66.00% 66.00%
Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets $ 235 $ 211
Other postretirement benefit plans | International equity    
Assets:    
Plan assets $ 172 $ 173
Other postretirement benefit plans | Fixed income    
Assets:    
Target Allocation 27.00% 28.00%
Actual Allocation 25.00% 27.00%
Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 37 $ 32
Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 50 44
Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 90 86
Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 14 15
Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 530 508
Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets $ 74 $ 57
Target Allocation 5.00% 5.00%
Actual Allocation 6.00% 5.00%
Other postretirement benefit plans | Special situations    
Assets:    
Plan assets $ 5 $ 4
Target Allocation 1.00% 1.00%
Actual Allocation 0.00% 0.00%
Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 42 $ 29
Target Allocation 3.00% 3.00%
Actual Allocation 3.00% 2.00%
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets:    
Plan assets $ 6,946 $ 6,410
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets:    
Plan assets 3,095 2,852
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets:    
Plan assets 2,740 2,660
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 405 356
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets:    
Plan assets 706 542
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets:    
Plan assets 230 214
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 123 113
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 73 71
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 14 15
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 20 15
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Pension plans    
Assets:    
Plan assets 6,477 6,154
Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets:    
Plan assets 1,326 1,247
Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets:    
Plan assets 1,402 1,497
Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 1,209 951
Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 10 9
Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets:    
Plan assets 1,752 1,673
Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets:    
Plan assets 771 772
Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 7 5
Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets:    
Plan assets 918 870
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 112 98
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 99 102
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 37 32
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 50 44
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 90 86
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 530 508
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Significant Unobservable Inputs (Level 3) | Pension plans    
Assets:    
Plan assets 3  
Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity    
Assets:    
Plan assets 0  
Significant Unobservable Inputs (Level 3) | Pension plans | International equity    
Assets:    
Plan assets 3  
Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0  
Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0  
Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0  
Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds    
Assets:    
Plan assets 0  
Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0  
Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments    
Assets:    
Plan assets 0  
Significant Unobservable Inputs (Level 3) | Pension plans | Special situations    
Assets:    
Plan assets 0  
Significant Unobservable Inputs (Level 3) | Pension plans | Private equity    
Assets:    
Plan assets 0  
Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets:    
Plan assets 3,799 2,866
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets:    
Plan assets 2,038 1,596
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets:    
Plan assets 171 166
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets:    
Plan assets 1,590 1,104
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets:    
Plan assets 101 75
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 54 42
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 5 4
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 42 29
Alabama Power | Pension plans    
Assets:    
Plan assets $ 4,141 $ 3,699
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Alabama Power | Pension plans | Equity    
Assets:    
Target Allocation 51.00% 51.00%
Actual Allocation 53.00% 56.00%
Alabama Power | Pension plans | Domestic equity    
Assets:    
Plan assets $ 1,062 $ 984
Alabama Power | Pension plans | International equity    
Assets:    
Plan assets $ 997 $ 997
Alabama Power | Pension plans | Fixed income    
Assets:    
Target Allocation 23.00% 23.00%
Actual Allocation 22.00% 23.00%
Alabama Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 291 $ 228
Alabama Power | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 2 2
Alabama Power | Pension plans | Corporate bonds    
Assets:    
Plan assets 421 401
Alabama Power | Pension plans | Pooled funds    
Assets:    
Plan assets 186 185
Alabama Power | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 99 86
Alabama Power | Pension plans | Real estate investments    
Assets:    
Plan assets $ 660 $ 512
Target Allocation 14.00% 14.00%
Actual Allocation 15.00% 13.00%
Alabama Power | Pension plans | Special situations    
Assets:    
Plan assets $ 41 $ 40
Target Allocation 3.00% 3.00%
Actual Allocation 1.00% 1.00%
Alabama Power | Pension plans | Private equity    
Assets:    
Plan assets $ 382 $ 264
Target Allocation 9.00% 9.00%
Actual Allocation 9.00% 7.00%
Alabama Power | Other postretirement benefit plans    
Assets:    
Plan assets $ 488 $ 459
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Alabama Power | Other postretirement benefit plans | Equity    
Assets:    
Target Allocation 71.00% 68.00%
Actual Allocation 69.00% 69.00%
Alabama Power | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets $ 37 $ 37
Alabama Power | Other postretirement benefit plans | International equity    
Assets:    
Plan assets $ 35 $ 36
Alabama Power | Other postretirement benefit plans | Fixed income    
Assets:    
Target Allocation 21.00% 24.00%
Actual Allocation 21.00% 25.00%
Alabama Power | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 10 $ 11
Alabama Power | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 18 14
Alabama Power | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 6 7
Alabama Power | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 3 5
Alabama Power | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 341 321
Alabama Power | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets $ 23 $ 18
Target Allocation 4.00% 4.00%
Actual Allocation 7.00% 4.00%
Alabama Power | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets $ 2 $ 1
Target Allocation 1.00% 1.00%
Actual Allocation 0.00% 0.00%
Alabama Power | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 13 $ 9
Target Allocation 3.00% 3.00%
Actual Allocation 3.00% 2.00%
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets:    
Plan assets $ 1,669 $ 1,538
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets:    
Plan assets 743 685
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets:    
Plan assets 659 638
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 97 85
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets:    
Plan assets 170 130
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets:    
Plan assets 58 59
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 26 26
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 23 23
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 3 5
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 6 5
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets:    
Plan assets 1,558 1,475
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets:    
Plan assets 319 299
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets:    
Plan assets 337 359
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 291 228
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 2 2
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets:    
Plan assets 421 401
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets:    
Plan assets 186 185
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 2 1
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets:    
Plan assets 398 377
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 11 11
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 12 13
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 10 11
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 18 14
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 6 7
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 341 321
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans    
Assets:    
Plan assets 1  
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity    
Assets:    
Plan assets 0  
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | International equity    
Assets:    
Plan assets 1  
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0  
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0  
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0  
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds    
Assets:    
Plan assets 0  
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0  
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments    
Assets:    
Plan assets 0  
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations    
Assets:    
Plan assets 0  
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity    
Assets:    
Plan assets 0  
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets:    
Plan assets 913 686
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets:    
Plan assets 490 382
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets:    
Plan assets 41 40
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets:    
Plan assets 382 264
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets:    
Plan assets 32 23
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 17 13
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 2 1
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 13 9
Georgia Power | Pension plans    
Assets:    
Plan assets $ 5,415 $ 4,864
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Georgia Power | Pension plans | Equity    
Assets:    
Target Allocation 51.00% 51.00%
Actual Allocation 53.00% 56.00%
Georgia Power | Pension plans | Domestic equity    
Assets:    
Plan assets $ 1,389 $ 1,292
Georgia Power | Pension plans | International equity    
Assets:    
Plan assets $ 1,303 $ 1,311
Georgia Power | Pension plans | Fixed income    
Assets:    
Target Allocation 23.00% 23.00%
Actual Allocation 22.00% 23.00%
Georgia Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 380 $ 300
Georgia Power | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 3 3
Georgia Power | Pension plans | Corporate bonds    
Assets:    
Plan assets 551 527
Georgia Power | Pension plans | Pooled funds    
Assets:    
Plan assets 243 243
Georgia Power | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 129 113
Georgia Power | Pension plans | Real estate investments    
Assets:    
Plan assets $ 863 $ 674
Target Allocation 14.00% 14.00%
Actual Allocation 15.00% 13.00%
Georgia Power | Pension plans | Special situations    
Assets:    
Plan assets $ 54 $ 53
Target Allocation 3.00% 3.00%
Actual Allocation 1.00% 1.00%
Georgia Power | Pension plans | Private equity    
Assets:    
Plan assets $ 500 $ 348
Target Allocation 9.00% 9.00%
Actual Allocation 9.00% 7.00%
Georgia Power | Other postretirement benefit plans    
Assets:    
Plan assets $ 450 $ 427
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Georgia Power | Other postretirement benefit plans | Equity    
Assets:    
Target Allocation 60.00% 60.00%
Actual Allocation 62.00% 64.00%
Georgia Power | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets $ 78 $ 68
Georgia Power | Other postretirement benefit plans | International equity    
Assets:    
Plan assets $ 72 $ 71
Georgia Power | Other postretirement benefit plans | Fixed income    
Assets:    
Target Allocation 33.00% 33.00%
Actual Allocation 30.00% 30.00%
Georgia Power | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 9 $ 8
Georgia Power | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 14 13
Georgia Power | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 46 46
Georgia Power | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 5 5
Georgia Power | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 189 188
Georgia Power | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets $ 23 $ 18
Target Allocation 4.00% 4.00%
Actual Allocation 5.00% 4.00%
Georgia Power | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets $ 1 $ 1
Target Allocation 1.00% 1.00%
Actual Allocation 0.00% 0.00%
Georgia Power | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 13 $ 9
Target Allocation 2.00% 2.00%
Actual Allocation 3.00% 2.00%
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets:    
Plan assets $ 2,182 $ 2,021
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets:    
Plan assets 972 899
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets:    
Plan assets 861 839
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 127 112
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets:    
Plan assets 222 171
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets:    
Plan assets 99 89
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 65 58
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 22 21
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 5 5
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 7 5
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets:    
Plan assets 2,037 1,939
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets:    
Plan assets 417 393
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets:    
Plan assets 441 472
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 380 300
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 3 3
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets:    
Plan assets 551 527
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets:    
Plan assets 243 243
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 2 1
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets:    
Plan assets 321 315
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 13 10
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 50 50
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 9 8
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 14 13
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 46 46
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 189 188
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans    
Assets:    
Plan assets 1  
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity    
Assets:    
Plan assets 0  
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | International equity    
Assets:    
Plan assets 1  
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0  
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0  
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0  
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds    
Assets:    
Plan assets 0  
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0  
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments    
Assets:    
Plan assets 0  
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations    
Assets:    
Plan assets 0  
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity    
Assets:    
Plan assets 0  
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets:    
Plan assets 1,195 904
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets:    
Plan assets 641 503
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets:    
Plan assets 54 53
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets:    
Plan assets 500 348
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets:    
Plan assets 30 23
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 16 13
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 1 1
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 13 9
Mississippi Power | Pension plans    
Assets:    
Plan assets $ 787 $ 704
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Mississippi Power | Pension plans | Equity    
Assets:    
Target Allocation 51.00% 51.00%
Actual Allocation 53.00% 56.00%
Mississippi Power | Pension plans | Domestic equity    
Assets:    
Plan assets $ 203 $ 188
Mississippi Power | Pension plans | International equity    
Assets:    
Plan assets $ 190 $ 190
Mississippi Power | Pension plans | Fixed income    
Assets:    
Target Allocation 23.00% 23.00%
Actual Allocation 22.00% 23.00%
Mississippi Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 55 $ 43
Mississippi Power | Pension plans | Corporate bonds    
Assets:    
Plan assets 80 76
Mississippi Power | Pension plans | Pooled funds    
Assets:    
Plan assets 35 35
Mississippi Power | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 18 16
Mississippi Power | Pension plans | Real estate investments    
Assets:    
Plan assets $ 125 $ 98
Target Allocation 14.00% 14.00%
Actual Allocation 15.00% 13.00%
Mississippi Power | Pension plans | Special situations    
Assets:    
Plan assets $ 8 $ 8
Target Allocation 3.00% 3.00%
Actual Allocation 1.00% 1.00%
Mississippi Power | Pension plans | Private equity    
Assets:    
Plan assets $ 73 $ 50
Target Allocation 9.00% 9.00%
Actual Allocation 9.00% 7.00%
Mississippi Power | Other postretirement benefit plans    
Assets:    
Plan assets $ 27 $ 26
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Mississippi Power | Other postretirement benefit plans | Equity    
Assets:    
Target Allocation 43.00% 43.00%
Actual Allocation 44.00% 46.00%
Mississippi Power | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets $ 6 $ 6
Mississippi Power | Other postretirement benefit plans | International equity    
Assets:    
Plan assets $ 6 $ 6
Mississippi Power | Other postretirement benefit plans | Fixed income    
Assets:    
Target Allocation 36.00% 37.00%
Actual Allocation 34.00% 36.00%
Mississippi Power | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 5 $ 5
Mississippi Power | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 2 2
Mississippi Power | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 1 1
Mississippi Power | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 1 1
Mississippi Power | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets $ 4 $ 3
Target Allocation 11.00% 11.00%
Actual Allocation 13.00% 11.00%
Mississippi Power | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets $ 0 $ 0
Target Allocation 2.00% 2.00%
Actual Allocation 1.00% 1.00%
Mississippi Power | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 2 $ 2
Target Allocation 8.00% 7.00%
Actual Allocation 8.00% 6.00%
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets:    
Plan assets $ 318 $ 294
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets:    
Plan assets 142 131
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets:    
Plan assets 126 122
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 18 16
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets:    
Plan assets 32 25
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets:    
Plan assets 10 10
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 4 4
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 4 4
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 1 1
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 1 1
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets:    
Plan assets 295 279
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets:    
Plan assets 61 57
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets:    
Plan assets 64 68
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 55 43
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets:    
Plan assets 80 76
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets:    
Plan assets 35 35
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets:    
Plan assets 12 12
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 2 2
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 2 2
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 5 5
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 2 2
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 1 1
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans    
Assets:    
Plan assets 0  
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity    
Assets:    
Plan assets 0  
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | International equity    
Assets:    
Plan assets 0  
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0  
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0  
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds    
Assets:    
Plan assets 0  
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0  
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments    
Assets:    
Plan assets 0  
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations    
Assets:    
Plan assets 0  
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity    
Assets:    
Plan assets 0  
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets:    
Plan assets 174 131
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets:    
Plan assets 93 73
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets:    
Plan assets 8 8
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets:    
Plan assets 73 50
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets:    
Plan assets 5 4
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 3 2
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 2 2
Southern Power | Pension plans    
Assets:    
Plan assets $ 213 $ 187
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Southern Power | Pension plans | Equity    
Assets:    
Target Allocation 51.00% 51.00%
Actual Allocation 53.00% 56.00%
Southern Power | Pension plans | Domestic equity    
Assets:    
Plan assets $ 54 $ 50
Southern Power | Pension plans | International equity    
Assets:    
Plan assets $ 51 $ 51
Southern Power | Pension plans | Fixed income    
Assets:    
Target Allocation 23.00% 23.00%
Actual Allocation 22.00% 23.00%
Southern Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 15 $ 12
Southern Power | Pension plans | Corporate bonds    
Assets:    
Plan assets 22 20
Southern Power | Pension plans | Pooled funds    
Assets:    
Plan assets 10 9
Southern Power | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 5 4
Southern Power | Pension plans | Real estate investments    
Assets:    
Plan assets $ 34 $ 26
Target Allocation 14.00% 14.00%
Actual Allocation 15.00% 13.00%
Southern Power | Pension plans | Special situations    
Assets:    
Plan assets $ 2 $ 2
Target Allocation 3.00% 3.00%
Actual Allocation 1.00% 1.00%
Southern Power | Pension plans | Private equity    
Assets:    
Plan assets $ 20 $ 13
Target Allocation 9.00% 9.00%
Actual Allocation 9.00% 7.00%
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets:    
Plan assets $ 86 $ 78
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets:    
Plan assets 38 35
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets:    
Plan assets 34 32
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 5 4
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets:    
Plan assets 9 7
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets:    
Plan assets 80 75
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets:    
Plan assets 16 15
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets:    
Plan assets 17 19
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 15 12
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets:    
Plan assets 22 20
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets:    
Plan assets 10 9
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans    
Assets:    
Plan assets 0  
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity    
Assets:    
Plan assets 0  
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | International equity    
Assets:    
Plan assets 0  
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0  
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0  
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds    
Assets:    
Plan assets 0  
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0  
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments    
Assets:    
Plan assets 0  
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations    
Assets:    
Plan assets 0  
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity    
Assets:    
Plan assets 0  
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets:    
Plan assets 47 34
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets:    
Plan assets 25 19
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets:    
Plan assets 2 2
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets:    
Plan assets 20 13
Southern Company Gas | Pension plans    
Assets:    
Plan assets $ 1,241 $ 1,128
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Southern Company Gas | Pension plans | Equity    
Assets:    
Target Allocation 51.00% 51.00%
Actual Allocation 53.00% 56.00%
Southern Company Gas | Pension plans | Domestic equity    
Assets:    
Plan assets $ 319 $ 300
Southern Company Gas | Pension plans | International equity    
Assets:    
Plan assets $ 298 $ 304
Southern Company Gas | Pension plans | Fixed income    
Assets:    
Target Allocation 23.00% 23.00%
Actual Allocation 22.00% 23.00%
Southern Company Gas | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 87 $ 69
Southern Company Gas | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 1 1
Southern Company Gas | Pension plans | Corporate bonds    
Assets:    
Plan assets 126 122
Southern Company Gas | Pension plans | Pooled funds    
Assets:    
Plan assets 56 56
Southern Company Gas | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 29 26
Southern Company Gas | Pension plans | Real estate investments    
Assets:    
Plan assets $ 198 $ 157
Target Allocation 14.00% 14.00%
Actual Allocation 15.00% 13.00%
Southern Company Gas | Pension plans | Special situations    
Assets:    
Plan assets $ 12 $ 12
Target Allocation 3.00% 3.00%
Actual Allocation 1.00% 1.00%
Southern Company Gas | Pension plans | Private equity    
Assets:    
Plan assets $ 115 $ 81
Target Allocation 9.00% 9.00%
Actual Allocation 9.00% 7.00%
Southern Company Gas | Other postretirement benefit plans    
Assets:    
Plan assets $ 143 $ 125
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Southern Company Gas | Other postretirement benefit plans | Equity    
Assets:    
Target Allocation 72.00% 72.00%
Actual Allocation 73.00% 76.00%
Southern Company Gas | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets $ 79 $ 68
Southern Company Gas | Other postretirement benefit plans | International equity    
Assets:    
Plan assets $ 26 $ 27
Southern Company Gas | Other postretirement benefit plans | Fixed income    
Assets:    
Target Allocation 26.00% 26.00%
Actual Allocation 24.00% 22.00%
Southern Company Gas | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 1 $ 1
Southern Company Gas | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 1 1
Southern Company Gas | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 30 25
Southern Company Gas | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 2 1
Southern Company Gas | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets $ 3 $ 1
Target Allocation 1.00% 1.00%
Actual Allocation 2.00% 1.00%
Southern Company Gas | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 1 $ 1
Target Allocation 1.00% 1.00%
Actual Allocation 1.00% 1.00%
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets:    
Plan assets $ 500 $ 470
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets:    
Plan assets 223 209
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets:    
Plan assets 197 195
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 29 26
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets:    
Plan assets 51 40
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets:    
Plan assets 8 5
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 3 2
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 2 2
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 2 1
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 1 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets:    
Plan assets 467 448
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets:    
Plan assets 96 91
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets:    
Plan assets 101 109
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 87 69
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 1 1
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets:    
Plan assets 126 122
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets:    
Plan assets 56 56
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets:    
Plan assets 132 118
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 76 66
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 24 25
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 1 1
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 1 1
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 30 25
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans    
Assets:    
Plan assets 0  
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity    
Assets:    
Plan assets 0  
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | International equity    
Assets:    
Plan assets 0  
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0  
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0  
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0  
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds    
Assets:    
Plan assets 0  
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0  
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments    
Assets:    
Plan assets 0  
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations    
Assets:    
Plan assets 0  
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity    
Assets:    
Plan assets 0  
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets:    
Plan assets 274 210
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets:    
Plan assets 147 117
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets:    
Plan assets 12 12
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets:    
Plan assets 115 81
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets:    
Plan assets 3 2
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 2 1
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 1 $ 1