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RETIREMENT BENEFITS - Fair Values of Pension Plan and Other Postretirement Benefit Plan Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Liabilities Fair Value    
Target plan asset allocations 100.00%  
Actual plan asset allocations 100.00%  
Pension plans    
Assets Fair Value    
Fair value, plan assets $ 14,119 $ 11,631
Liabilities Fair Value    
Derivatives (1)  
Total $ 14,118  
Target plan asset allocations   100.00%
Actual plan asset allocations   100.00%
Pension plans | Equity    
Liabilities Fair Value    
Target plan asset allocations 51.00% 51.00%
Actual plan asset allocations 51.00% 53.00%
Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 3,118 $ 3,132
Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 3,646 $ 2,669
Pension plans | Fixed income    
Liabilities Fair Value    
Target plan asset allocations 23.00% 23.00%
Actual plan asset allocations 29.00% 24.00%
Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 965 $ 930
Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 9 7
Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 1,315 1,195
Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 684 654
Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 1,317 272
Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 1,957 $ 1,780
Liabilities Fair Value    
Target plan asset allocations 14.00% 14.00%
Actual plan asset allocations 12.00% 15.00%
Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 155 $ 171
Liabilities Fair Value    
Target plan asset allocations 3.00% 3.00%
Actual plan asset allocations 1.00% 1.00%
Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 953 $ 821
Liabilities Fair Value    
Target plan asset allocations 9.00% 9.00%
Actual plan asset allocations 7.00% 7.00%
Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets $ 1,060 $ 926
Liabilities Fair Value    
Target plan asset allocations 100.00% 100.00%
Actual plan asset allocations 100.00% 100.00%
Other postretirement benefit plans | Equity    
Liabilities Fair Value    
Target plan asset allocations 63.00% 62.00%
Actual plan asset allocations 64.00% 62.00%
Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 176 $ 176
Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 149 $ 120
Other postretirement benefit plans | Fixed income    
Liabilities Fair Value    
Target plan asset allocations 28.00% 29.00%
Actual plan asset allocations 30.00% 30.00%
Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 31 $ 34
Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 35 35
Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 82 81
Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 42 13
Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 463 386
Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 53 $ 53
Liabilities Fair Value    
Target plan asset allocations 5.00% 5.00%
Actual plan asset allocations 4.00% 5.00%
Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 4 $ 4
Liabilities Fair Value    
Target plan asset allocations 1.00% 1.00%
Actual plan asset allocations 0.00% 0.00%
Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 25 $ 24
Liabilities Fair Value    
Target plan asset allocations 3.00% 3.00%
Actual plan asset allocations 2.00% 3.00%
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 6,436 $ 4,135
Liabilities Fair Value    
Derivatives (1)  
Total 6,435  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 2,220 2,102
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 2,360 1,344
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 1,317 270
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 539 419
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 221 171
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 95 100
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 69 45
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 42 13
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 15 13
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Significant Other Observable Inputs (Level 2) | Pension plans    
Assets Fair Value    
Fair value, plan assets 5,157 5,143
Liabilities Fair Value    
Derivatives 0  
Total 5,157  
Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 898 1,030
Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 1,286 1,325
Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 965 930
Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 9 7
Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 1,315 1,195
Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 684 654
Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 2
Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 0 0
Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 772 687
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 81 76
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 80 75
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 31 34
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 35 35
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 82 81
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 463 386
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
Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets Fair Value    
Fair value, plan assets 2,526 2,353
Liabilities Fair Value    
Derivatives 0  
Total 2,526  
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 1,418 1,361
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 155 171
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 953 821
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 67 68
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 38 40
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 4 4
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets 25 24
Alabama Power | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 3,372 $ 2,579
Liabilities Fair Value    
Target plan asset allocations 100.00% 100.00%
Actual plan asset allocations 100.00% 100.00%
Alabama Power | Pension plans | Equity    
Liabilities Fair Value    
Target plan asset allocations 51.00% 51.00%
Actual plan asset allocations 51.00% 53.00%
Alabama Power | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 744 $ 694
Alabama Power | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 871 $ 591
Alabama Power | Pension plans | Fixed income    
Liabilities Fair Value    
Target plan asset allocations 23.00% 23.00%
Actual plan asset allocations 29.00% 24.00%
Alabama Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 230 $ 206
Alabama Power | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 2 2
Alabama Power | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 314 265
Alabama Power | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 163 145
Alabama Power | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 315 61
Alabama Power | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 468 $ 395
Liabilities Fair Value    
Target plan asset allocations 14.00% 14.00%
Actual plan asset allocations 12.00% 15.00%
Alabama Power | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 37 $ 38
Liabilities Fair Value    
Target plan asset allocations 3.00% 3.00%
Actual plan asset allocations 1.00% 1.00%
Alabama Power | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 228 $ 182
Liabilities Fair Value    
Target plan asset allocations 9.00% 9.00%
Actual plan asset allocations 7.00% 7.00%
Alabama Power | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets $ 412 $ 359
Liabilities Fair Value    
Target plan asset allocations 100.00% 100.00%
Actual plan asset allocations 100.00% 100.00%
Alabama Power | Other postretirement benefit plans | Equity    
Liabilities Fair Value    
Target plan asset allocations 68.00% 64.00%
Actual plan asset allocations 67.00% 66.00%
Alabama Power | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 34 $ 45
Alabama Power | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 32 $ 24
Alabama Power | Other postretirement benefit plans | Fixed income    
Liabilities Fair Value    
Target plan asset allocations 24.00% 28.00%
Actual plan asset allocations 27.00% 28.00%
Alabama Power | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 10 $ 10
Alabama Power | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 11 11
Alabama Power | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 6 6
Alabama Power | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 12 3
Alabama Power | Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 281 233
Alabama Power | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 17 $ 17
Liabilities Fair Value    
Target plan asset allocations 4.00% 4.00%
Actual plan asset allocations 4.00% 4.00%
Alabama Power | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 1 $ 2
Liabilities Fair Value    
Target plan asset allocations 1.00% 1.00%
Actual plan asset allocations 0.00% 0.00%
Alabama Power | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 8 $ 8
Liabilities Fair Value    
Target plan asset allocations 3.00% 3.00%
Actual plan asset allocations 2.00% 2.00%
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 1,538 $ 917
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 530 466
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 564 298
Alabama Power | 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 | 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 | 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 | 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 | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 315 60
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 129 93
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | 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 | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 64 54
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 26 35
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 21 12
Alabama Power | 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 | 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 | 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 | 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 12 3
Alabama Power | 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 | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 5 4
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | 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 | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets Fair Value    
Fair value, plan assets 1,230 1,140
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 214 228
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 307 293
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 230 206
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 2 2
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 314 265
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 163 145
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 1
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 327 282
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 8 10
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 11 12
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 10 10
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 11 11
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 6 6
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 281 233
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets Fair Value    
Fair value, plan assets 604 522
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 339 302
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 37 38
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 228 182
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 21 23
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, 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 Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 12 13
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 1 2
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets 8 8
Georgia Power | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 4,461 $ 3,669
Liabilities Fair Value    
Target plan asset allocations 100.00%  
Actual plan asset allocations 100.00% 100.00%
Georgia Power | Pension plans | Equity    
Liabilities Fair Value    
Target plan asset allocations 51.00% 51.00%
Actual plan asset allocations 51.00% 53.00%
Georgia Power | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 985 $ 988
Georgia Power | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 1,153 $ 842
Georgia Power | Pension plans | Fixed income    
Liabilities Fair Value    
Target plan asset allocations 23.00%  
Actual plan asset allocations 29.00% 24.00%
Georgia Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 305 $ 294
Georgia Power | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 3 2
Georgia Power | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 415 377
Georgia Power | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 216 206
Georgia Power | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 416 86
Georgia Power | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 618 $ 561
Liabilities Fair Value    
Target plan asset allocations 14.00%  
Actual plan asset allocations 12.00% 15.00%
Georgia Power | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 49 $ 54
Liabilities Fair Value    
Target plan asset allocations 3.00%  
Actual plan asset allocations 1.00% 1.00%
Georgia Power | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 301 $ 259
Liabilities Fair Value    
Target plan asset allocations 9.00%  
Actual plan asset allocations 7.00% 7.00%
Georgia Power | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets $ 402 $ 342
Liabilities Fair Value    
Target plan asset allocations 100.00% 100.00%
Actual plan asset allocations 100.00% 100.00%
Georgia Power | Other postretirement benefit plans | Equity    
Liabilities Fair Value    
Target plan asset allocations 60.00% 60.00%
Actual plan asset allocations 61.00% 59.00%
Georgia Power | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 55 $ 50
Georgia Power | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 61 $ 49
Georgia Power | Other postretirement benefit plans | Fixed income    
Liabilities Fair Value    
Target plan asset allocations 33.00% 33.00%
Actual plan asset allocations 34.00% 35.00%
Georgia Power | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 7 $ 7
Georgia Power | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 11 10
Georgia Power | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 45 44
Georgia Power | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 16 5
Georgia Power | Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 182 153
Georgia Power | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 16 $ 15
Liabilities Fair Value    
Target plan asset allocations 4.00% 4.00%
Actual plan asset allocations 3.00% 4.00%
Georgia Power | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 1 $ 2
Liabilities Fair Value    
Target plan asset allocations 1.00% 1.00%
Actual plan asset allocations 0.00% 0.00%
Georgia Power | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 8 $ 7
Liabilities Fair Value    
Target plan asset allocations 2.00% 2.00%
Actual plan asset allocations 2.00% 2.00%
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 2,033 $ 1,304
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 701 663
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 746 424
Georgia Power | 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 | 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 | 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 | 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 | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 416 85
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 170 132
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | 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 | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 94 67
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 48 41
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 25 17
Georgia Power | 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 | 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 | 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 | 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 16 5
Georgia Power | 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 | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 5 4
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | 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 | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets Fair Value    
Fair value, plan assets 1,630 1,623
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 284 325
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 407 418
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 305 294
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 3 2
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 415 377
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 216 206
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 1
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 288 255
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 7 9
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 36 32
Georgia Power | 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 7
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 11 10
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 45 44
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 182 153
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets Fair Value    
Fair value, plan assets 798 742
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 448 429
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 49 54
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 301 259
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 20 20
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, 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 Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 11 11
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 1 2
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets 8 7
Mississippi Power | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 644 $ 506
Liabilities Fair Value    
Target plan asset allocations 100.00% 100.00%
Actual plan asset allocations 100.00% 100.00%
Mississippi Power | Pension plans | Equity    
Liabilities Fair Value    
Target plan asset allocations 51.00%  
Actual plan asset allocations 51.00%  
Mississippi Power | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 142 $ 136
Mississippi Power | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 167 $ 118
Mississippi Power | Pension plans | Fixed income    
Liabilities Fair Value    
Target plan asset allocations 23.00% 23.00%
Actual plan asset allocations 29.00% 24.00%
Mississippi Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 44 $ 40
Mississippi Power | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 60 52
Mississippi Power | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 31 28
Mississippi Power | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 60 12
Mississippi Power | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 90 $ 77
Liabilities Fair Value    
Target plan asset allocations 14.00% 14.00%
Actual plan asset allocations 12.00% 15.00%
Mississippi Power | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 7 $ 7
Liabilities Fair Value    
Target plan asset allocations 3.00% 3.00%
Actual plan asset allocations 1.00% 1.00%
Mississippi Power | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 43 $ 36
Liabilities Fair Value    
Target plan asset allocations 9.00% 9.00%
Actual plan asset allocations 7.00% 7.00%
Mississippi Power | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets $ 25 $ 23
Liabilities Fair Value    
Target plan asset allocations 100.00% 100.00%
Actual plan asset allocations 100.00% 100.00%
Mississippi Power | Other postretirement benefit plans | Equity    
Liabilities Fair Value    
Target plan asset allocations 43.00% 41.00%
Actual plan asset allocations 41.00% 42.00%
Mississippi Power | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 4 $ 5
Mississippi Power | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 6 $ 4
Mississippi Power | Other postretirement benefit plans | Fixed income    
Liabilities Fair Value    
Target plan asset allocations 37.00% 38.00%
Actual plan asset allocations 42.00% 39.00%
Mississippi Power | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 6 $ 6
Mississippi Power | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 2 2
Mississippi Power | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 1 1
Mississippi Power | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 2 1
Mississippi Power | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 3 $ 3
Liabilities Fair Value    
Target plan asset allocations 11.00% 11.00%
Actual plan asset allocations 10.00% 12.00%
Mississippi Power | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 0 $ 0
Liabilities Fair Value    
Target plan asset allocations 2.00% 3.00%
Actual plan asset allocations 1.00% 1.00%
Mississippi Power | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 1 $ 1
Liabilities Fair Value    
Target plan asset allocations 7.00% 7.00%
Actual plan asset allocations 6.00% 6.00%
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 294 $ 180
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 101 91
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 108 59
Mississippi Power | 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 | 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 | 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 | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 60 12
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 25 18
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | 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 | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 10 7
Mississippi Power | 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
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 4 2
Mississippi Power | 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 | 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 | 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 | 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 | 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 | 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
Mississippi Power | 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 | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets Fair Value    
Fair value, plan assets 235 224
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 41 45
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 59 59
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 44 40
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 60 52
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 31 28
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 12 13
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 1 2
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 2 2
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 6 6
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 2 2
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 1 1
Mississippi Power | 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 | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets Fair Value    
Fair value, plan assets 115 102
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 65 59
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 7 7
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 43 36
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 3 3
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, 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 Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 2 2
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets 1 1
Southern Power | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 170 $ 124
Liabilities Fair Value    
Target plan asset allocations 100.00% 100.00%
Actual plan asset allocations 100.00% 100.00%
Southern Power | Pension plans | Equity    
Liabilities Fair Value    
Target plan asset allocations 51.00% 51.00%
Actual plan asset allocations 51.00% 53.00%
Southern Power | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 38 $ 33
Southern Power | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 44 $ 28
Southern Power | Pension plans | Fixed income    
Liabilities Fair Value    
Target plan asset allocations 23.00% 23.00%
Actual plan asset allocations 29.00% 24.00%
Southern Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 12 $ 10
Southern Power | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 16 13
Southern Power | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 8 7
Southern Power | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 16 3
Southern Power | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 23 $ 19
Liabilities Fair Value    
Target plan asset allocations 14.00% 14.00%
Actual plan asset allocations 12.00% 15.00%
Southern Power | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 2 $ 2
Liabilities Fair Value    
Target plan asset allocations 3.00% 3.00%
Actual plan asset allocations 1.00% 1.00%
Southern Power | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 11 $ 9
Liabilities Fair Value    
Target plan asset allocations 9.00% 9.00%
Actual plan asset allocations 7.00% 7.00%
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 77 $ 43
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 27 22
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 28 14
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 16 3
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 6 4
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets Fair Value    
Fair value, plan assets 63 55
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 11 11
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 16 14
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 12 10
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 16 13
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 8 7
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets Fair Value    
Fair value, plan assets 30 26
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 17 15
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 2 2
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 11 9
Southern Company Gas | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 1,054 $ 800
Liabilities Fair Value    
Target plan asset allocations 100.00% 100.00%
Actual plan asset allocations 100.00% 100.00%
Southern Company Gas | Pension plans | Equity    
Liabilities Fair Value    
Target plan asset allocations 51.00% 51.00%
Actual plan asset allocations 51.00% 53.00%
Southern Company Gas | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 233 $ 216
Southern Company Gas | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 272 $ 183
Southern Company Gas | Pension plans | Fixed income    
Liabilities Fair Value    
Target plan asset allocations 23.00% 23.00%
Actual plan asset allocations 29.00% 24.00%
Southern Company Gas | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 72 $ 64
Southern Company Gas | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 1  
Southern Company Gas | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 98 82
Southern Company Gas | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 51 45
Southern Company Gas | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 98 19
Southern Company Gas | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 146 $ 123
Liabilities Fair Value    
Target plan asset allocations 14.00% 14.00%
Actual plan asset allocations 12.00% 15.00%
Southern Company Gas | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 12 $ 12
Liabilities Fair Value    
Target plan asset allocations 3.00% 3.00%
Actual plan asset allocations 1.00% 1.00%
Southern Company Gas | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 71 $ 56
Liabilities Fair Value    
Target plan asset allocations 9.00% 9.00%
Actual plan asset allocations 7.00% 7.00%
Southern Company Gas | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets $ 114 $ 96
Liabilities Fair Value    
Target plan asset allocations 100.00% 100.00%
Actual plan asset allocations 100.00% 100.00%
Southern Company Gas | Other postretirement benefit plans | Equity    
Liabilities Fair Value    
Target plan asset allocations 72.00% 71.00%
Actual plan asset allocations 73.00% 69.00%
Southern Company Gas | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 60 $ 49
Southern Company Gas | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 23 $ 18
Southern Company Gas | Other postretirement benefit plans | Fixed income    
Liabilities Fair Value    
Target plan asset allocations 26.00% 25.00%
Actual plan asset allocations 25.00% 28.00%
Southern Company Gas | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 1 $ 1
Southern Company Gas | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 1 1
Southern Company Gas | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 25 24
Southern Company Gas | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 2 1
Southern Company Gas | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 1 $ 1
Liabilities Fair Value    
Target plan asset allocations 1.00% 2.00%
Actual plan asset allocations 1.00% 2.00%
Southern Company Gas | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets   $ 0
Liabilities Fair Value    
Target plan asset allocations   1.00%
Actual plan asset allocations   0.00%
Southern Company Gas | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 1 $ 1
Liabilities Fair Value    
Target plan asset allocations 1.00% 1.00%
Actual plan asset allocations 1.00% 1.00%
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 480 $ 285
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 166 145
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 176 92
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | 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  
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 98 19
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 40 29
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 6 4
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 2 2
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 2 1
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 Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | 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
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 2 1
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets   0
Southern Company Gas | 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
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets Fair Value    
Fair value, plan assets 385 353
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 67 71
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 96 91
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 72 64
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 1  
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 98 82
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 51 45
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 106 90
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 58 47
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 21 17
Southern Company Gas | 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
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 1 1
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 25 24
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets   0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets Fair Value    
Fair value, plan assets 189 162
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets Fair Value    
Fair value, 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 Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 0  
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 106 94
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 12 12
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 71 56
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 2 2
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, 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 Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 1 1
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets   0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 1 $ 1