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Retirement Benefits - Fair Values of Pension Plan and Other Postretirement Benefit Plan Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Assets Fair Value    
Target plan asset allocations (as percent) 100.00%  
Actual plan asset allocations (as percent) 100.00%  
Pension plans    
Assets Fair Value    
Fair value, plan assets $ 11,631 $ 12,901
Target plan asset allocations (as percent)   100.00%
Actual plan asset allocations (as percent)   100.00%
Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 3,132 $ 4,041
Target plan asset allocations (as percent) 26.00% 26.00%
Actual plan asset allocations (as percent) 28.00% 31.00%
Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 2,669 $ 3,124
Target plan asset allocations (as percent) 25.00% 25.00%
Actual plan asset allocations (as percent) 25.00% 25.00%
Pension plans | Fixed income    
Assets Fair Value    
Target plan asset allocations (as percent) 23.00% 23.00%
Actual plan asset allocations (as percent) 24.00% 24.00%
Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 930 $ 926
Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 7 8
Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 1,195 1,241
Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 654 650
Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 272 385
Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 1,780 $ 1,676
Target plan asset allocations (as percent) 14.00% 14.00%
Actual plan asset allocations (as percent) 15.00% 13.00%
Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 171 $ 180
Target plan asset allocations (as percent) 3.00% 3.00%
Actual plan asset allocations (as percent) 1.00% 1.00%
Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 821 $ 670
Target plan asset allocations (as percent) 9.00% 9.00%
Actual plan asset allocations (as percent) 7.00% 6.00%
Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets $ 926 $ 1,048
Target plan asset allocations (as percent) 100.00% 100.00%
Actual plan asset allocations (as percent) 100.00% 100.00%
Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 176 $ 239
Target plan asset allocations (as percent) 39.00% 37.00%
Actual plan asset allocations (as percent) 40.00% 40.00%
Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 120 $ 145
Target plan asset allocations (as percent) 23.00% 23.00%
Actual plan asset allocations (as percent) 22.00% 23.00%
Other postretirement benefit plans | Fixed income    
Assets Fair Value    
Target plan asset allocations (as percent) 29.00% 30.00%
Actual plan asset allocations (as percent) 30.00% 29.00%
Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 34 $ 32
Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 35 37
Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 81 79
Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 13 13
Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 386 426
Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 53 $ 52
Target plan asset allocations (as percent) 5.00% 5.00%
Actual plan asset allocations (as percent) 5.00% 5.00%
Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 4 $ 5
Target plan asset allocations (as percent) 1.00% 1.00%
Actual plan asset allocations (as percent) 0.00% 1.00%
Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 24 $ 20
Target plan asset allocations (as percent) 3.00% 4.00%
Actual plan asset allocations (as percent) 3.00% 2.00%
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 4,135 $ 4,887
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 2,102 2,559
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 1,344 1,555
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 270 301
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 419 472
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 171 210
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 100 135
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 45 47
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 13 12
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 13 16
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 5,143 5,912
Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 1,030 1,482
Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 1,325 1,569
Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 930 926
Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 7 8
Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 1,195 1,241
Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 654 650
Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 2 36
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 687 776
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 76 104
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 75 98
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 34 32
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 35 37
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 81 79
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 386 426
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 | Pension plans    
Assets Fair Value    
Fair value, plan assets 2,353 2,102
Net Asset Value as a Practical Expedient | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient | 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 | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 48
Net Asset Value as a Practical Expedient | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 1,361 1,204
Net Asset Value as a Practical Expedient | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 171 180
Net Asset Value as a Practical Expedient | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 821 670
Net Asset Value as a Practical Expedient | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 68 62
Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient | 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 | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 1
Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 0 0
Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 40 36
Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 4 5
Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets 24 20
Alabama Power | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 2,579 $ 2,844
Target plan asset allocations (as percent) 100.00% 100.00%
Actual plan asset allocations (as percent) 100.00% 100.00%
Alabama Power | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 694 $ 848
Target plan asset allocations (as percent) 26.00% 26.00%
Actual plan asset allocations (as percent) 28.00% 31.00%
Alabama Power | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 591 $ 703
Target plan asset allocations (as percent) 25.00% 25.00%
Actual plan asset allocations (as percent) 25.00% 25.00%
Alabama Power | Pension plans | Fixed income    
Assets Fair Value    
Target plan asset allocations (as percent) 23.00% 23.00%
Actual plan asset allocations (as percent) 24.00% 24.00%
Alabama Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 206 $ 200
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 265 286
Alabama Power | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 145 155
Alabama Power | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 61 54
Alabama Power | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 395 $ 394
Target plan asset allocations (as percent) 14.00% 14.00%
Actual plan asset allocations (as percent) 15.00% 13.00%
Alabama Power | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 38 $ 43
Target plan asset allocations (as percent) 3.00% 3.00%
Actual plan asset allocations (as percent) 1.00% 1.00%
Alabama Power | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 182 $ 159
Target plan asset allocations (as percent) 9.00% 9.00%
Actual plan asset allocations (as percent) 7.00% 6.00%
Alabama Power | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets $ 359 $ 405
Target plan asset allocations (as percent) 100.00% 100.00%
Actual plan asset allocations (as percent) 100.00% 100.00%
Alabama Power | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 45 $ 64
Target plan asset allocations (as percent) 43.00% 42.00%
Actual plan asset allocations (as percent) 45.00% 44.00%
Alabama Power | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 24 $ 30
Target plan asset allocations (as percent) 21.00% 22.00%
Actual plan asset allocations (as percent) 21.00% 22.00%
Alabama Power | Other postretirement benefit plans | Fixed income    
Assets Fair Value    
Target plan asset allocations (as percent) 28.00% 28.00%
Actual plan asset allocations (as percent) 28.00% 28.00%
Alabama Power | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 10 $ 11
Alabama Power | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 11 12
Alabama Power | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 6 7
Alabama Power | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 3 2
Alabama Power | Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 233 253
Alabama Power | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 17 $ 17
Target plan asset allocations (as percent) 4.00% 4.00%
Actual plan asset allocations (as percent) 4.00% 4.00%
Alabama Power | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 2 $ 2
Target plan asset allocations (as percent) 1.00% 1.00%
Actual plan asset allocations (as percent) 0.00% 0.00%
Alabama Power | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 8 $ 7
Target plan asset allocations (as percent) 3.00% 3.00%
Actual plan asset allocations (as percent) 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 $ 917 $ 1,104
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 466 572
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 298 370
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 60 51
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 93 111
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 54 75
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 35 52
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 12 16
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 3 2
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 4 5
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 1,140 1,255
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 228 276
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 293 333
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 206 200
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 265 286
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 145 155
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 1 3
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 282 309
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 10 12
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 12 14
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 11
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 11 12
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 6 7
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 233 253
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 | Pension plans    
Assets Fair Value    
Fair value, plan assets 522 485
Alabama Power | Net Asset Value as a Practical Expedient | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient | 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 | 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 | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 302 283
Alabama Power | Net Asset Value as a Practical Expedient | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 38 43
Alabama Power | Net Asset Value as a Practical Expedient | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 182 159
Alabama Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 23 21
Alabama Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient | 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 | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient | 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 | 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 | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 13 12
Alabama Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 2 2
Alabama Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets 8 7
Georgia Power | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 3,669 $ 4,069
Target plan asset allocations (as percent) 100.00% 100.00%
Actual plan asset allocations (as percent) 100.00% 100.00%
Georgia Power | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 988 $ 1,213
Target plan asset allocations (as percent) 26.00% 26.00%
Actual plan asset allocations (as percent) 28.00% 31.00%
Georgia Power | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 842 $ 1,006
Target plan asset allocations (as percent) 25.00% 25.00%
Actual plan asset allocations (as percent) 25.00% 25.00%
Georgia Power | Pension plans | Fixed income    
Assets Fair Value    
Target plan asset allocations (as percent) 23.00% 23.00%
Actual plan asset allocations (as percent) 24.00% 24.00%
Georgia Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 294 $ 286
Georgia Power | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 2 3
Georgia Power | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 377 409
Georgia Power | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 206 221
Georgia Power | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 86 78
Georgia Power | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 561 $ 564
Target plan asset allocations (as percent) 14.00% 14.00%
Actual plan asset allocations (as percent) 15.00% 13.00%
Georgia Power | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 54 $ 61
Target plan asset allocations (as percent) 3.00% 3.00%
Actual plan asset allocations (as percent) 1.00% 1.00%
Georgia Power | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 259 $ 228
Target plan asset allocations (as percent) 9.00% 9.00%
Actual plan asset allocations (as percent) 7.00% 6.00%
Georgia Power | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets $ 342 $ 384
Target plan asset allocations (as percent) 100.00% 100.00%
Actual plan asset allocations (as percent) 100.00% 100.00%
Georgia Power | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 50 $ 64
Target plan asset allocations (as percent) 36.00% 36.00%
Actual plan asset allocations (as percent) 35.00% 38.00%
Georgia Power | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 49 $ 60
Target plan asset allocations (as percent) 24.00% 24.00%
Actual plan asset allocations (as percent) 24.00% 24.00%
Georgia Power | Other postretirement benefit plans | Fixed income    
Assets Fair Value    
Target plan asset allocations (as percent) 33.00% 33.00%
Actual plan asset allocations (as percent) 35.00% 31.00%
Georgia Power | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 7 $ 6
Georgia Power | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 10 11
Georgia Power | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 44 41
Georgia Power | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 5 4
Georgia Power | Other postretirement benefit plans | Trust-owned life insurance    
Assets Fair Value    
Fair value, plan assets 153 173
Georgia Power | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 15 $ 17
Target plan asset allocations (as percent) 4.00% 4.00%
Actual plan asset allocations (as percent) 4.00% 4.00%
Georgia Power | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 2 $ 2
Target plan asset allocations (as percent) 1.00% 1.00%
Actual plan asset allocations (as percent) 0.00% 1.00%
Georgia Power | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 7 $ 6
Target plan asset allocations (as percent) 2.00% 2.00%
Actual plan asset allocations (as percent) 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 $ 1,304 $ 1,582
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 663 819
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 424 529
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 85 74
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 132 160
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 67 77
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 41 53
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 17 14
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 5 4
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 4 6
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 1,623 1,794
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 325 394
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 418 477
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 294 286
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets Fair Value    
Fair value, plan assets 2 3
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 377 409
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 206 221
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 1 4
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 255 288
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 9 11
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 32 46
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 6
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 10 11
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 44 41
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 153 173
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 | Pension plans    
Assets Fair Value    
Fair value, plan assets 742 693
Georgia Power | Net Asset Value as a Practical Expedient | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient | 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 | 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 | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 429 404
Georgia Power | Net Asset Value as a Practical Expedient | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 54 61
Georgia Power | Net Asset Value as a Practical Expedient | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 259 228
Georgia Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 20 19
Georgia Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient | 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 | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient | 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 | 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 | 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 | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 2 2
Georgia Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets 7 6
Mississippi Power | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 506 $ 564
Target plan asset allocations (as percent) 100.00% 100.00%
Actual plan asset allocations (as percent) 100.00% 100.00%
Mississippi Power | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 136 $ 168
Target plan asset allocations (as percent) 26.00% 26.00%
Actual plan asset allocations (as percent) 28.00% 31.00%
Mississippi Power | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 118 $ 139
Target plan asset allocations (as percent) 25.00% 25.00%
Actual plan asset allocations (as percent) 25.00% 25.00%
Mississippi Power | Pension plans | Fixed income    
Assets Fair Value    
Target plan asset allocations (as percent) 23.00% 23.00%
Actual plan asset allocations (as percent) 24.00% 24.00%
Mississippi Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 40 $ 40
Mississippi Power | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 52 56
Mississippi Power | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 28 31
Mississippi Power | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 12 11
Mississippi Power | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 77 $ 78
Target plan asset allocations (as percent) 14.00% 14.00%
Actual plan asset allocations (as percent) 15.00% 13.00%
Mississippi Power | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 7 $ 9
Target plan asset allocations (as percent) 3.00% 3.00%
Actual plan asset allocations (as percent) 1.00% 1.00%
Mississippi Power | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 36 $ 32
Target plan asset allocations (as percent) 9.00% 9.00%
Actual plan asset allocations (as percent) 7.00% 6.00%
Mississippi Power | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets $ 23 $ 24
Target plan asset allocations (as percent) 100.00% 100.00%
Actual plan asset allocations (as percent) 100.00% 100.00%
Mississippi Power | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 5 $ 6
Target plan asset allocations (as percent) 21.00% 21.00%
Actual plan asset allocations (as percent) 22.00% 25.00%
Mississippi Power | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 4 $ 5
Target plan asset allocations (as percent) 20.00% 21.00%
Actual plan asset allocations (as percent) 20.00% 20.00%
Mississippi Power | Other postretirement benefit plans | Fixed income    
Assets Fair Value    
Target plan asset allocations (as percent) 38.00% 37.00%
Actual plan asset allocations (as percent) 39.00% 38.00%
Mississippi Power | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 6 $ 5
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 1 1
Mississippi Power | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 3 $ 3
Target plan asset allocations (as percent) 11.00% 12.00%
Actual plan asset allocations (as percent) 12.00% 11.00%
Mississippi Power | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 0 $ 0
Target plan asset allocations (as percent) 3.00% 2.00%
Actual plan asset allocations (as percent) 1.00% 1.00%
Mississippi Power | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 1 $ 1
Target plan asset allocations (as percent) 7.00% 7.00%
Actual plan asset allocations (as percent) 6.00% 5.00%
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 180 $ 218
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 91 113
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 59 73
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 12 10
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 18 22
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 7 9
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 4
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 2 3
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 1 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 224 249
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 45 55
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 59 66
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 40 40
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 52 56
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 28 31
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 1
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 13 12
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 2 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 5
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 | Pension plans    
Assets Fair Value    
Fair value, plan assets 102 97
Mississippi Power | Net Asset Value as a Practical Expedient | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient | 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 | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 59 56
Mississippi Power | Net Asset Value as a Practical Expedient | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 7 9
Mississippi Power | Net Asset Value as a Practical Expedient | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 36 32
Mississippi Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 3 3
Mississippi Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient | 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 | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient | 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 | 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 | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient | 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 $ 124 $ 138
Target plan asset allocations (as percent) 100.00% 100.00%
Actual plan asset allocations (as percent) 100.00% 100.00%
Southern Power | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 33 $ 41
Target plan asset allocations (as percent) 26.00% 26.00%
Actual plan asset allocations (as percent) 28.00% 31.00%
Southern Power | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 28 $ 34
Target plan asset allocations (as percent) 25.00% 25.00%
Actual plan asset allocations (as percent) 25.00% 25.00%
Southern Power | Pension plans | Fixed income    
Assets Fair Value    
Target plan asset allocations (as percent) 23.00% 23.00%
Actual plan asset allocations (as percent) 24.00% 24.00%
Southern Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 10 $ 10
Southern Power | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 13 14
Southern Power | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 7 8
Southern Power | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 3 2
Southern Power | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 19 $ 19
Target plan asset allocations (as percent) 14.00% 14.00%
Actual plan asset allocations (as percent) 15.00% 13.00%
Southern Power | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 2 $ 2
Target plan asset allocations (as percent) 3.00% 3.00%
Actual plan asset allocations (as percent) 1.00% 1.00%
Southern Power | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 9 $ 8
Target plan asset allocations (as percent) 9.00% 9.00%
Actual plan asset allocations (as percent) 7.00% 6.00%
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 43 $ 53
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 22 28
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 14 18
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 3 2
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 4 5
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 55 61
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 11 13
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 14 16
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 10 10
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 13 14
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 7 8
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 | Pension plans    
Assets Fair Value    
Fair value, plan assets 26 24
Southern Power | Net Asset Value as a Practical Expedient | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient | 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 | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 15 14
Southern Power | Net Asset Value as a Practical Expedient | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 2 2
Southern Power | Net Asset Value as a Practical Expedient | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 9 8
Southern Company Gas | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 800 945
Target plan asset allocations (as percent) 100.00%  
Actual plan asset allocations (as percent) 100.00%  
Southern Company Gas | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 216 478
Target plan asset allocations (as percent) 26.00%  
Actual plan asset allocations (as percent) 28.00%  
Southern Company Gas | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 183 166
Target plan asset allocations (as percent) 25.00%  
Actual plan asset allocations (as percent) 25.00%  
Southern Company Gas | Pension plans | Fixed income    
Assets Fair Value    
Target plan asset allocations (as percent) 23.00%  
Actual plan asset allocations (as percent) 24.00%  
Southern Company Gas | Pension plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 64 85
Southern Company Gas | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 82 39
Southern Company Gas | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 45  
Southern Company Gas | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 19 157
Southern Company Gas | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 123 19
Target plan asset allocations (as percent) 14.00%  
Actual plan asset allocations (as percent) 15.00%  
Southern Company Gas | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 12  
Target plan asset allocations (as percent) 3.00%  
Actual plan asset allocations (as percent) 1.00%  
Southern Company Gas | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 56 1
Target plan asset allocations (as percent) 9.00%  
Actual plan asset allocations (as percent) 7.00%  
Southern Company Gas | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets $ 96 $ 121
Target plan asset allocations (as percent) 100.00% 100.00%
Actual plan asset allocations (as percent) 100.00% 100.00%
Southern Company Gas | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets $ 49 $ 72
Target plan asset allocations (as percent) 51.00%  
Actual plan asset allocations (as percent) 51.00%  
Southern Company Gas | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets $ 18 $ 22
Target plan asset allocations (as percent) 20.00%  
Actual plan asset allocations (as percent) 18.00%  
Southern Company Gas | Other postretirement benefit plans | Fixed income    
Assets Fair Value    
Target plan asset allocations (as percent) 25.00% 24.00%
Actual plan asset allocations (as percent) 28.00% 20.00%
Southern Company Gas | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets $ 1  
Southern Company Gas | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 1  
Southern Company Gas | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 24 $ 24
Southern Company Gas | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 1 $ 3
Southern Company Gas | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets $ 1  
Target plan asset allocations (as percent) 2.00%  
Actual plan asset allocations (as percent) 2.00%  
Southern Company Gas | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets $ 0  
Target plan asset allocations (as percent) 1.00%  
Actual plan asset allocations (as percent) 0.00%  
Southern Company Gas | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 1  
Target plan asset allocations (as percent) 1.00%  
Actual plan asset allocations (as percent) 1.00%  
Southern Company Gas | Other postretirement benefit plans | Equity securities    
Assets Fair Value    
Target plan asset allocations (as percent)   72.00%
Actual plan asset allocations (as percent)   76.00%
Southern Company Gas | Other postretirement benefit plans | Cash    
Assets Fair Value    
Target plan asset allocations (as percent)   1.00%
Actual plan asset allocations (as percent)   2.00%
Southern Company Gas | Other postretirement benefit plans | Other Types Of Investments    
Assets Fair Value    
Target plan asset allocations (as percent)   3.00%
Actual plan asset allocations (as percent)   2.00%
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets Fair Value    
Fair value, plan assets $ 285 $ 242
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 145 155
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 92 0
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 | 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  
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 19 84
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 29 3
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  
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 4 5
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 3
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 1 0
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  
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  
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 1 2
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  
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  
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets Fair Value    
Fair value, plan assets 353 638
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 71 323
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 91 166
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 64 85
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 82 39
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 45  
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 25
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  
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 90 115
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 47 69
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets Fair Value    
Fair value, plan assets 17 22
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  
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 1  
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 24 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  
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  
Southern Company Gas | Net Asset Value as a Practical Expedient | Pension plans    
Assets Fair Value    
Fair value, plan assets 162 65
Southern Company Gas | Net Asset Value as a Practical Expedient | Pension plans | Domestic equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient | Pension plans | International equity    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient | 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 | Pension plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient | Pension plans | Pooled funds    
Assets Fair Value    
Fair value, plan assets 0  
Southern Company Gas | Net Asset Value as a Practical Expedient | Pension plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 48
Southern Company Gas | Net Asset Value as a Practical Expedient | Pension plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 94 16
Southern Company Gas | Net Asset Value as a Practical Expedient | Pension plans | Special situations    
Assets Fair Value    
Fair value, plan assets 12  
Southern Company Gas | Net Asset Value as a Practical Expedient | Pension plans | Private equity    
Assets Fair Value    
Fair value, plan assets 56 1
Southern Company Gas | Net Asset Value as a Practical Expedient | Other postretirement benefit plans    
Assets Fair Value    
Fair value, plan assets 2 1
Southern Company Gas | Net Asset Value as a Practical Expedient | 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 | 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 | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets Fair Value    
Fair value, plan assets 0  
Southern Company Gas | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Corporate bonds    
Assets Fair Value    
Fair value, plan assets 0  
Southern Company Gas | Net Asset Value as a Practical Expedient | 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 | Other postretirement benefit plans | Cash equivalents and other    
Assets Fair Value    
Fair value, plan assets 0 $ 1
Southern Company Gas | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Real estate investments    
Assets Fair Value    
Fair value, plan assets 1  
Southern Company Gas | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Special situations    
Assets Fair Value    
Fair value, plan assets 0  
Southern Company Gas | Net Asset Value as a Practical Expedient | Other postretirement benefit plans | Private equity    
Assets Fair Value    
Fair value, plan assets $ 1  
AGL Resources Inc. Retirement Plan | Southern Company Gas | Pension plans    
Assets Fair Value    
Target plan asset allocations (as percent) 100.00%  
Actual plan asset allocations (as percent)   100.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas | Pension plans | Fixed income    
Assets Fair Value    
Target plan asset allocations (as percent) 15.00%  
Actual plan asset allocations (as percent)   19.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas | Pension plans | Equity securities    
Assets Fair Value    
Target plan asset allocations (as percent) 53.00%  
Actual plan asset allocations (as percent)   65.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas | Pension plans | Cash    
Assets Fair Value    
Target plan asset allocations (as percent) 2.00%  
Actual plan asset allocations (as percent)   6.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas | Pension plans | Other Types Of Investments    
Assets Fair Value    
Target plan asset allocations (as percent) 30.00%  
Actual plan asset allocations (as percent)   10.00%