XML 248 R205.htm IDEA: XBRL DOCUMENT v3.22.4
RETIREMENT BENEFITS - Fair Values of Pension Plan and Other Postretirement Benefit Plan Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2022
Dec. 31, 2021
Assets:    
Target Allocation 100.00%  
Actual Allocation 100.00%  
Pension plans    
Assets:    
Plan assets $ 14,197 $ 17,225
Target Allocation   100.00%
Actual Allocation   100.00%
Derivatives (4)  
Defined Benefit Plan, Plan Assets, Amount, Excluding Cash, Investment Income And Pending Investment Sales Receivables, And Pending Investment Purchase Payables $ 14,193  
Pension plans | Equity    
Assets:    
Target Allocation 45.00% 51.00%
Actual Allocation 43.00% 53.00%
Pension plans | Domestic equity    
Assets:    
Plan assets $ 2,769 $ 4,421
Pension plans | International equity    
Assets:    
Plan assets $ 3,256 $ 4,145
Pension plans | Fixed income    
Assets:    
Target Allocation 30.00% 23.00%
Actual Allocation 28.00% 22.00%
Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 1,469 $ 1,209
Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 29 10
Pension plans | Corporate bonds    
Assets:    
Plan assets 1,494 1,752
Pension plans | Pooled funds    
Assets:    
Plan assets 607 771
Pension plans | Cash equivalents and other    
Assets:    
Plan assets 406 412
Pension plans | Real estate investments    
Assets:    
Plan assets $ 2,263 $ 2,744
Target Allocation 13.00% 14.00%
Actual Allocation 15.00% 15.00%
Pension plans | Special situations    
Assets:    
Plan assets $ 187 $ 171
Target Allocation 3.00% 3.00%
Actual Allocation 2.00% 1.00%
Pension plans | Private equity    
Assets:    
Plan assets $ 1,717 $ 1,590
Target Allocation 9.00% 9.00%
Actual Allocation 12.00% 9.00%
Other postretirement benefit plans    
Assets:    
Plan assets $ 998 $ 1,249
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Other postretirement benefit plans | Equity    
Assets:    
Target Allocation 61.00% 64.00%
Actual Allocation 59.00% 66.00%
Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets $ 159 $ 235
Other postretirement benefit plans | International equity    
Assets:    
Plan assets $ 137 $ 172
Other postretirement benefit plans | Fixed income    
Assets:    
Target Allocation 30.00% 27.00%
Actual Allocation 28.00% 25.00%
Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 43 $ 37
Other postretirement benefit plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 1  
Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 40 50
Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 79 90
Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 19 14
Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 406 530
Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets $ 62 $ 74
Target Allocation 5.00% 5.00%
Actual Allocation 7.00% 6.00%
Other postretirement benefit plans | Special situations    
Assets:    
Plan assets $ 6 $ 5
Target Allocation 1.00% 1.00%
Actual Allocation 1.00% 0.00%
Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 46 $ 42
Target Allocation 3.00% 3.00%
Actual Allocation 5.00% 3.00%
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets:    
Plan assets $ 5,019 $ 6,946
Derivatives (4)  
Defined Benefit Plan, Plan Assets, Amount, Excluding Cash, Investment Income And Pending Investment Sales Receivables, And Pending Investment Purchase Payables 5,015  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets:    
Plan assets 2,078 3,095
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets:    
Plan assets 2,166 2,740
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 399 405
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets:    
Plan assets 376 706
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets:    
Plan assets 173 230
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 85 123
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 58 73
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 19 14
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 11 20
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Pension plans    
Assets:    
Plan assets 5,387 6,477
Derivatives 0  
Defined Benefit Plan, Plan Assets, Amount, Excluding Cash, Investment Income And Pending Investment Sales Receivables, And Pending Investment Purchase Payables 5,387  
Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets:    
Plan assets 691 1,326
Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets:    
Plan assets 1,090 1,402
Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 1,469 1,209
Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 29 10
Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets:    
Plan assets 1,494 1,752
Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets:    
Plan assets 607 771
Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 7 7
Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets:    
Plan assets 722 918
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 74 112
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 79 99
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 43 37
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 1  
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 40 50
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 79 90
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 406 530
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Significant Unobservable Inputs (Level 3) | Pension plans    
Assets:    
Plan assets 0 3
Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Significant Unobservable Inputs (Level 3) | Pension plans | International equity    
Assets:    
Plan assets 0 3
Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Significant Unobservable Inputs (Level 3) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Significant Unobservable Inputs (Level 3) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets:    
Plan assets 3,791 3,799
Derivatives 0  
Defined Benefit Plan, Plan Assets, Amount, Excluding Cash, Investment Income And Pending Investment Sales Receivables, And Pending Investment Purchase Payables 3,791  
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets:    
Plan assets 1,887 2,038
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets:    
Plan assets 187 171
Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets:    
Plan assets 1,717 1,590
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets:    
Plan assets 103 101
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0  
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 0 0
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 51 54
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 6 5
Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 46 42
Alabama Power    
Assets:    
Target Allocation 100.00%  
Actual Allocation 100.00%  
Alabama Power | Pension plans    
Assets:    
Plan assets $ 3,422 $ 4,141
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Derivatives $ (1)  
Defined Benefit Plan, Plan Assets, Amount, Excluding Cash, Investment Income And Pending Investment Sales Receivables, And Pending Investment Purchase Payables $ 3,421  
Alabama Power | Pension plans | Equity    
Assets:    
Target Allocation 45.00% 51.00%
Actual Allocation 43.00% 53.00%
Alabama Power | Pension plans | Domestic equity    
Assets:    
Plan assets $ 667 $ 1,062
Alabama Power | Pension plans | International equity    
Assets:    
Plan assets $ 785 $ 997
Alabama Power | Pension plans | Fixed income    
Assets:    
Target Allocation 30.00% 23.00%
Actual Allocation 28.00% 22.00%
Alabama Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 354 $ 291
Alabama Power | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 7 2
Alabama Power | Pension plans | Corporate bonds    
Assets:    
Plan assets 360 421
Alabama Power | Pension plans | Pooled funds    
Assets:    
Plan assets 146 186
Alabama Power | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 98 99
Alabama Power | Pension plans | Real estate investments    
Assets:    
Plan assets $ 546 $ 660
Target Allocation 13.00% 14.00%
Actual Allocation 15.00% 15.00%
Alabama Power | Pension plans | Special situations    
Assets:    
Plan assets $ 45 $ 41
Target Allocation 3.00% 3.00%
Actual Allocation 2.00% 1.00%
Alabama Power | Pension plans | Private equity    
Assets:    
Plan assets $ 414 $ 382
Target Allocation 9.00% 9.00%
Actual Allocation 12.00% 9.00%
Alabama Power | Other postretirement benefit plans    
Assets:    
Plan assets $ 371 $ 488
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Alabama Power | Other postretirement benefit plans | Equity    
Assets:    
Target Allocation 69.00% 71.00%
Actual Allocation 65.00% 69.00%
Alabama Power | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets $ 23 $ 37
Alabama Power | Other postretirement benefit plans | International equity    
Assets:    
Plan assets $ 27 $ 35
Alabama Power | Other postretirement benefit plans | Fixed income    
Assets:    
Target Allocation 23.00% 21.00%
Actual Allocation 23.00% 21.00%
Alabama Power | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 12 $ 10
Alabama Power | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 12 18
Alabama Power | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 7 6
Alabama Power | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 3 3
Alabama Power | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 252 341
Alabama Power | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets $ 19 $ 23
Target Allocation 4.00% 4.00%
Actual Allocation 7.00% 7.00%
Alabama Power | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets $ 2 $ 2
Target Allocation 1.00% 1.00%
Actual Allocation 1.00% 0.00%
Alabama Power | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 14 $ 13
Target Allocation 3.00% 3.00%
Actual Allocation 4.00% 3.00%
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets:    
Plan assets $ 1,209 $ 1,669
Derivatives (1)  
Defined Benefit Plan, Plan Assets, Amount, Excluding Cash, Investment Income And Pending Investment Sales Receivables, And Pending Investment Purchase Payables 1,208  
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets:    
Plan assets 500 743
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets:    
Plan assets 522 659
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 96 97
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets:    
Plan assets 91 170
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets:    
Plan assets 41 58
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 17 26
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 18 23
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 3 3
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 3 6
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Alabama Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets:    
Plan assets 1,299 1,558
Derivatives 0  
Defined Benefit Plan, Plan Assets, Amount, Excluding Cash, Investment Income And Pending Investment Sales Receivables, And Pending Investment Purchase Payables 1,299  
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets:    
Plan assets 167 319
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets:    
Plan assets 263 337
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 354 291
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 7 2
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets:    
Plan assets 360 421
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets:    
Plan assets 146 186
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 2 2
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets:    
Plan assets 298 398
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 6 11
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 9 12
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 12 10
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 12 18
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 7 6
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 252 341
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Alabama Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans    
Assets:    
Plan assets 0 1
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | International equity    
Assets:    
Plan assets 0 1
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Alabama Power | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets:    
Plan assets 914 913
Derivatives 0  
Defined Benefit Plan, Plan Assets, Amount, Excluding Cash, Investment Income And Pending Investment Sales Receivables, And Pending Investment Purchase Payables 914  
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets:    
Plan assets 455 490
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets:    
Plan assets 45 41
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets:    
Plan assets 414 382
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets:    
Plan assets 32 32
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 0 0
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 16 17
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 2 2
Alabama Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 14 13
Georgia Power    
Assets:    
Target Allocation 100.00%  
Actual Allocation 100.00%  
Georgia Power | Pension plans    
Assets:    
Plan assets $ 4,448 $ 5,415
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Derivatives $ (1)  
Defined Benefit Plan, Plan Assets, Amount, Excluding Cash, Investment Income And Pending Investment Sales Receivables, And Pending Investment Purchase Payables $ 4,447  
Georgia Power | Pension plans | Equity    
Assets:    
Target Allocation 45.00% 51.00%
Actual Allocation 43.00% 53.00%
Georgia Power | Pension plans | Domestic equity    
Assets:    
Plan assets $ 868 $ 1,389
Georgia Power | Pension plans | International equity    
Assets:    
Plan assets $ 1,020 $ 1,303
Georgia Power | Pension plans | Fixed income    
Assets:    
Target Allocation 30.00% 23.00%
Actual Allocation 28.00% 22.00%
Georgia Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 460 $ 380
Georgia Power | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 9 3
Georgia Power | Pension plans | Corporate bonds    
Assets:    
Plan assets 468 551
Georgia Power | Pension plans | Pooled funds    
Assets:    
Plan assets 190 243
Georgia Power | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 127 129
Georgia Power | Pension plans | Real estate investments    
Assets:    
Plan assets $ 709 $ 863
Target Allocation 13.00% 14.00%
Actual Allocation 15.00% 15.00%
Georgia Power | Pension plans | Special situations    
Assets:    
Plan assets $ 59 $ 54
Target Allocation 3.00% 3.00%
Actual Allocation 2.00% 1.00%
Georgia Power | Pension plans | Private equity    
Assets:    
Plan assets $ 538 $ 500
Target Allocation 9.00% 9.00%
Actual Allocation 12.00% 9.00%
Georgia Power | Other postretirement benefit plans    
Assets:    
Plan assets $ 368 $ 450
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Georgia Power | Other postretirement benefit plans | Equity    
Assets:    
Target Allocation 58.00% 60.00%
Actual Allocation 56.00% 62.00%
Georgia Power | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets $ 52 $ 78
Georgia Power | Other postretirement benefit plans | International equity    
Assets:    
Plan assets $ 56 $ 72
Georgia Power | Other postretirement benefit plans | Fixed income    
Assets:    
Target Allocation 35.00% 33.00%
Actual Allocation 34.00% 30.00%
Georgia Power | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 10 $ 9
Georgia Power | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 12 14
Georgia Power | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 40 46
Georgia Power | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 9 5
Georgia Power | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 154 189
Georgia Power | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets $ 19 $ 23
Target Allocation 4.00% 4.00%
Actual Allocation 5.00% 5.00%
Georgia Power | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets $ 2 $ 1
Target Allocation 1.00% 1.00%
Actual Allocation 1.00% 0.00%
Georgia Power | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 14 $ 13
Target Allocation 2.00% 2.00%
Actual Allocation 4.00% 3.00%
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets:    
Plan assets $ 1,572 $ 2,182
Derivatives (1)  
Defined Benefit Plan, Plan Assets, Amount, Excluding Cash, Investment Income And Pending Investment Sales Receivables, And Pending Investment Purchase Payables 1,571  
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets:    
Plan assets 651 972
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets:    
Plan assets 678 861
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 125 127
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets:    
Plan assets 118 222
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets:    
Plan assets 76 99
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 46 65
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 17 22
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 9 5
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 4 7
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Georgia Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets:    
Plan assets 1,688 2,037
Derivatives 0  
Defined Benefit Plan, Plan Assets, Amount, Excluding Cash, Investment Income And Pending Investment Sales Receivables, And Pending Investment Purchase Payables 1,688  
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets:    
Plan assets 217 417
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets:    
Plan assets 342 441
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 460 380
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 9 3
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets:    
Plan assets 468 551
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets:    
Plan assets 190 243
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 2 2
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets:    
Plan assets 261 321
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 6 13
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 39 50
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 10 9
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 12 14
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 40 46
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 154 189
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Georgia Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans    
Assets:    
Plan assets 0 1
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | International equity    
Assets:    
Plan assets 0 1
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Georgia Power | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets:    
Plan assets 1,188 1,195
Derivatives 0  
Defined Benefit Plan, Plan Assets, Amount, Excluding Cash, Investment Income And Pending Investment Sales Receivables, And Pending Investment Purchase Payables 1,188  
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets:    
Plan assets 591 641
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets:    
Plan assets 59 54
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets:    
Plan assets 538 500
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets:    
Plan assets 31 30
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Trust-owned life insurance    
Assets:    
Plan assets 0 0
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 15 16
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 2 1
Georgia Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 14 13
Mississippi Power | Pension plans    
Assets:    
Plan assets $ 648 $ 787
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Mississippi Power | Pension plans | Equity    
Assets:    
Target Allocation 45.00% 51.00%
Actual Allocation 43.00% 53.00%
Mississippi Power | Pension plans | Domestic equity    
Assets:    
Plan assets $ 127 $ 203
Mississippi Power | Pension plans | International equity    
Assets:    
Plan assets $ 149 $ 190
Mississippi Power | Pension plans | Fixed income    
Assets:    
Target Allocation 30.00% 23.00%
Actual Allocation 28.00% 22.00%
Mississippi Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 67 $ 55
Mississippi Power | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 1  
Mississippi Power | Pension plans | Corporate bonds    
Assets:    
Plan assets 68 80
Mississippi Power | Pension plans | Pooled funds    
Assets:    
Plan assets 28 35
Mississippi Power | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 18 18
Mississippi Power | Pension plans | Real estate investments    
Assets:    
Plan assets $ 103 $ 125
Target Allocation 13.00% 14.00%
Actual Allocation 15.00% 15.00%
Mississippi Power | Pension plans | Special situations    
Assets:    
Plan assets $ 9 $ 8
Target Allocation 3.00% 3.00%
Actual Allocation 2.00% 1.00%
Mississippi Power | Pension plans | Private equity    
Assets:    
Plan assets $ 78 $ 73
Target Allocation 9.00% 9.00%
Actual Allocation 12.00% 9.00%
Mississippi Power | Other postretirement benefit plans    
Assets:    
Plan assets $ 24 $ 27
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Mississippi Power | Other postretirement benefit plans | Equity    
Assets:    
Target Allocation 37.00% 43.00%
Actual Allocation 35.00% 44.00%
Mississippi Power | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets $ 4 $ 6
Mississippi Power | Other postretirement benefit plans | International equity    
Assets:    
Plan assets $ 5 $ 6
Mississippi Power | Other postretirement benefit plans | Fixed income    
Assets:    
Target Allocation 43.00% 36.00%
Actual Allocation 41.00% 34.00%
Mississippi Power | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 4 $ 5
Mississippi Power | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 2 2
Mississippi Power | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 1 1
Mississippi Power | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 2 1
Mississippi Power | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets $ 4 $ 4
Target Allocation 11.00% 11.00%
Actual Allocation 12.00% 13.00%
Mississippi Power | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets $ 0 $ 0
Target Allocation 2.00% 2.00%
Actual Allocation 2.00% 1.00%
Mississippi Power | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 2 $ 2
Target Allocation 7.00% 8.00%
Actual Allocation 10.00% 8.00%
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets:    
Plan assets $ 229 $ 318
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets:    
Plan assets 95 142
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets:    
Plan assets 99 126
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0  
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 18 18
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets:    
Plan assets 17 32
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets:    
Plan assets 9 10
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 3 4
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 3 4
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 2 1
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 1 1
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Mississippi Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets:    
Plan assets 246 295
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets:    
Plan assets 32 61
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets:    
Plan assets 50 64
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 67 55
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 1  
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets:    
Plan assets 68 80
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets:    
Plan assets 28 35
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets:    
Plan assets 10 12
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 1 2
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 2 2
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 4 5
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 2 2
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 1 1
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0  
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Mississippi Power | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets:    
Plan assets 173 174
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0  
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets:    
Plan assets 86 93
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets:    
Plan assets 9 8
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets:    
Plan assets 78 73
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets:    
Plan assets 5 5
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 3 3
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Special situations    
Assets:    
Plan assets 0 0
Mississippi Power | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 2 2
Southern Power | Pension plans    
Assets:    
Plan assets $ 177 $ 213
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Southern Power | Pension plans | Equity    
Assets:    
Target Allocation 45.00% 51.00%
Actual Allocation 43.00% 53.00%
Southern Power | Pension plans | Domestic equity    
Assets:    
Plan assets $ 34 $ 54
Southern Power | Pension plans | International equity    
Assets:    
Plan assets $ 41 $ 51
Southern Power | Pension plans | Fixed income    
Assets:    
Target Allocation 30.00% 23.00%
Actual Allocation 28.00% 22.00%
Southern Power | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 18 $ 15
Southern Power | Pension plans | Corporate bonds    
Assets:    
Plan assets 19 22
Southern Power | Pension plans | Pooled funds    
Assets:    
Plan assets 8 10
Southern Power | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 5 5
Southern Power | Pension plans | Real estate investments    
Assets:    
Plan assets $ 29 $ 34
Target Allocation 13.00% 14.00%
Actual Allocation 15.00% 15.00%
Southern Power | Pension plans | Special situations    
Assets:    
Plan assets $ 2 $ 2
Target Allocation 3.00% 3.00%
Actual Allocation 2.00% 1.00%
Southern Power | Pension plans | Private equity    
Assets:    
Plan assets $ 21 $ 20
Target Allocation 9.00% 9.00%
Actual Allocation 12.00% 9.00%
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets:    
Plan assets $ 62 $ 86
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets:    
Plan assets 25 38
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets:    
Plan assets 27 34
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 5 5
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets:    
Plan assets 5 9
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Southern Power | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets:    
Plan assets 68 80
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets:    
Plan assets 9 16
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets:    
Plan assets 14 17
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 18 15
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets:    
Plan assets 19 22
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets:    
Plan assets 8 10
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Southern Power | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans    
Assets:    
Plan assets 0 0
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Southern Power | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets:    
Plan assets 47 47
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets:    
Plan assets 24 25
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets:    
Plan assets 2 2
Southern Power | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets:    
Plan assets 21 20
Southern Company Gas | Pension plans    
Assets:    
Plan assets $ 1,001 $ 1,241
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Southern Company Gas | Pension plans | Equity    
Assets:    
Target Allocation 45.00% 51.00%
Actual Allocation 43.00% 53.00%
Southern Company Gas | Pension plans | Domestic equity    
Assets:    
Plan assets $ 195 $ 319
Southern Company Gas | Pension plans | International equity    
Assets:    
Plan assets $ 229 $ 298
Southern Company Gas | Pension plans | Fixed income    
Assets:    
Target Allocation 30.00% 23.00%
Actual Allocation 28.00% 22.00%
Southern Company Gas | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 104 $ 87
Southern Company Gas | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 2 1
Southern Company Gas | Pension plans | Corporate bonds    
Assets:    
Plan assets 105 126
Southern Company Gas | Pension plans | Pooled funds    
Assets:    
Plan assets 43 56
Southern Company Gas | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 29 29
Southern Company Gas | Pension plans | Real estate investments    
Assets:    
Plan assets $ 160 $ 198
Target Allocation 13.00% 14.00%
Actual Allocation 15.00% 15.00%
Southern Company Gas | Pension plans | Special situations    
Assets:    
Plan assets $ 13 $ 12
Target Allocation 3.00% 3.00%
Actual Allocation 2.00% 1.00%
Southern Company Gas | Pension plans | Private equity    
Assets:    
Plan assets $ 121 $ 115
Target Allocation 9.00% 9.00%
Actual Allocation 12.00% 9.00%
Southern Company Gas | Other postretirement benefit plans    
Assets:    
Plan assets $ 113 $ 143
Target Allocation 100.00% 100.00%
Actual Allocation 100.00% 100.00%
Southern Company Gas | Other postretirement benefit plans | Equity    
Assets:    
Target Allocation 72.00% 72.00%
Actual Allocation 70.00% 73.00%
Southern Company Gas | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets $ 58 $ 79
Southern Company Gas | Other postretirement benefit plans | International equity    
Assets:    
Plan assets $ 22 $ 26
Southern Company Gas | Other postretirement benefit plans | Fixed income    
Assets:    
Target Allocation 26.00% 26.00%
Actual Allocation 27.00% 24.00%
Southern Company Gas | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets $ 1 $ 1
Southern Company Gas | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 1 1
Southern Company Gas | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 27 30
Southern Company Gas | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 1 2
Southern Company Gas | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets $ 2 $ 3
Target Allocation 1.00% 1.00%
Actual Allocation 2.00% 2.00%
Southern Company Gas | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 1 $ 1
Target Allocation 1.00% 1.00%
Actual Allocation 1.00% 1.00%
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans    
Assets:    
Plan assets $ 353 $ 500
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Domestic equity    
Assets:    
Plan assets 146 223
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | International equity    
Assets:    
Plan assets 152 197
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 28 29
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Real estate investments    
Assets:    
Plan assets 27 51
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans    
Assets:    
Plan assets 5 8
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 2 3
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 2 2
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 1 2
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 0 1
Southern Company Gas | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans    
Assets:    
Plan assets 381 467
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Domestic equity    
Assets:    
Plan assets 49 96
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | International equity    
Assets:    
Plan assets 77 101
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 104 87
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 2 1
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Corporate bonds    
Assets:    
Plan assets 105 126
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Pooled funds    
Assets:    
Plan assets 43 56
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 1 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans    
Assets:    
Plan assets 105 132
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 56 76
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 20 24
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 1 1
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 1 1
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 27 30
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Other Observable Inputs (Level 2) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Real estate investments    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Special situations    
Assets:    
Plan assets 0 0
Southern Company Gas | Significant Unobservable Inputs (Level 3) | Pension plans | Private equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans    
Assets:    
Plan assets 267 274
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Domestic equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | International equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Mortgage- and asset-backed securities    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Real estate investments    
Assets:    
Plan assets 133 147
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Special situations    
Assets:    
Plan assets 13 12
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Pension plans | Private equity    
Assets:    
Plan assets 121 115
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans    
Assets:    
Plan assets 3 3
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Domestic equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | International equity    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | U.S. Treasury, government, and agency bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Corporate bonds    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Pooled funds    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Cash equivalents and other    
Assets:    
Plan assets 0 0
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Real estate investments    
Assets:    
Plan assets 2 2
Southern Company Gas | Net Asset Value as a Practical Expedient (NAV) | Other postretirement benefit plans | Private equity    
Assets:    
Plan assets $ 1 $ 1