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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Assets:    
Fair Value $ 33  
Liabilities:    
Collateral already posted, aggregate fair value 183  
Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives 453  
Foreign currency derivatives 132  
Nuclear decommissioning trusts 1,829  
Cash equivalents 1,449  
Other investments 10  
Total 3,873  
Liabilities:    
Energy-related derivatives 573  
Interest rate derivatives 68  
Foreign currency derivatives 23  
Contingent consideration 22  
Total 686  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Energy-related derivatives 287  
Foreign currency derivatives 0  
Nuclear decommissioning trusts 798  
Cash equivalents 1,449  
Other investments 9  
Total 2,543  
Liabilities:    
Energy-related derivatives 422  
Interest rate derivatives 0  
Foreign currency derivatives 0  
Contingent consideration 0  
Total 422  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Energy-related derivatives 166  
Foreign currency derivatives 132  
Nuclear decommissioning trusts 998  
Cash equivalents 0  
Other investments 0  
Total 1,296  
Liabilities:    
Energy-related derivatives 151  
Interest rate derivatives 68  
Foreign currency derivatives 23  
Contingent consideration 0  
Total 242  
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)    
Assets:    
Energy-related derivatives 0  
Foreign currency derivatives 0  
Nuclear decommissioning trusts 0  
Cash equivalents 0  
Other investments 1  
Total 1  
Liabilities:    
Energy-related derivatives 0  
Interest rate derivatives 0  
Foreign currency derivatives 0  
Contingent consideration 22  
Total 22  
ALABAMA POWER CO    
Assets:    
Energy-related derivatives 8 $ 4
Fair Value 33  
Liabilities:    
Energy-related derivatives 9 10
ALABAMA POWER CO | Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives 8  
Cash equivalents 495  
Total 1,407  
Liabilities:    
Energy-related derivatives 9  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Domestic equity    
Assets:    
Nuclear decommissioning trusts 526  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Foreign equity    
Assets:    
Nuclear decommissioning trusts 116  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | U.S. Treasury and government agency securities    
Assets:    
Nuclear decommissioning trusts 18  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Corporate bonds    
Assets:    
Nuclear decommissioning trusts 186  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Mortgage and asset backed securities    
Assets:    
Nuclear decommissioning trusts 18  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Private equity    
Assets:    
Nuclear decommissioning trusts 33  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Other    
Assets:    
Nuclear decommissioning trusts 7  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Energy-related derivatives 0  
Cash equivalents 495  
Total 1,031  
Liabilities:    
Energy-related derivatives 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity    
Assets:    
Nuclear decommissioning trusts 440  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign equity    
Assets:    
Nuclear decommissioning trusts 61  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury and government agency securities    
Assets:    
Nuclear decommissioning trusts 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds    
Assets:    
Nuclear decommissioning trusts 28  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage and asset backed securities    
Assets:    
Nuclear decommissioning trusts 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Private equity    
Assets:    
Nuclear decommissioning trusts 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other    
Assets:    
Nuclear decommissioning trusts 7  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Energy-related derivatives 8  
Cash equivalents 0  
Total 343  
Liabilities:    
Energy-related derivatives 9  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Domestic equity    
Assets:    
Nuclear decommissioning trusts 86  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Foreign equity    
Assets:    
Nuclear decommissioning trusts 55  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury and government agency securities    
Assets:    
Nuclear decommissioning trusts 18  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate bonds    
Assets:    
Nuclear decommissioning trusts 158  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Mortgage and asset backed securities    
Assets:    
Nuclear decommissioning trusts 18  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Private equity    
Assets:    
Nuclear decommissioning trusts 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Other    
Assets:    
Nuclear decommissioning trusts 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)    
Assets:    
Energy-related derivatives 0  
Cash equivalents 0  
Total 0  
Liabilities:    
Energy-related derivatives 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Domestic equity    
Assets:    
Nuclear decommissioning trusts 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Foreign equity    
Assets:    
Nuclear decommissioning trusts 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury and government agency securities    
Assets:    
Nuclear decommissioning trusts 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate bonds    
Assets:    
Nuclear decommissioning trusts 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Mortgage and asset backed securities    
Assets:    
Nuclear decommissioning trusts 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Private equity    
Assets:    
Nuclear decommissioning trusts 0  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Other    
Assets:    
Nuclear decommissioning trusts 0  
GEORGIA POWER CO    
Assets:    
Energy-related derivatives 7 6
Nuclear decommissioning trusts 63  
Liabilities:    
Energy-related derivatives 25 24
GEORGIA POWER CO | Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives 7  
Total 931  
Liabilities:    
Energy-related derivatives 19  
Interest rate derivatives 6  
Total 25  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Domestic equity    
Assets:    
Nuclear decommissioning trusts 243  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Foreign equity    
Assets:    
Nuclear decommissioning trusts 135  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | U.S. Treasury and government agency securities    
Assets:    
Nuclear decommissioning trusts 239  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Municipal bonds    
Assets:    
Nuclear decommissioning trusts 78  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Corporate bonds    
Assets:    
Nuclear decommissioning trusts 164  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Mortgage and asset backed securities    
Assets:    
Nuclear decommissioning trusts 41  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Other    
Assets:    
Nuclear decommissioning trusts 24  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Energy-related derivatives 0  
Total 261  
Liabilities:    
Energy-related derivatives 0  
Interest rate derivatives 0  
Total 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity    
Assets:    
Nuclear decommissioning trusts 242  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign equity    
Assets:    
Nuclear decommissioning trusts 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury and government agency securities    
Assets:    
Nuclear decommissioning trusts 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds    
Assets:    
Nuclear decommissioning trusts 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds    
Assets:    
Nuclear decommissioning trusts 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage and asset backed securities    
Assets:    
Nuclear decommissioning trusts 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other    
Assets:    
Nuclear decommissioning trusts 19  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Energy-related derivatives 7  
Total 670  
Liabilities:    
Energy-related derivatives 19  
Interest rate derivatives 6  
Total 25  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Domestic equity    
Assets:    
Nuclear decommissioning trusts 1  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Foreign equity    
Assets:    
Nuclear decommissioning trusts 135  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury and government agency securities    
Assets:    
Nuclear decommissioning trusts 239  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds    
Assets:    
Nuclear decommissioning trusts 78  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate bonds    
Assets:    
Nuclear decommissioning trusts 164  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Mortgage and asset backed securities    
Assets:    
Nuclear decommissioning trusts 41  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Other    
Assets:    
Nuclear decommissioning trusts 5  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)    
Assets:    
Energy-related derivatives 0  
Total 0  
Liabilities:    
Energy-related derivatives 0  
Interest rate derivatives 0  
Total 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Domestic equity    
Assets:    
Nuclear decommissioning trusts 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Foreign equity    
Assets:    
Nuclear decommissioning trusts 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury and government agency securities    
Assets:    
Nuclear decommissioning trusts 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Municipal bonds    
Assets:    
Nuclear decommissioning trusts 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate bonds    
Assets:    
Nuclear decommissioning trusts 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Mortgage and asset backed securities    
Assets:    
Nuclear decommissioning trusts 0  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Other    
Assets:    
Nuclear decommissioning trusts 0  
GULF POWER CO    
Assets:    
Energy-related derivatives 0 0
Liabilities:    
Energy-related derivatives 12 21
GULF POWER CO | Fair Value, Measurements, Recurring    
Assets:    
Cash equivalents 27  
Liabilities:    
Energy-related derivatives 12  
GULF POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Cash equivalents 27  
Liabilities:    
Energy-related derivatives 0  
GULF POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Cash equivalents 0  
Liabilities:    
Energy-related derivatives 12  
GULF POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)    
Assets:    
Cash equivalents 0  
Liabilities:    
Energy-related derivatives 0  
MISSISSIPPI POWER CO    
Assets:    
Energy-related derivatives 3 3
Liabilities:    
Energy-related derivatives 9 9
MISSISSIPPI POWER CO | Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives 3  
Cash equivalents 205  
Total 208  
Liabilities:    
Energy-related derivatives 9  
MISSISSIPPI POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Energy-related derivatives 0  
Cash equivalents 205  
Total 205  
Liabilities:    
Energy-related derivatives 0  
MISSISSIPPI POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Energy-related derivatives 3  
Cash equivalents 0  
Total 3  
Liabilities:    
Energy-related derivatives 9  
MISSISSIPPI POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)    
Assets:    
Energy-related derivatives 0  
Cash equivalents 0  
Total 0  
Liabilities:    
Energy-related derivatives 0  
SOUTHERN POWER CO    
Assets:    
Energy-related derivatives 135 132
Liabilities:    
Energy-related derivatives 26 36
SOUTHERN POWER CO | Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives 3  
Foreign currency derivatives 132  
Total 135  
Liabilities:    
Energy-related derivatives 3  
Foreign currency derivatives 23  
Contingent consideration 22  
Total 48  
SOUTHERN POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Energy-related derivatives 0  
Foreign currency derivatives 0  
Total 0  
Liabilities:    
Energy-related derivatives 0  
Foreign currency derivatives 0  
Contingent consideration 0  
Total 0  
SOUTHERN POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Energy-related derivatives 3  
Foreign currency derivatives 132  
Total 135  
Liabilities:    
Energy-related derivatives 3  
Foreign currency derivatives 23  
Contingent consideration 0  
Total 26  
SOUTHERN POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)    
Assets:    
Energy-related derivatives 0  
Foreign currency derivatives 0  
Total 0  
Liabilities:    
Energy-related derivatives 0  
Foreign currency derivatives 0  
Contingent consideration 22  
Total 22  
SOUTHERN Co GAS    
Assets:    
Energy-related derivatives 431 554
Liabilities:    
Energy-related derivatives 521 660
Collateral already posted, aggregate fair value 183 $ 193
SOUTHERN Co GAS | Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives 431  
Cash equivalents 1  
Total 432  
Liabilities:    
Energy-related derivatives 521  
SOUTHERN Co GAS | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Energy-related derivatives 287  
Cash equivalents 1  
Total 288  
Liabilities:    
Energy-related derivatives 422  
SOUTHERN Co GAS | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Energy-related derivatives 144  
Cash equivalents 0  
Total 144  
Liabilities:    
Energy-related derivatives 99  
SOUTHERN Co GAS | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)    
Assets:    
Energy-related derivatives 0  
Cash equivalents 0  
Total 0  
Liabilities:    
Energy-related derivatives 0  
Nuclear Decommissioning Trusts    
Assets:    
Fair Value 33  
Nuclear Decommissioning Trusts | ALABAMA POWER CO    
Assets:    
Fair Value 33  
Weather Derivative | Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives $ 3