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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Assets:    
Fair Value $ 32  
Liabilities:    
Collateral already posted, aggregate fair value 223  
Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives 532  
Foreign currency derivatives 182  
Nuclear decommissioning trusts 1,827  
Cash equivalents 1,664  
Other investments 10  
Total 4,215  
Liabilities:    
Energy-related derivatives 642  
Interest rate derivatives 61  
Foreign currency derivatives 22  
Contingent consideration 22  
Total 747  
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Energy-related derivatives 360  
Foreign currency derivatives 0  
Nuclear decommissioning trusts 776  
Cash equivalents 1,664  
Other investments 9  
Total 2,809  
Liabilities:    
Energy-related derivatives 506  
Interest rate derivatives 0  
Foreign currency derivatives 0  
Contingent consideration 0  
Total 506  
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Energy-related derivatives 172  
Foreign currency derivatives 182  
Nuclear decommissioning trusts 1,019  
Cash equivalents 0  
Other investments 0  
Total 1,373  
Liabilities:    
Energy-related derivatives 136  
Interest rate derivatives 61  
Foreign currency derivatives 22  
Contingent consideration 0  
Total 219  
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 3 $ 4
Fair Value 32  
Liabilities:    
Energy-related derivatives 8 10
ALABAMA POWER CO | Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives 3  
Cash equivalents 459  
Total 1,360  
Liabilities:    
Energy-related derivatives 8  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Domestic equity    
Assets:    
Nuclear decommissioning trusts 520  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Foreign equity    
Assets:    
Nuclear decommissioning trusts 121  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | U.S. Treasury and government agency securities    
Assets:    
Nuclear decommissioning trusts 19  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Corporate bonds    
Assets:    
Nuclear decommissioning trusts 183  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Mortgage and asset backed securities    
Assets:    
Nuclear decommissioning trusts 17  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Private Equity    
Assets:    
Nuclear decommissioning trusts 32  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Other    
Assets:    
Nuclear decommissioning trusts 6  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Energy-related derivatives 0  
Cash equivalents 459  
Total 986  
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 437  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign equity    
Assets:    
Nuclear decommissioning trusts 63  
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 21  
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 6  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Energy-related derivatives 3  
Cash equivalents 0  
Total 342  
Liabilities:    
Energy-related derivatives 8  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Domestic equity    
Assets:    
Nuclear decommissioning trusts 83  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Foreign equity    
Assets:    
Nuclear decommissioning trusts 58  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury and government agency securities    
Assets:    
Nuclear decommissioning trusts 19  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate bonds    
Assets:    
Nuclear decommissioning trusts 162  
ALABAMA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Mortgage and asset backed securities    
Assets:    
Nuclear decommissioning trusts 17  
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 5 6
Nuclear decommissioning trusts 76  
Liabilities:    
Energy-related derivatives 26 24
GEORGIA POWER CO | Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives 5  
Cash equivalents 1,055  
Total 1,990  
Liabilities:    
Energy-related derivatives 18  
Interest rate derivatives 8  
Total 26  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Domestic equity    
Assets:    
Nuclear decommissioning trusts 239  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Foreign equity    
Assets:    
Nuclear decommissioning trusts 141  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | U.S. Treasury and government agency securities    
Assets:    
Nuclear decommissioning trusts 241  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Corporate bonds    
Assets:    
Nuclear decommissioning trusts 168  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Mortgage and asset backed securities    
Assets:    
Nuclear decommissioning trusts 40  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Other    
Assets:    
Nuclear decommissioning trusts 25  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Municipal bonds    
Assets:    
Nuclear decommissioning trusts 76  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Energy-related derivatives 0  
Cash equivalents 1,055  
Total 1,304  
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 238  
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) | 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 11  
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 | Significant Other Observable Inputs (Level 2)    
Assets:    
Energy-related derivatives 5  
Cash equivalents 0  
Total 686  
Liabilities:    
Energy-related derivatives 18  
Interest rate derivatives 8  
Total 26  
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 141  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury and government agency securities    
Assets:    
Nuclear decommissioning trusts 241  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate bonds    
Assets:    
Nuclear decommissioning trusts 168  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Mortgage and asset backed securities    
Assets:    
Nuclear decommissioning trusts 40  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Other    
Assets:    
Nuclear decommissioning trusts 14  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds    
Assets:    
Nuclear decommissioning trusts 76  
GEORGIA 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  
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) | 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  
GEORGIA POWER CO | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Municipal bonds    
Assets:    
Nuclear decommissioning trusts 0  
GULF POWER CO    
Assets:    
Energy-related derivatives 0 0
Liabilities:    
Energy-related derivatives 17 21
GULF POWER CO | Fair Value, Measurements, Recurring    
Assets:    
Cash equivalents 27  
Liabilities:    
Energy-related derivatives 17  
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 17  
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 2 3
Liabilities:    
Energy-related derivatives 8 9
MISSISSIPPI POWER CO | Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives 2  
Cash equivalents 103  
Total 105  
Liabilities:    
Energy-related derivatives 8  
MISSISSIPPI POWER CO | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Energy-related derivatives 0  
Cash equivalents 103  
Total 103  
Liabilities:    
Energy-related derivatives 0  
MISSISSIPPI POWER CO | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Energy-related derivatives 2  
Cash equivalents 0  
Total 2  
Liabilities:    
Energy-related derivatives 8  
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 186 132
Liabilities:    
Energy-related derivatives 25 36
SOUTHERN POWER CO | Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives 4  
Foreign currency derivatives 182  
Total 186  
Liabilities:    
Energy-related derivatives 3  
Foreign currency derivatives 22  
Contingent consideration 22  
Total 47  
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 4  
Foreign currency derivatives 182  
Total 186  
Liabilities:    
Energy-related derivatives 3  
Foreign currency derivatives 22  
Contingent consideration 0  
Total 25  
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 518 554
Liabilities:    
Energy-related derivatives 588 660
Collateral already posted, aggregate fair value 223 $ 193
SOUTHERN Co GAS | Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives 518  
Liabilities:    
Energy-related derivatives 588  
SOUTHERN Co GAS | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Energy-related derivatives 360  
Liabilities:    
Energy-related derivatives 506  
SOUTHERN Co GAS | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Energy-related derivatives 158  
Liabilities:    
Energy-related derivatives 82  
SOUTHERN Co GAS | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)    
Assets:    
Energy-related derivatives 0  
Liabilities:    
Energy-related derivatives 0  
Nuclear Decommissioning Trusts    
Assets:    
Fair Value 32  
Nuclear Decommissioning Trusts | ALABAMA POWER CO    
Assets:    
Fair Value 32  
Weather Derivative | Fair Value, Measurements, Recurring    
Assets:    
Energy-related derivatives $ 4