v3.10.0.1
Contingencies - Other Matters (Details)
 12 Months Ended
May 14, 2018
USD ($)
Dec. 31, 2018
USD ($)
cavern
Dec. 31, 2017
USD ($)
Schedule of Regulatory Assets and Liabilities [Line Items]      
Property, plant and equipment   $ 80,797,000,000 $ 79,872,000,000
Natural Gas Storage - Salt Dome Caverns      
Schedule of Regulatory Assets and Liabilities [Line Items]      
Number of salt dome caverns | cavern   2  
Property, plant and equipment   $ 109,000,000  
Property, Plant and Equipment | Natural Gas Storage - Salt Dome Caverns      
Schedule of Regulatory Assets and Liabilities [Line Items]      
Concentration risk (as percent)   20.00%  
Mississippi Power      
Schedule of Regulatory Assets and Liabilities [Line Items]      
Property, plant and equipment   $ 3,574,000,000 3,532,000,000
Southern Company Gas      
Schedule of Regulatory Assets and Liabilities [Line Items]      
Property, plant and equipment   11,357,000,000 $ 11,728,000,000
Impairment of long-lived assets   $ 0  
Deepwater Horizon Oil Spill Litigation Case | Mississippi Power      
Schedule of Regulatory Assets and Liabilities [Line Items]      
Litigation settlement awarded from other party $ 18,000,000