v3.10.0.1
Financing - Bank Credit Arrangements (Details) - USD ($)
Dec. 31, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]    
Expires, 2019 $ 163,000,000  
Expires, 2020 500,000,000  
Expires, 2022 7,200,000,000  
Total 7,863,000,000  
Unused 7,820,000,000  
Executable term-loans, one year 0  
Executable term-loans, two years 0  
Due within one year, term out 0  
Due within one year, no term out 163,000,000  
Southern Company    
Line of Credit Facility [Line Items]    
Expires, 2019 0  
Expires, 2020 0  
Expires, 2022 2,000,000,000  
Total 2,000,000,000  
Unused 1,999,000,000  
Executable term-loans, one year 0  
Executable term-loans, two years 0  
Due within one year, term out 0  
Due within one year, no term out 0  
Alabama Power    
Line of Credit Facility [Line Items]    
Expires, 2019 33,000,000  
Expires, 2020 500,000,000  
Expires, 2022 800,000,000  
Total 1,333,000,000  
Unused 1,333,000,000  
Executable term-loans, one year 0  
Executable term-loans, two years 0  
Due within one year, term out 0  
Due within one year, no term out 33,000,000  
Georgia Power    
Line of Credit Facility [Line Items]    
Expires, 2019 0  
Expires, 2020 0  
Expires, 2022 1,750,000,000  
Total 1,750,000,000  
Unused 1,736,000,000  
Executable term-loans, one year 0  
Executable term-loans, two years 0  
Due within one year, term out 0  
Due within one year, no term out 0  
Mississippi Power    
Line of Credit Facility [Line Items]    
Expires, 2019 100,000,000  
Expires, 2020 0  
Expires, 2022 0  
Total 100,000,000  
Unused 100,000,000  
Executable term-loans, one year 0  
Executable term-loans, two years 0  
Due within one year, term out 0  
Due within one year, no term out 100,000,000  
Southern Power    
Line of Credit Facility [Line Items]    
Expires, 2019 0  
Expires, 2020 0  
Expires, 2022 750,000,000  
Total 750,000,000  
Unused 727,000,000  
Executable term-loans, one year 0  
Executable term-loans, two years 0  
Due within one year, term out 0  
Due within one year, no term out 0  
Southern Power | Continuing Letter of Credit Facility    
Line of Credit Facility [Line Items]    
Total 120,000,000  
Unused 103,000,000 $ 19,000,000
Southern Company Gas    
Line of Credit Facility [Line Items]    
Expires, 2019 0  
Expires, 2020 0  
Expires, 2022 1,900,000,000  
Total 1,900,000,000  
Unused 1,895,000,000  
Executable term-loans, one year 0  
Executable term-loans, two years 0  
Due within one year, term out 0  
Due within one year, no term out 0  
Other Subsidiaries    
Line of Credit Facility [Line Items]    
Expires, 2019 30,000,000  
Expires, 2020 0  
Expires, 2022 0  
Total 30,000,000  
Unused 30,000,000  
Executable term-loans, one year 0  
Executable term-loans, two years 0  
Due within one year, term out 0  
Due within one year, no term out 30,000,000  
Nicor Gas    
Line of Credit Facility [Line Items]    
Total 500,000,000  
Southern Company Gas Capital    
Line of Credit Facility [Line Items]    
Total 1,400,000,000.0  
Unused 1,395,000,000  
Gulf Power    
Line of Credit Facility [Line Items]    
Total $ 280,000,000