v3.10.0.1
Retirement Benefits - Amounts Recognized in Balance Sheets and Amounts in AOCI (Details) - USD ($)
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]      
Other regulatory assets, deferred $ 8,308,000,000 $ 6,943,000,000  
Other deferred charges and assets 1,463,000,000 1,577,000,000  
Other current liabilities (852,000,000) (874,000,000)  
Employee benefit obligations (2,147,000,000) (2,256,000,000)  
Other regulatory liabilities, deferred (169,000,000) (239,000,000)  
Pension plans      
Defined Benefit Plan Disclosure [Line Items]      
Prepaid pension costs 0 0  
Other regulatory assets, deferred 3,566,000,000 3,273,000,000  
Other deferred charges and assets 0 0  
Other current liabilities (55,000,000) (53,000,000)  
Employee benefit obligations (1,097,000,000) (763,000,000)  
Other regulatory liabilities, deferred (108,000,000) (118,000,000)  
AOCI 97,000,000 107,000,000  
Other postretirement benefit plans      
Defined Benefit Plan Disclosure [Line Items]      
Other regulatory assets, deferred 99,000,000 382,000,000  
Other current liabilities (6,000,000) (5,000,000)  
Employee benefit obligations (931,000,000) (1,281,000,000)  
Other regulatory liabilities, deferred (77,000,000) (41,000,000)  
AOCI (4,000,000) 4,000,000  
Alabama Power      
Defined Benefit Plan Disclosure [Line Items]      
Other regulatory assets, deferred 1,386,000,000 1,272,000,000  
Other deferred charges and assets 189,000,000 189,000,000  
Other current liabilities (161,000,000) (53,000,000)  
Employee benefit obligations (314,000,000) (304,000,000)  
Other regulatory liabilities, deferred (69,000,000) (84,000,000)  
Alabama Power | Pension plans      
Defined Benefit Plan Disclosure [Line Items]      
Prepaid pension costs 0 0  
Other regulatory assets, deferred 955,000,000 890,000,000  
Other deferred charges and assets 0 0  
Other current liabilities (12,000,000) (12,000,000)  
Employee benefit obligations (229,000,000) (150,000,000)  
Other regulatory liabilities, deferred 0 0  
AOCI 0 0  
Alabama Power | Other postretirement benefit plans      
Defined Benefit Plan Disclosure [Line Items]      
Other regulatory assets, deferred 0 63,000,000  
Other current liabilities 0 0  
Employee benefit obligations (43,000,000) (111,000,000)  
Other regulatory liabilities, deferred (8,000,000) (7,000,000)  
AOCI 0 0  
Georgia Power      
Defined Benefit Plan Disclosure [Line Items]      
Other regulatory assets, deferred 4,902,000,000 2,932,000,000  
Other deferred charges and assets 514,000,000 548,000,000  
Other current liabilities (183,000,000) (198,000,000)  
Employee benefit obligations (599,000,000) (659,000,000)  
Georgia Power | Pension plans      
Defined Benefit Plan Disclosure [Line Items]      
Prepaid pension costs 0 23,000,000  
Other regulatory assets, deferred 1,230,000,000 1,105,000,000  
Other deferred charges and assets 0 0  
Other current liabilities (15,000,000) (15,000,000)  
Employee benefit obligations (227,000,000) (138,000,000)  
Other regulatory liabilities, deferred 0 0  
AOCI 0 0  
Georgia Power | Other postretirement benefit plans      
Defined Benefit Plan Disclosure [Line Items]      
Other regulatory assets, deferred 60,000,000 202,000,000  
Other current liabilities 0 0  
Employee benefit obligations (331,000,000) (477,000,000)  
Other regulatory liabilities, deferred 0 0  
AOCI 0 0  
Mississippi Power      
Defined Benefit Plan Disclosure [Line Items]      
Other regulatory assets, deferred 474,000,000 437,000,000  
Other deferred charges and assets 3,000,000 33,000,000  
Other current liabilities (40,000,000) (47,000,000)  
Employee benefit obligations (115,000,000) (116,000,000)  
Other regulatory liabilities, deferred (81,000,000) (79,000,000)  
Mississippi Power | Pension plans      
Defined Benefit Plan Disclosure [Line Items]      
Prepaid pension costs 0 0  
Other regulatory assets, deferred 167,000,000 158,000,000  
Other deferred charges and assets 0 0  
Other current liabilities (3,000,000) (3,000,000)  
Employee benefit obligations (49,000,000) (36,000,000)  
Other regulatory liabilities, deferred 0 0  
AOCI 0 0  
Mississippi Power | Other postretirement benefit plans      
Defined Benefit Plan Disclosure [Line Items]      
Other regulatory assets, deferred 6,000,000 18,000,000  
Other current liabilities 0 0  
Employee benefit obligations (58,000,000) (72,000,000)  
Other regulatory liabilities, deferred (2,000,000) (1,000,000)  
AOCI 0 0  
Southern Power      
Defined Benefit Plan Disclosure [Line Items]      
Other deferred charges and assets 373,000,000 566,000,000  
Other current liabilities (142,000,000) (148,000,000)  
Southern Power | Pension plans      
Defined Benefit Plan Disclosure [Line Items]      
Prepaid pension costs 1,000,000 0  
Other regulatory assets, deferred 0 0  
Other deferred charges and assets 0 0  
Other current liabilities 0 0  
Employee benefit obligations (1,000,000) (1,000,000)  
Other regulatory liabilities, deferred 0 0  
AOCI 26,000,000 33,000,000  
Southern Power | Other postretirement benefit plans      
Defined Benefit Plan Disclosure [Line Items]      
Other regulatory assets, deferred 0 0  
Other current liabilities 0 0  
Employee benefit obligations (9,000,000) (11,000,000)  
Other regulatory liabilities, deferred 0 0  
AOCI 1,000,000 3,000,000  
Southern Company Gas      
Defined Benefit Plan Disclosure [Line Items]      
Other regulatory assets, deferred 669,000,000 901,000,000  
Other deferred charges and assets 193,000,000 218,000,000  
Other current liabilities (130,000,000) (159,000,000)  
Employee benefit obligations (357,000,000) (415,000,000)  
Southern Company Gas | Pension plans      
Defined Benefit Plan Disclosure [Line Items]      
Prepaid pension costs 0 0  
Other regulatory assets, deferred 160,000,000 217,000,000  
Other deferred charges and assets 74,000,000 85,000,000  
Other current liabilities (3,000,000) (3,000,000)  
Employee benefit obligations (179,000,000) (198,000,000)  
Other regulatory liabilities, deferred 0 0  
AOCI (44,000,000) (42,000,000)  
Regulatory asset     $ 268,000,000
Southern Company Gas | Other postretirement benefit plans      
Defined Benefit Plan Disclosure [Line Items]      
Other regulatory assets, deferred (4,000,000) 46,000,000  
Other current liabilities 0 0  
Employee benefit obligations 146,000,000 (185,000,000)  
Other regulatory liabilities, deferred 0 0  
AOCI $ (4,000,000) $ (3,000,000)  
Regulatory asset     $ 57,000,000