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