Income Taxes - Changes in Unrecognized Tax Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits at beginning of year	$ 484,000,000	$ 433,000,000	$ 170,000,000
Tax positions increase from current periods	10,000,000	45,000,000	43,000,000
Tax positions increase from prior periods	10,000,000	21,000,000	240,000,000
Tax positions decrease from prior periods	(196,000,000)	(15,000,000)	(20,000,000)
Reductions due to settlements	(290,000,000)	0	0
Balance at end of year	18,000,000	484,000,000	433,000,000
GEORGIA POWER CO
Changes in unrecognized tax benefits [Roll Forward]
Balance at end of year	0
MISSISSIPPI POWER CO
Changes in unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits at beginning of year	465,000,000	421,000,000	421,000,000
Tax positions increase from current periods	0	26,000,000	32,000,000
Tax positions increase from prior periods	2,000,000	18,000,000	224,000,000
Tax positions decrease from prior periods	(177,000,000)	0	0
Reductions due to settlements	(290,000,000)	0	0
Balance at end of year	0	465,000,000	421,000,000
SOUTHERN POWER CO
Changes in unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits at beginning of year	17,000,000	8,000,000	5,000,000
Tax positions increase from current periods	0	17,000,000	9,000,000
Tax positions decrease from prior periods	(17,000,000)	(8,000,000)	(6,000,000)
Balance at end of year	$ 0	$ 17,000,000	$ 8,000,000