Income Taxes - Changes in Unrecognized Tax Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Changes in unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits at beginning of year	$ 18,000,000	$ 484,000,000	$ 433,000,000
Increase from current periods		10,000,000	45,000,000
Increase from prior periods		10,000,000	21,000,000
Decrease from prior periods	(18,000,000)	(196,000,000)	(15,000,000)
Reductions due to settlements		(290,000,000)
Balance at end of year	0	18,000,000	484,000,000
Mississippi Power
Changes in unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits at beginning of year	0	465,000,000	421,000,000
Increase from current periods		0	26,000,000
Increase from prior periods		2,000,000	18,000,000
Decrease from prior periods	0	(177,000,000)	0
Reductions due to settlements		(290,000,000)
Balance at end of year	0	0	465,000,000
Southern Power
Changes in unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits at beginning of year	0	17,000,000	8,000,000
Increase from current periods		0	17,000,000
Increase from prior periods		0	0
Decrease from prior periods	0	(17,000,000)	(8,000,000)
Reductions due to settlements		0
Balance at end of year	$ 0	$ 0	$ 17,000,000