Income Taxes - Changes in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits at beginning of year	$ 433	$ 170	$ 7
Tax positions increase from current periods	45	43	64
Tax positions increase from prior periods	21	240	102
Tax positions decrease from prior periods	(15)	(20)	(3)
Unrecognized tax benefits at end of year	484	433	170
Mississippi Power [Member]
Changes in unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits at beginning of year	421	165	4
Tax positions increase from current periods	26	32	58
Tax positions increase from prior periods	18	224	103
Unrecognized tax benefits at end of year	465	421	165
Southern Power [Member]
Changes in unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits at beginning of year	8	5	2
Tax positions increase from current periods	17	9	5
Tax positions decrease from prior periods	(8)	(6)	(2)
Unrecognized tax benefits at end of year	$ 17	$ 8	$ 5