Summary of Significant Accounting Policies - Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	$ 4,514	$ 3,759
Liabilities incurred	16	66
Liabilities settled	(177)	(171)
Liabilities settled	(177)	(171)	$ (37)
Accretion	179	162
Cash flow revisions	292	698
Balance at end of year	4,824	4,514	3,759
ALABAMA POWER CO
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	1,533	1,448
Liabilities incurred	0	5
Liabilities settled	(26)	(25)
Accretion	77	73
Cash flow revisions	125	32
Balance at end of year	1,709	1,533	1,448
GEORGIA POWER CO
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	2,532	1,916
Liabilities incurred	4	0
Liabilities settled	(120)	(123)	(29)
Accretion	89	77
Cash flow revisions	133	662
Balance at end of year	2,638	2,532	1,916
GULF POWER CO
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	136	130
Liabilities incurred	0	1
Liabilities settled	(8)	(1)
Accretion	2	4
Cash flow revisions	12	2
Balance at end of year	142	136	130
MISSISSIPPI POWER CO
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	179	177
Liabilities incurred	0	15
Liabilities settled	(23)	(23)
Accretion	5	5
Cash flow revisions	13	5
Balance at end of year	174	179	177
SOUTHERN POWER CO
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	64	21
Liabilities incurred	6	42
Accretion	4	1
Cash flow revisions	4	0
Balance at end of year	$ 78	$ 64	$ 21