Summary of Significant Accounting Policies - Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	$ 2,201	$ 2,018
Liabilities incurred	662	18
Liabilities settled	(37)	(17)
Accretion	115	102
Cash flow revisions	818	80
Balance at end of year	3,759	2,201
Alabama Power [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	829	730
Liabilities incurred	402	1
Liabilities settled	(3)	(3)
Accretion	53	45
Cash flow revisions	167	56
Balance at end of year	1,448	829
Georgia Power [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	1,255	1,222
Liabilities incurred	6	9
Liabilities settled	(30)	(12)
Accretion	56	53
Cash flow revisions	629	(17)
Balance at end of year	1,916	1,255
Gulf Power [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	17	16
Liabilities incurred	105	0
Liabilities settled	(1)	0
Accretion	2	1
Cash flow revisions	7	0
Balance at end of year	130	17
Mississippi Power [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	48	42
Liabilities incurred	101	0
Liabilities settled	(3)	(3)
Accretion	4	2
Cash flow revisions	27	7
Balance at end of year	177	48
Southern Power [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	13	4
Liabilities incurred	7	8
Accretion	1	1
Balance at end of year	$ 21	$ 13