ASSET RETIREMENT OBLIGATIONS - Details of AROs Included in the Balance Sheets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	$ 9,786	$ 9,394
Liabilities incurred	19	37
Liabilities settled	(442)	(328)
Accretion	409	402
Cash flow revisions	912	281
Balance at end of year	10,684	9,786
Alabama Power
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	3,540	3,210
Liabilities incurred	0	0
Liabilities settled	(219)	(127)
Accretion	152	145
Cash flow revisions	501	312
Balance at end of year	3,974	3,540
Georgia Power
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	5,784	5,829
Liabilities incurred	10	35
Liabilities settled	(185)	(151)
Accretion	238	243
Cash flow revisions	418	(172)
Balance at end of year	6,265	5,784
Mississippi Power
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	190	160
Liabilities incurred	0	1
Liabilities settled	(22)	(35)
Accretion	8	7
Cash flow revisions	0	57
Balance at end of year	176	190
Southern Power
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	89	84
Liabilities incurred	9	1
Liabilities settled	0	0
Accretion	4	4
Cash flow revisions	(7)	0
Balance at end of year	$ 95	$ 89