ASSET RETIREMENT OBLIGATIONS - Details of AROs Included in the Balance Sheets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	$ 9,394	$ 4,824
Liabilities incurred	37	29
Liabilities settled	(328)	(244)
Accretion	402	217
Cash flow revisions	281	4,737
Reclassification to held for sale		(169)
Balance at end of year	9,786	9,394
Alabama Power
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	3,210	1,709
Liabilities incurred	0	0
Liabilities settled	(127)	(55)
Accretion	145	106
Cash flow revisions	312	1,450
Reclassification to held for sale		0
Balance at end of year	3,540	3,210
Georgia Power
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	5,829	2,638
Liabilities incurred	35	27
Liabilities settled	(151)	(116)
Accretion	243	94
Cash flow revisions	(172)	3,186
Reclassification to held for sale		0
Balance at end of year	5,784	5,829
Mississippi Power
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	160	174
Liabilities incurred	1	0
Liabilities settled	(35)	(35)
Accretion	7	5
Cash flow revisions	57	16
Reclassification to held for sale		0
Balance at end of year	190	160
Southern Power
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	84	78
Liabilities incurred	1	2
Liabilities settled	0	0
Accretion	4	4
Cash flow revisions	0	0
Reclassification to held for sale		0
Balance at end of year	$ 89	$ 84