SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of AOCI (Loss) Balances, Net of Tax Effects (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 29,039	$ 25,528	$ 26,612
Current period change	(118)	26	(9)
Ending balance	31,759	29,039	25,528
Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(121)
Ending balance		(121)
Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Current period change	(58)
Ending balance	(179)
Pension and Other Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(82)
Current period change	(60)
Ending balance	(142)	(82)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(203)	(189)	(180)
Current period change	(118)
Ending balance	(321)	(203)	(189)
Southern Power
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	7,284	6,498	5,675
Current period change	(42)	14	(10)
Ending balance	6,622	7,284	6,498
Southern Power | Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	36
Ending balance		36
Southern Power | Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Current period change	(25)
Ending balance	11
Southern Power | Pension and Other Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(20)
Current period change	(17)
Ending balance	(37)	(20)
Southern Power | Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	16	(2)	35
Current period change	(42)
Ending balance	(26)	16	(2)
Southern Company Gas
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	8,570	9,022	9,109
Current period change	(19)	2	(5)
Ending balance	9,506	8,570	9,022
Southern Company Gas | Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(3)
Ending balance		(3)
Southern Company Gas | Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Current period change	(3)
Ending balance	(6)
Southern Company Gas | Pension and Other Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	29
Current period change	(16)
Ending balance	13	29
Southern Company Gas | Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	26	20	26
Current period change	(19)
Ending balance	$ 7	$ 26	$ 20