SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of AOCI (Loss) Balances, Net of Tax Effects (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 32,234	$ 31,759	$ 29,039
Current period change	158	(75)	(118)
Ending balance	32,276	32,234	31,759
Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(209)
Current period change	47
Ending balance	(162)	(209)
Pension and Other Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(187)
Current period change	111
Ending balance	(76)	(187)
Accumulated Other Comprehensive Income (Loss)(*)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(395)	(321)	(203)
Current period change	158
Ending balance	(237)	(395)	(321)
Southern Power
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	6,631	6,622	7,284
Current period change	40	(42)	(42)
Ending balance	6,598	6,631	6,622
Southern Power | Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(21)
Current period change	22
Ending balance	1	(21)
Southern Power | Pension and Other Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(47)
Current period change	18
Ending balance	(29)	(47)
Southern Power | Accumulated Other Comprehensive Income (Loss)(*)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(67)	(26)	16
Current period change	40
Ending balance	(27)	(67)	(26)
Southern Company Gas
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	9,767	9,506	8,570
Current period change	46	(29)	(19)
Ending balance	9,916	9,767	9,506
Southern Company Gas | Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(20)
Current period change	6
Ending balance	(14)	(20)
Southern Company Gas | Pension and Other Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(2)
Current period change	40
Ending balance	38	(2)
Southern Company Gas | Accumulated Other Comprehensive Income (Loss)(*)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(22)	7	26
Current period change	46
Ending balance	$ 24	$ (22)	$ 7