RETIREMENT BENEFITS - Changes in Regulatory Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 5,833
Total other comprehensive income (loss)	(10)	$ 71	$ 158
Ending balance	6,523	5,833
Regulatory asset	66	216
Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Total other comprehensive income (loss)	(38)
Pension plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	2,371
Ending balance	2,913	2,371
Pension plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	2,371	2,801
Reclassification adjustments	(7)
Total other comprehensive income (loss)	55	(76)
Ending balance	2,913	2,371	2,801
Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(34)	(247)
Total other comprehensive income (loss)	542	(430)
Pension plans | Net gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	62	(82)
Reclassification adjustments	(8)	6
Pension plans | Net gain | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	576	(183)
Reclassification adjustments	(33)	(246)
Pension plans | Change in prior service costs
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	1
Pension plans | Change in prior service costs | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(1)	(1)
Pension plans | Amortization of regulatory assets | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Other Postretirement Benefits Plan
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	136
Reclassification adjustments	7	(1)
Ending balance	203	136
Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(136)	(74)
Total other comprehensive income (loss)	(5)	(4)
Ending balance	(208)	(136)	(74)
Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	5	2
Total other comprehensive income (loss)	(72)	(62)
Other Postretirement Benefits Plan | Net gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	(12)	(3)
Other Postretirement Benefits Plan | Net gain | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	77	64
Reclassification adjustments	6	1
Other Postretirement Benefits Plan | Change in prior service costs | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(1)	1
Other Postretirement Benefits Plan | Amortization of regulatory assets | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Alabama Power
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	2,340
Total other comprehensive income (loss)	2	4	6
Ending balance	2,720	2,340
Regulatory asset	76	97
Alabama Power | Pension plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	679
Ending balance	821	679
Alabama Power | Pension plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	679	809
Ending balance	821	679	809
Alabama Power | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(11)	(63)
Total other comprehensive income (loss)	142	(130)
Alabama Power | Pension plans | Net gain | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	153	(67)
Reclassification adjustments	(10)	(62)
Alabama Power | Pension plans | Change in prior service costs | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(1)	(1)
Alabama Power | Pension plans | Amortization of regulatory assets | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Alabama Power | Other Postretirement Benefits Plan
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	21
Ending balance	48	21
Alabama Power | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(21)	(62)
Ending balance	(48)	(21)	(62)
Alabama Power | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	3	0
Total other comprehensive income (loss)	(27)	41
Alabama Power | Other Postretirement Benefits Plan | Net gain | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	30	(41)
Reclassification adjustments	3	0
Alabama Power | Other Postretirement Benefits Plan | Change in prior service costs | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Alabama Power | Other Postretirement Benefits Plan | Amortization of regulatory assets | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Georgia Power
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	4,663
Total other comprehensive income (loss)	3	28	6
Ending balance	5,090	4,663
Regulatory asset		83
Georgia Power | Pension plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	887
Ending balance	1,051	887
Georgia Power | Pension plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	887	971
Ending balance	1,051	887	971
Georgia Power | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(14)	(75)
Total other comprehensive income (loss)	164	(84)
Georgia Power | Pension plans | Net gain | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	178	(9)
Reclassification adjustments	(13)	(74)
Georgia Power | Pension plans | Change in prior service costs | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(1)	(1)
Georgia Power | Pension plans | Amortization of regulatory assets | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Georgia Power | Other Postretirement Benefits Plan
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	39
Ending balance	74	39
Georgia Power | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(39)	(10)
Ending balance	(74)	(39)	(10)
Georgia Power | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	3	(2)
Total other comprehensive income (loss)	(35)	(29)
Georgia Power | Other Postretirement Benefits Plan | Net gain | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	38	27
Reclassification adjustments	4	(2)
Georgia Power | Other Postretirement Benefits Plan | Change in prior service costs | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(1)	0
Georgia Power | Other Postretirement Benefits Plan | Amortization of regulatory assets | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Mississippi Power
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	4
Total other comprehensive income (loss)	0	0	1
Ending balance	90	4
Regulatory asset	44	36
Mississippi Power | Pension plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	123
Ending balance	152	123
Mississippi Power | Pension plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	123	146
Ending balance	152	123	146
Mississippi Power | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(2)	(11)
Total other comprehensive income (loss)	29	(23)
Mississippi Power | Pension plans | Net gain | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	31	(12)
Reclassification adjustments	(2)	(11)
Mississippi Power | Pension plans | Change in prior service costs | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Mississippi Power | Pension plans | Amortization of regulatory assets | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Mississippi Power | Other Postretirement Benefits Plan
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(9)
Ending balance	10	(9)
Mississippi Power | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(9)	1
Ending balance	(10)	(9)	1
Mississippi Power | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Total other comprehensive income (loss)	(1)	(10)
Mississippi Power | Other Postretirement Benefits Plan | Net gain | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	1	10
Reclassification adjustments	0	0
Mississippi Power | Other Postretirement Benefits Plan | Change in prior service costs | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Mississippi Power | Other Postretirement Benefits Plan | Amortization of regulatory assets | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Southern Power
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Total other comprehensive income (loss)	1	10	40
Southern Power | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Total other comprehensive income (loss)	(7)
Southern Power | Pension plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0
Total other comprehensive income (loss)	9	(24)
Southern Power | Pension plans | Net gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	9	(22)
Reclassification adjustments	0	(2)
Southern Power | Pension plans | Change in prior service costs
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0
Southern Power | Other Postretirement Benefits Plan
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Southern Power | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Total other comprehensive income (loss)	1	(2)
Southern Power | Other Postretirement Benefits Plan | Net gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	1	(2)
Southern Company Gas
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(1,891)
Total other comprehensive income (loss)	(15)	7	46
Ending balance	(2,225)	(1,891)
Southern Company Gas | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Total other comprehensive income (loss)	(16)
Southern Company Gas | Pension plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	111
Ending balance	143	111
Regulatory asset	173	190
Southern Company Gas | Pension plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	111	91
Reclassification adjustments	1
Total other comprehensive income (loss)	30	(30)
Ending balance	143	111	91
Southern Company Gas | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(2)	(7)
Total other comprehensive income (loss)	32	20
Southern Company Gas | Pension plans | Net gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	29	(30)
Reclassification adjustments	0	0
Southern Company Gas | Pension plans | Net gain | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	34	27
Reclassification adjustments	0	1
Southern Company Gas | Pension plans | Change in prior service costs
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	1
Southern Company Gas | Pension plans | Change in prior service costs | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	2	2
Southern Company Gas | Pension plans | Amortization of regulatory assets | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(4)	(10)
Southern Company Gas | Other Postretirement Benefits Plan
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	58
Reclassification adjustments	(8)	3
Ending balance	68	58
Regulatory asset	24	32
Southern Company Gas | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(58)	(34)
Total other comprehensive income (loss)	(8)	3
Ending balance	(68)	(58)	$ (34)
Southern Company Gas | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(10)	(11)
Total other comprehensive income (loss)	(10)	(24)
Southern Company Gas | Other Postretirement Benefits Plan | Net gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	0	0
Southern Company Gas | Other Postretirement Benefits Plan | Net gain | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain (loss)	0	13
Reclassification adjustments	0	0
Southern Company Gas | Other Postretirement Benefits Plan | Change in prior service costs | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Southern Company Gas | Other Postretirement Benefits Plan | Amortization of regulatory assets | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	$ (10)	$ (11)