RETIREMENT BENEFITS - Components of Accumulated OCI (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	$ 34,532	$ 32,276	$ 32,234
Total other comprehensive income (loss)	(10)	71	158
Ending balance	35,225	34,532	32,276
Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(18)
Total other comprehensive income (loss)	(38)
Ending balance	(56)	(18)
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	(1)	(2)
Net loss	80	26
AOCI	79	24
Pension plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	24	100
Reclassification adjustments	(7)
Total other comprehensive income (loss)	55	(76)
Ending balance	79	24	100
Pension plans | Net gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain	62	(82)
Reclassification adjustments	(8)	6
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	1	1
Net loss	(10)	(5)
AOCI	(9)	(4)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	7	(1)
Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(4)	0
Total other comprehensive income (loss)	(5)	(4)
Ending balance	(9)	(4)	0
Other Postretirement Benefits Plan | Net gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain	(12)	(3)
Southern Power
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	6,916	6,598	6,631
Total other comprehensive income (loss)	1	10	40
Ending balance	6,698	6,916	6,598
Southern Power | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(9)
Total other comprehensive income (loss)	(7)
Ending balance	(16)	(9)
Southern Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	0	0
Net loss	20	11
AOCI	20	11
Southern Power | Pension plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	11	35
Reclassification adjustments	0
Total other comprehensive income (loss)	9	(24)
Ending balance	20	11	35
Southern Power | Pension plans | Net gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain	9	(22)
Reclassification adjustments	0	(2)
Southern Power | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	0	0
Net loss	1	0
AOCI	1	0
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]
Beginning balance	0	2
Total other comprehensive income (loss)	1	(2)
Ending balance	1	0	2
Southern Power | Other Postretirement Benefits Plan | Net gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain	1	(2)
Southern Company Gas
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Total other comprehensive income (loss)	(15)	7	46
Southern Company Gas | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	56
Total other comprehensive income (loss)	(16)
Ending balance	40	56
Southern Company Gas | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	(2)	(3)
Net loss	(43)	(72)
AOCI	(45)	(75)
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	(75)	(45)
Reclassification adjustments	1
Total other comprehensive income (loss)	30	(30)
Ending balance	(45)	(75)	(45)
Southern Company Gas | Pension plans | Net gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain	29	(30)
Reclassification adjustments	0	0
Southern Company Gas | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	0	0
Net loss	(10)	(2)
AOCI	(10)	(2)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(8)	3
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	(2)	(5)
Total other comprehensive income (loss)	(8)	3
Ending balance	(10)	(2)	$ (5)
Southern Company Gas | Other Postretirement Benefits Plan | Net gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Net gain	$ 0	$ 0