RETIREMENT BENEFITS - Components of Accumulated OCI and Changes in Regulatory Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 32,234	$ 31,759	$ 29,039
Total other comprehensive income (loss)	158	(75)	(118)
Ending balance	32,276	32,234	31,759
Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(187)
Total other comprehensive income (loss)	111
Ending balance	(76)	(187)
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	(2)	(3)
Net loss	102	248
Total AOCI	100	245
Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	11	11
Net loss	2,790	4,610
Regulatory amortization	0	0
Regulatory asset	2,801	4,621
Pension plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	245	185
Reclassification adjustments	(17)	(14)
Total other comprehensive income (loss)	(145)	60
Ending balance	100	245	185
Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	4,621	3,993
Reclassification adjustments	(297)	(256)
Total other comprehensive income (loss)	(1,820)	628
Ending balance	2,801	4,621	3,993
Pension plans | Net loss
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	128	(74)
Reclassification adjustments	(17)	(14)
Pension plans | Net loss | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	1,523	(884)
Reclassification adjustments	(296)	(255)
Pension plans | Change in prior service costs
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments		0
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 benefit plans
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	1	1
Net loss	(1)	7
Total AOCI	0	8
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	3	4
Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	13	12
Net loss	(87)	39
Regulatory amortization	0	0
Regulatory asset		51
Regulatory liabilities	(74)
Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	8	2
Total other comprehensive income (loss)	(8)	6
Ending balance	0	8	2
Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	51	121
Reclassification adjustments	(5)	(5)
Total other comprehensive income (loss)	(125)	(70)
Ending balance	(74)	51	121
Other postretirement benefit plans | Net loss
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	(11)	2
Other postretirement benefit plans | Net loss | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	120	65
Reclassification adjustments	(6)	(6)
Other postretirement benefit plans | 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 benefit 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
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	9,810	8,955	7,477
Total other comprehensive income (loss)	6	4	4
Ending balance	10,713	9,810	8,955
Alabama Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Total AOCI	0	0
Alabama Power | Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	5	5
Net loss	804	1,281
Regulatory amortization	0	0
Regulatory asset	809	1,286
Alabama Power | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	1,286	1,130
Reclassification adjustments	(83)	(72)
Total other comprehensive income (loss)	(477)	156
Ending balance	809	1,286	1,130
Alabama Power | Pension plans | Net loss | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	394	(228)
Reclassification adjustments	(82)	(71)
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 benefit plans
Defined Benefit Plan Disclosure [Line Items]
Total AOCI	0	0
Alabama Power | Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	3	3
Net loss	(65)	(24)
Regulatory amortization	0	0
Regulatory liabilities	(62)	(21)
Alabama Power | Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(21)	1
Reclassification adjustments	0	0
Total other comprehensive income (loss)	(41)	(22)
Ending balance	(62)	(21)	1
Alabama Power | Other postretirement benefit plans | Net loss | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	41	22
Reclassification adjustments	0	0
Alabama Power | Other postretirement benefit plans | 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 benefit 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
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	16,501	15,065	14,323
Total other comprehensive income (loss)	6	4	(42)
Ending balance	17,234	16,501	15,065
Georgia Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Total AOCI	0	0
Georgia Power | Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	8	9
Net loss	963	1,589
Regulatory amortization	0	0
Regulatory asset	971	1,598
Georgia Power | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	1,598	1,416
Reclassification adjustments	(100)	(87)
Total other comprehensive income (loss)	(627)	182
Ending balance	971	1,598	1,416
Georgia Power | Pension plans | Net loss | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	527	(269)
Reclassification adjustments	(99)	(86)
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 benefit plans
Defined Benefit Plan Disclosure [Line Items]
Total AOCI	0	0
Georgia Power | Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	5	5
Net loss	(15)	42
Regulatory amortization	0	0
Regulatory asset		47
Regulatory liabilities	(10)
Georgia Power | Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	47	96
Reclassification adjustments	(2)	(2)
Total other comprehensive income (loss)	(57)	(49)
Ending balance	(10)	47	96
Georgia Power | Other postretirement benefit plans | Net loss | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	55	47
Reclassification adjustments	(3)	(3)
Georgia Power | Other postretirement benefit 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 | Other postretirement benefit 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
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	1,742	1,652	1,609
Total other comprehensive income (loss)	1	1	1
Ending balance	1,867	1,742	1,652
Mississippi Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Total AOCI	0	0
Mississippi Power | Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	1	2
Net loss	145	233
Regulatory amortization	0	0
Regulatory asset	146	235
Mississippi Power | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	235	203
Reclassification adjustments	(15)	(13)
Total other comprehensive income (loss)	(89)	32
Ending balance	146	235	203
Mississippi Power | Pension plans | Net loss | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	74	(45)
Reclassification adjustments	(15)	(13)
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 benefit plans
Defined Benefit Plan Disclosure [Line Items]
Total AOCI	0	0
Mississippi Power | Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	1	0
Net loss	0	5
Regulatory amortization	0	0
Regulatory asset	1	5
Mississippi Power | Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	5	10
Reclassification adjustments	0	0
Total other comprehensive income (loss)	(4)	(5)
Ending balance	1	5	10
Mississippi Power | Other postretirement benefit plans | Net loss | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	4	5
Reclassification adjustments	0	0
Mississippi Power | Other postretirement benefit 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 | Other postretirement benefit plans | 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]
Beginning balance	6,631	6,622	7,284
Total other comprehensive income (loss)	40	(42)	(42)
Ending balance	6,598	6,631	6,622
Southern Power | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(47)
Total other comprehensive income (loss)	18
Ending balance	(29)	(47)
Southern Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	0	0
Net loss	35	60
Total AOCI	35	60
Southern Power | Pension plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	60	46
Reclassification adjustments	(3)	(2)
Total other comprehensive income (loss)	(25)	14
Ending balance	35	60	46
Southern Power | Pension plans | Net loss
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	22	(16)
Reclassification adjustments	(3)	(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 benefit plans
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	0	0
Net loss	2	3
Total AOCI	2	3
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	0	0
Southern Power | Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	3	2
Total other comprehensive income (loss)	(1)	1
Ending balance	2	3	2
Southern Power | Other postretirement benefit plans | Net loss
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	(1)	1
Southern Company Gas
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	9,767	9,506	8,570
Total other comprehensive income (loss)	46	(29)	(19)
Ending balance	9,916	9,767	9,506
Southern Company Gas | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(2)
Total other comprehensive income (loss)	40
Ending balance	38	(2)
Southern Company Gas | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	(3)	(4)
Net loss	(42)	5
Total AOCI	(45)	1
Regulatory asset	210	224
Southern Company Gas | Pension plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	(11)	(13)
Net loss	38	135
Regulatory amortization	64	83
Regulatory asset	91	205
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	1	(14)
Reclassification adjustments	1	0
Total other comprehensive income (loss)	(46)	15
Ending balance	(45)	1	(14)
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]
Beginning balance	205	172
Reclassification adjustments	(17)	(12)
Total other comprehensive income (loss)	(114)	33
Ending balance	91	205	172
Southern Company Gas | Pension plans | Net loss
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	47	(15)
Reclassification adjustments	1	(1)
Southern Company Gas | Pension plans | Net loss | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	97	(45)
Reclassification adjustments	(9)	(8)
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	(10)	(6)
Southern Company Gas | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	1	1
Net loss	(6)	(1)
Total AOCI	(5)	0
Regulatory asset	40	47
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	(5)	4
Southern Company Gas | Other postretirement benefit plans | Regulatory Assets
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	1	1
Net loss	(51)	(49)
Regulatory amortization	16	25
Regulatory liabilities	(34)	(23)
Southern Company Gas | Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	0	(4)
Total other comprehensive income (loss)	(5)	4
Ending balance	(5)	0	(4)
Southern Company Gas | Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(23)	(11)
Reclassification adjustments	(9)	(7)
Total other comprehensive income (loss)	(11)	(12)
Ending balance	(34)	(23)	$ (11)
Southern Company Gas | Other postretirement benefit plans | Net loss
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	0	0
Southern Company Gas | Other postretirement benefit plans | Net loss | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive (income) loss	2	5
Reclassification adjustments	0	0
Southern Company Gas | Other postretirement benefit plans | 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 benefit plans | Amortization of regulatory assets | Regulatory Assets
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Reclassification adjustments	$ (9)	$ (7)