RETIREMENT BENEFITS - Amounts Recognized in Balance Sheets and Amounts in AOCI (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	$ 5,577	$ 7,198
Other current liabilities	(872)	(968)
Employee benefit obligations	(1,550)	(2,213)
Other regulatory liabilities, deferred	(485)	(251)
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	1,657	0
Other regulatory assets, deferred	2,920	4,655
Other current liabilities	(55)	(52)
Employee benefit obligations	(759)	(1,227)
Other regulatory liabilities, deferred	(119)	(34)
AOCI	100	245
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension and other postretirement benefit costs	0
Other regulatory assets, deferred	97	137
Other current liabilities	(5)	(5)
Employee benefit obligations	(593)	(785)
Other regulatory liabilities, deferred	(171)	(86)
AOCI	0	8
Alabama Power
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension and other postretirement benefit costs	513	0
Other regulatory assets, deferred	1,807	2,162
Other current liabilities	(125)	(107)
Employee benefit obligations	(171)	(214)
Other regulatory liabilities, deferred	(210)	(124)
Alabama Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	464	0
Other regulatory assets, deferred	809	1,286
Other current liabilities	(9)	(9)
Employee benefit obligations	(120)	(161)
Other regulatory liabilities, deferred	0	0
AOCI	0	0
Alabama Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension and other postretirement benefit costs	49
Other regulatory assets, deferred	0	0
Other current liabilities	0	0
Employee benefit obligations	0	(5)
Other regulatory liabilities, deferred	(62)	(21)
AOCI	0	0
Georgia Power
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	1,964	2,692
Other current liabilities	(254)	(254)
Employee benefit obligations	(402)	(586)
Georgia Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	563	0
Other regulatory assets, deferred	971	1,598
Other current liabilities	(12)	(10)
Employee benefit obligations	(148)	(273)
Other regulatory liabilities, deferred	0	0
AOCI	0	0
Georgia Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension and other postretirement benefit costs	0
Other regulatory assets, deferred	30	47
Other current liabilities	0	0
Employee benefit obligations	(206)	(272)
Other regulatory liabilities, deferred	(40)	0
AOCI	0	0
Mississippi Power
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	79	0
Other regulatory assets, deferred	317	388
Other current liabilities	(49)	(40)
Employee benefit obligations	(88)	(113)
Other regulatory liabilities, deferred	(64)	(15)
Mississippi Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	78	0
Other regulatory assets, deferred	146	235
Other current liabilities	(2)	(2)
Employee benefit obligations	(33)	(51)
Other regulatory liabilities, deferred	0	0
AOCI	0	0
Mississippi Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension and other postretirement benefit costs	0
Other regulatory assets, deferred	2	5
Other current liabilities	0	0
Employee benefit obligations	(47)	(54)
Other regulatory liabilities, deferred	(1)	0
AOCI	0	0
Southern Power
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	(140)	(132)
Southern Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	20	0
Other regulatory assets, deferred	0	0
Other current liabilities	(2)	(2)
Employee benefit obligations	(27)	(29)
Other regulatory liabilities, deferred	0	0
AOCI	35	60
Southern Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension and other postretirement benefit costs	0
Other regulatory assets, deferred	0	0
Other current liabilities	0	0
Employee benefit obligations	(11)	(12)
Other regulatory liabilities, deferred	0	0
AOCI	2	3
Southern Company Gas
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	689	615
Other current liabilities	(155)	(150)
Employee benefit obligations	(176)	(283)
Southern Company Gas | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	175	70
Other regulatory assets, deferred	91	205
Other current liabilities	(2)	(2)
Employee benefit obligations	(66)	(134)
Other regulatory liabilities, deferred	0	0
AOCI	(45)	1
Regulatory asset	210	224
Southern Company Gas | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension and other postretirement benefit costs	0
Other regulatory assets, deferred	0	(23)
Other current liabilities	0	0
Employee benefit obligations	(94)	(120)
Other regulatory liabilities, deferred	(34)	0
AOCI	(5)	0
Regulatory asset	$ 40	$ 47