RETIREMENT BENEFITS - Amounts Recognized in Balance Sheets and Amounts in AOCI (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	$ 7,198	$ 6,805
Other deferred charges and assets	1,288	1,071
Other current liabilities	(968)	(830)
Employee benefit obligations	(2,213)	(1,814)
Other regulatory liabilities, deferred	(251)	(256)
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs		2
Other regulatory assets, deferred	4,655	4,072
Other deferred charges and assets	0	0
Other current liabilities	(52)	(54)
Employee benefit obligations	(1,227)	(679)
Other regulatory liabilities, deferred	(34)	(79)
AOCI	245	185
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	137	183
Other current liabilities	(5)	(5)
Employee benefit obligations	(785)	(919)
Other regulatory liabilities, deferred	(86)	(62)
AOCI	8	2
Alabama Power
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	2,162	2,016
Other deferred charges and assets	273	269
Other current liabilities	(107)	(105)
Employee benefit obligations	(214)	(206)
Other regulatory liabilities, deferred	(124)	(146)
Alabama Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs		71
Other regulatory assets, deferred	1,286	1,130
Other deferred charges and assets	0	0
Other current liabilities	(9)	(8)
Employee benefit obligations	(161)	(110)
Other regulatory liabilities, deferred	0	0
AOCI	0	0
Alabama Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	0	3
Other current liabilities	0	0
Employee benefit obligations	(5)	(49)
Other regulatory liabilities, deferred	(21)	(2)
AOCI	0	0
Georgia Power
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	2,692	2,716
Other deferred charges and assets	479	500
Other current liabilities	(254)	(187)
Employee benefit obligations	(586)	(540)
Georgia Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs		0
Other regulatory assets, deferred	1,598	1,416
Other deferred charges and assets	0	0
Other current liabilities	(10)	(11)
Employee benefit obligations	(273)	(157)
Other regulatory liabilities, deferred	0	0
AOCI	0	0
Georgia Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	47	96
Other current liabilities	0	0
Employee benefit obligations	(272)	(339)
Other regulatory liabilities, deferred	0	0
AOCI	0	0
Mississippi Power
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	388	360
Other deferred charges and assets	55	34
Other current liabilities	(40)	(64)
Employee benefit obligations	(113)	(99)
Other regulatory liabilities, deferred	(15)	(76)
Mississippi Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs		2
Other regulatory assets, deferred	235	204
Other deferred charges and assets	0	0
Other current liabilities	(2)	(2)
Employee benefit obligations	(51)	(30)
Other regulatory liabilities, deferred	0	0
AOCI	0	0
Mississippi Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	5	10
Other current liabilities	0	0
Employee benefit obligations	(54)	(61)
Other regulatory liabilities, deferred	0	0
AOCI	0	0
Southern Power
Defined Benefit Plan Disclosure [Line Items]
Other deferred charges and assets	293	216
Other current liabilities	(132)	(132)
Southern Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs		10
Other regulatory assets, deferred	0	0
Other deferred charges and assets	0	0
Other current liabilities	(2)	(2)
Employee benefit obligations	(29)	(24)
Other regulatory liabilities, deferred	0	0
AOCI	60	46
Southern Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	0	0
Other current liabilities	0	0
Employee benefit obligations	(12)	(11)
Other regulatory liabilities, deferred	0	0
AOCI	3	2
Southern Company Gas
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	615	618
Other deferred charges and assets	235	207
Other current liabilities	(150)	(149)
Employee benefit obligations	(283)	(265)
Southern Company Gas | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs		0
Other regulatory assets, deferred	205	172
Other deferred charges and assets	70	82
Other current liabilities	(2)	(2)
Employee benefit obligations	(134)	(97)
Other regulatory liabilities, deferred	0	0
AOCI	1	(14)
Regulatory asset	224	252
Southern Company Gas | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	(23)	(11)
Other current liabilities	0	0
Employee benefit obligations	(120)	(135)
Other regulatory liabilities, deferred	0	0
AOCI	0	(4)
Regulatory asset	$ 47	$ 50