RETIREMENT BENEFITS - Amounts Recognized in Balance Sheets and Amounts in AOCI (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	$ 6,805	$ 5,375
Other deferred charges and assets	1,071	1,463
Other current liabilities	(830)	(852)
Employee benefit obligations	(1,814)	(2,147)
Other regulatory liabilities, deferred	(256)	(169)
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	2	0
Other regulatory assets, deferred	4,072	3,566
Other deferred charges and assets	0	0
Other current liabilities	(54)	(55)
Employee benefit obligations	(679)	(1,097)
Other regulatory liabilities, deferred	(79)	(108)
AOCI	185	97
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	183	99
Other current liabilities	(5)	(6)
Employee benefit obligations	(919)	(931)
Other regulatory liabilities, deferred	(62)	(77)
AOCI	2	(4)
Alabama Power
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	1,976	1,240
Other deferred charges and assets	269	188
Other current liabilities	(105)	(45)
Employee benefit obligations	(206)	(314)
Other regulatory liabilities, deferred	(146)	(69)
Alabama Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	71	0
Other regulatory assets, deferred	1,130	955
Other deferred charges and assets	0	0
Other current liabilities	(8)	(12)
Employee benefit obligations	(110)	(229)
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	3	0
Other current liabilities	0	0
Employee benefit obligations	(49)	(43)
Other regulatory liabilities, deferred	(2)	(8)
AOCI	0	0
Georgia Power
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	2,716	2,258
Other deferred charges and assets	500	514
Other current liabilities	(187)	(183)
Employee benefit obligations	(540)	(599)
Georgia Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	0	0
Other regulatory assets, deferred	1,416	1,230
Other deferred charges and assets	0	0
Other current liabilities	(11)	(15)
Employee benefit obligations	(157)	(227)
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	96	60
Other current liabilities	0	0
Employee benefit obligations	(339)	(331)
Other regulatory liabilities, deferred	0	0
AOCI	0	0
Mississippi Power
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	360	331
Other deferred charges and assets	34	3
Other current liabilities	(49)	(28)
Employee benefit obligations	(99)	(115)
Other regulatory liabilities, deferred	(76)	(81)
Mississippi Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	2	0
Other regulatory assets, deferred	204	167
Other deferred charges and assets	0	0
Other current liabilities	(2)	(3)
Employee benefit obligations	(30)	(49)
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	10	6
Other current liabilities	0	0
Employee benefit obligations	(61)	(58)
Other regulatory liabilities, deferred	0	(2)
AOCI	0	0
Southern Power
Defined Benefit Plan Disclosure [Line Items]
Other deferred charges and assets	216	373
Other current liabilities	(132)	(121)
Southern Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	10	1
Other regulatory assets, deferred	0	0
Other deferred charges and assets	0	0
Other current liabilities	(2)	0
Employee benefit obligations	(24)	(1)
Other regulatory liabilities, deferred	0	0
AOCI	46	26
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	(11)	(9)
Other regulatory liabilities, deferred	0	0
AOCI	2	1
Southern Company Gas
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	618	669
Other deferred charges and assets	207	193
Other current liabilities	(128)	(130)
Employee benefit obligations	(265)	(357)
Southern Company Gas | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension costs	0	0
Other regulatory assets, deferred	172	160
Other deferred charges and assets	82	74
Other current liabilities	(2)	(3)
Employee benefit obligations	(97)	(179)
Other regulatory liabilities, deferred	0	0
AOCI	(14)	(44)
Regulatory asset	252	268
Southern Company Gas | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Other regulatory assets, deferred	(11)	(4)
Other current liabilities	0	0
Employee benefit obligations	(135)	146
Other regulatory liabilities, deferred	0	0
AOCI	(4)	(4)
Regulatory asset	$ 50	$ 57