RETIREMENT BENEFITS - Changes in Projected Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	$ 12,602,000,000	$ 16,382,000,000
Service cost	275,000,000	412,000,000	$ 434,000,000
Interest cost	626,000,000	408,000,000	346,000,000
Benefits paid	(744,000,000)	(692,000,000)
Actuarial loss	493,000,000	(3,908,000,000)
Balance at end of year	13,252,000,000	12,602,000,000	16,382,000,000
Change in plan assets
Fair value of plan assets at beginning of year	14,218,000,000	17,225,000,000
Actual loss on plan assets	1,092,000,000	(2,376,000,000)
Employer contributions	52,000,000	61,000,000
Benefits paid	(744,000,000)	(692,000,000)
Fair value of plan assets at end of year	14,618,000,000	14,218,000,000	17,225,000,000
Accrued asset	1,366,000,000	1,616,000,000
Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	1,441,000,000	1,849,000,000
Service cost	15,000,000	23,000,000	24,000,000
Interest cost	70,000,000	42,000,000	35,000,000
Benefits paid	(107,000,000)	(109,000,000)
Actuarial loss	(33,000,000)	(365,000,000)
Retiree drug subsidy		1,000,000
Balance at end of year	1,386,000,000	1,441,000,000	1,849,000,000
Change in plan assets
Fair value of plan assets at beginning of year	998,000,000	1,251,000,000
Actual loss on plan assets	131,000,000	(218,000,000)
Employer contributions	73,000,000	73,000,000
Benefits paid	(107,000,000)	(108,000,000)
Fair value of plan assets at end of year	1,095,000,000	998,000,000	1,251,000,000
Accrued asset	(291,000,000)	(443,000,000)
Alabama Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	2,906,000,000	3,806,000,000
Service cost	64,000,000	99,000,000	102,000,000
Interest cost	145,000,000	96,000,000	82,000,000
Benefits paid	(155,000,000)	(144,000,000)
Actuarial loss	116,000,000	(951,000,000)
Balance at end of year	3,076,000,000	2,906,000,000	3,806,000,000
Change in plan assets
Fair value of plan assets at beginning of year	3,427,000,000	4,141,000,000
Actual loss on plan assets	260,000,000	(579,000,000)
Employer contributions	11,000,000	9,000,000
Benefits paid	(154,000,000)	(144,000,000)
Fair value of plan assets at end of year	3,544,000,000	3,427,000,000	4,141,000,000
Accrued asset	468,000,000	521,000,000
Alabama Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	344,000,000	440,000,000
Service cost	4,000,000	6,000,000	6,000,000
Interest cost	17,000,000	10,000,000	9,000,000
Benefits paid	(24,000,000)	(23,000,000)
Actuarial loss	(12,000,000)	(89,000,000)
Retiree drug subsidy		0
Balance at end of year	329,000,000	344,000,000	440,000,000
Change in plan assets
Fair value of plan assets at beginning of year	372,000,000	489,000,000
Actual loss on plan assets	52,000,000	(98,000,000)
Employer contributions	3,000,000	4,000,000
Benefits paid	(24,000,000)	(23,000,000)
Fair value of plan assets at end of year	403,000,000	372,000,000	489,000,000
Accrued asset	74,000,000	28,000,000
Georgia Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	3,851,000,000	5,012,000,000
Service cost	68,000,000	103,000,000	112,000,000
Interest cost	191,000,000	123,000,000	104,000,000
Benefits paid	(224,000,000)	(226,000,000)
Actuarial loss	123,000,000	(1,161,000,000)
Balance at end of year	4,009,000,000	3,851,000,000	5,012,000,000
Change in plan assets
Fair value of plan assets at beginning of year	4,456,000,000	5,415,000,000
Actual loss on plan assets	331,000,000	(753,000,000)
Employer contributions	9,000,000	20,000,000
Benefits paid	(225,000,000)	(226,000,000)
Fair value of plan assets at end of year	4,571,000,000	4,456,000,000	5,415,000,000
Accrued asset	562,000,000	605,000,000
Georgia Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	514,000,000	656,000,000
Service cost	4,000,000	6,000,000	7,000,000
Interest cost	25,000,000	15,000,000	12,000,000
Benefits paid	(36,000,000)	(38,000,000)
Actuarial loss	(18,000,000)	(125,000,000)
Retiree drug subsidy		0
Balance at end of year	489,000,000	514,000,000	656,000,000
Change in plan assets
Fair value of plan assets at beginning of year	368,000,000	450,000,000
Actual loss on plan assets	51,000,000	(71,000,000)
Employer contributions	27,000,000	27,000,000
Benefits paid	(36,000,000)	(38,000,000)
Fair value of plan assets at end of year	410,000,000	368,000,000	450,000,000
Accrued asset	(79,000,000)	(146,000,000)
Mississippi Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	569,000,000	743,000,000
Service cost	11,000,000	17,000,000	18,000,000
Interest cost	28,000,000	18,000,000	16,000,000
Benefits paid	(32,000,000)	(30,000,000)
Actuarial loss	23,000,000	(179,000,000)
Balance at end of year	599,000,000	569,000,000	743,000,000
Change in plan assets
Fair value of plan assets at beginning of year	649,000,000	786,000,000
Actual loss on plan assets	50,000,000	(110,000,000)
Employer contributions	2,000,000	3,000,000
Benefits paid	(32,000,000)	(30,000,000)
Fair value of plan assets at end of year	669,000,000	649,000,000	786,000,000
Accrued asset	70,000,000	80,000,000
Mississippi Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	59,000,000	76,000,000
Service cost	1,000,000	1,000,000	1,000,000
Interest cost	3,000,000	2,000,000	1,000,000
Benefits paid	(4,000,000)	(4,000,000)
Actuarial loss	(2,000,000)	(16,000,000)
Retiree drug subsidy		0
Balance at end of year	57,000,000	59,000,000	76,000,000
Change in plan assets
Fair value of plan assets at beginning of year	24,000,000	29,000,000
Actual loss on plan assets	2,000,000	(4,000,000)
Employer contributions	3,000,000	3,000,000
Benefits paid	(4,000,000)	(4,000,000)
Fair value of plan assets at end of year	25,000,000	24,000,000	29,000,000
Accrued asset	(32,000,000)	(35,000,000)
Southern Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	163,000,000	222,000,000
Service cost	6,000,000	9,000,000	10,000,000
Interest cost	8,000,000	6,000,000	5,000,000
Benefits paid	(6,000,000)	(5,000,000)
Actuarial loss	6,000,000	(69,000,000)
Balance at end of year	177,000,000	163,000,000	222,000,000
Change in plan assets
Fair value of plan assets at beginning of year	178,000,000	213,000,000
Actual loss on plan assets	12,000,000	(31,000,000)
Employer contributions	2,000,000	1,000,000
Benefits paid	(7,000,000)	(5,000,000)
Fair value of plan assets at end of year	185,000,000	178,000,000	213,000,000
Accrued asset	8,000,000	15,000,000
Southern Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	9,000,000	11,000,000
Service cost	0	0	0
Interest cost	0	0	0
Benefits paid	(1,000,000)	(1,000,000)
Actuarial loss	1,000,000	(1,000,000)
Retiree drug subsidy		0
Balance at end of year	9,000,000	9,000,000	11,000,000
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Actual loss on plan assets	0	0
Employer contributions	1,000,000	1,000,000
Benefits paid	(1,000,000)	(1,000,000)
Fair value of plan assets at end of year	0	0	0
Accrued asset	(9,000,000)	(9,000,000)
Southern Company Gas | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	868,000,000	1,134,000,000
Service cost	24,000,000	34,000,000	37,000,000
Interest cost	42,000,000	28,000,000	24,000,000
Benefits paid	(104,000,000)	(75,000,000)
Actuarial loss	52,000,000	(253,000,000)
Balance at end of year	882,000,000	868,000,000	1,134,000,000
Change in plan assets
Fair value of plan assets at beginning of year	1,002,000,000	1,241,000,000
Actual loss on plan assets	79,000,000	(167,000,000)
Employer contributions	3,000,000	3,000,000
Benefits paid	(104,000,000)	(75,000,000)
Fair value of plan assets at end of year	980,000,000	1,002,000,000	1,241,000,000
Accrued asset	98,000,000	134,000,000
Southern Company Gas | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	179,000,000	237,000,000
Service cost	1,000,000	1,000,000	2,000,000
Interest cost	9,000,000	5,000,000	4,000,000
Benefits paid	(18,000,000)	(18,000,000)
Actuarial loss	1,000,000	(46,000,000)
Retiree drug subsidy		0
Balance at end of year	172,000,000	179,000,000	237,000,000
Change in plan assets
Fair value of plan assets at beginning of year	113,000,000	143,000,000
Actual loss on plan assets	19,000,000	(25,000,000)
Employer contributions	14,000,000	13,000,000
Benefits paid	(18,000,000)	(18,000,000)
Fair value of plan assets at end of year	128,000,000	113,000,000	$ 143,000,000
Accrued asset	(44,000,000)	$ (66,000,000)
Qualified pension plan | Pension plans
Change in benefit obligation
Balance at end of year	12,540,000,000
Change in plan assets
Employer contributions	0
Qualified pension plan | Alabama Power | Pension plans
Change in benefit obligation
Balance at end of year	2,959,000,000
Qualified pension plan | Georgia Power | Pension plans
Change in benefit obligation
Balance at end of year	3,865,000,000
Qualified pension plan | Mississippi Power | Pension plans
Change in benefit obligation
Balance at end of year	569,000,000
Qualified pension plan | Southern Power | Pension plans
Change in benefit obligation
Balance at end of year	154,000,000
Qualified pension plan | Southern Company Gas | Pension plans
Change in benefit obligation
Balance at end of year	822,000,000
Non-qualified pension plan | Pension plans
Change in benefit obligation
Balance at end of year	713,000,000
Non-qualified pension plan | Alabama Power | Pension plans
Change in benefit obligation
Balance at end of year	117,000,000
Non-qualified pension plan | Georgia Power | Pension plans
Change in benefit obligation
Balance at end of year	145,000,000
Non-qualified pension plan | Mississippi Power | Pension plans
Change in benefit obligation
Balance at end of year	29,000,000
Non-qualified pension plan | Southern Power | Pension plans
Change in benefit obligation
Balance at end of year	22,000,000
Non-qualified pension plan | Southern Company Gas | Pension plans
Change in benefit obligation
Balance at end of year	$ 60,000,000