RETIREMENT BENEFITS - Changes in Projected Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	$ 16,382,000,000	$ 16,646,000,000
Service cost	412,000,000	434,000,000	$ 376,000,000
Interest cost	408,000,000	346,000,000	432,000,000
Benefits paid	(692,000,000)	(651,000,000)
Actuarial gain	(3,908,000,000)	(393,000,000)
Balance at end of year	12,602,000,000	16,382,000,000	16,646,000,000
Change in plan assets
Fair value of plan assets at beginning of year	17,225,000,000	15,367,000,000
Actual return on plan assets	(2,376,000,000)	2,449,000,000
Employer contributions	61,000,000	60,000,000
Benefits paid	(692,000,000)	(651,000,000)
Fair value of plan assets at end of year	14,218,000,000	17,225,000,000	15,367,000,000
Accrued asset	1,616,000,000	843,000,000
Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	1,849,000,000	1,948,000,000
Service cost	23,000,000	24,000,000	22,000,000
Interest cost	42,000,000	35,000,000	54,000,000
Benefits paid	(109,000,000)	(105,000,000)
Actuarial gain	(365,000,000)	(54,000,000)
Retiree drug subsidy	1,000,000	1,000,000
Balance at end of year	1,441,000,000	1,849,000,000	1,948,000,000
Change in plan assets
Fair value of plan assets at beginning of year	1,251,000,000	1,158,000,000
Actual return on plan assets	(218,000,000)	154,000,000
Employer contributions	73,000,000	43,000,000
Benefits paid	(108,000,000)	(104,000,000)
Fair value of plan assets at end of year	998,000,000	1,251,000,000	1,158,000,000
Accrued asset	(443,000,000)	(598,000,000)
Alabama Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	3,806,000,000	3,854,000,000
Service cost	99,000,000	102,000,000	89,000,000
Interest cost	96,000,000	82,000,000	100,000,000
Benefits paid	(144,000,000)	(137,000,000)
Actuarial gain	(951,000,000)	(95,000,000)
Balance at end of year	2,906,000,000	3,806,000,000	3,854,000,000
Change in plan assets
Fair value of plan assets at beginning of year	4,141,000,000	3,684,000,000
Actual return on plan assets	(579,000,000)	586,000,000
Employer contributions	9,000,000	8,000,000
Benefits paid	(144,000,000)	(137,000,000)
Fair value of plan assets at end of year	3,427,000,000	4,141,000,000	3,684,000,000
Accrued asset	521,000,000	335,000,000
Alabama Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	440,000,000	463,000,000
Service cost	6,000,000	6,000,000	6,000,000
Interest cost	10,000,000	9,000,000	13,000,000
Benefits paid	(23,000,000)	(22,000,000)
Actuarial gain	(89,000,000)	(16,000,000)
Retiree drug subsidy	0	0
Balance at end of year	344,000,000	440,000,000	463,000,000
Change in plan assets
Fair value of plan assets at beginning of year	489,000,000	458,000,000
Actual return on plan assets	(98,000,000)	55,000,000
Employer contributions	4,000,000	(2,000,000)
Benefits paid	(23,000,000)	(22,000,000)
Fair value of plan assets at end of year	372,000,000	489,000,000	458,000,000
Accrued asset	28,000,000	49,000,000
Georgia Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	5,012,000,000	5,127,000,000
Service cost	103,000,000	112,000,000	96,000,000
Interest cost	123,000,000	104,000,000	133,000,000
Benefits paid	(226,000,000)	(210,000,000)
Actuarial gain	(1,161,000,000)	(121,000,000)
Balance at end of year	3,851,000,000	5,012,000,000	5,127,000,000
Change in plan assets
Fair value of plan assets at beginning of year	5,415,000,000	4,844,000,000
Actual return on plan assets	(753,000,000)	781,000,000
Employer contributions	20,000,000	0
Benefits paid	(226,000,000)	(210,000,000)
Fair value of plan assets at end of year	4,456,000,000	5,415,000,000	4,844,000,000
Accrued asset	605,000,000	403,000,000
Georgia Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	656,000,000	699,000,000
Service cost	6,000,000	7,000,000	6,000,000
Interest cost	15,000,000	12,000,000	20,000,000
Benefits paid	(38,000,000)	(36,000,000)
Actuarial gain	(125,000,000)	(26,000,000)
Retiree drug subsidy	0	0
Balance at end of year	514,000,000	656,000,000	699,000,000
Change in plan assets
Fair value of plan assets at beginning of year	450,000,000	427,000,000
Actual return on plan assets	(71,000,000)	55,000,000
Employer contributions	27,000,000	4,000,000
Benefits paid	(38,000,000)	(36,000,000)
Fair value of plan assets at end of year	368,000,000	450,000,000	427,000,000
Accrued asset	(146,000,000)	(206,000,000)
Mississippi Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	743,000,000	754,000,000
Service cost	17,000,000	18,000,000	15,000,000
Interest cost	18,000,000	16,000,000	20,000,000
Benefits paid	(30,000,000)	(28,000,000)
Actuarial gain	(179,000,000)	(17,000,000)
Balance at end of year	569,000,000	743,000,000	754,000,000
Change in plan assets
Fair value of plan assets at beginning of year	786,000,000	701,000,000
Actual return on plan assets	(110,000,000)	111,000,000
Employer contributions	3,000,000	2,000,000
Benefits paid	(30,000,000)	(28,000,000)
Fair value of plan assets at end of year	649,000,000	786,000,000	701,000,000
Accrued asset	80,000,000	43,000,000
Mississippi Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	76,000,000	81,000,000
Service cost	1,000,000	1,000,000	1,000,000
Interest cost	2,000,000	1,000,000	2,000,000
Benefits paid	(4,000,000)	(5,000,000)
Actuarial gain	(16,000,000)	(2,000,000)
Retiree drug subsidy	0	0
Balance at end of year	59,000,000	76,000,000	81,000,000
Change in plan assets
Fair value of plan assets at beginning of year	29,000,000	27,000,000
Actual return on plan assets	(4,000,000)	4,000,000
Employer contributions	3,000,000	3,000,000
Benefits paid	(4,000,000)	(5,000,000)
Fair value of plan assets at end of year	24,000,000	29,000,000	27,000,000
Accrued asset	(35,000,000)	(47,000,000)
Southern Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	222,000,000	217,000,000
Service cost	9,000,000	10,000,000	8,000,000
Interest cost	6,000,000	5,000,000	6,000,000
Benefits paid	(5,000,000)	(4,000,000)
Actuarial gain	(69,000,000)	(6,000,000)
Balance at end of year	163,000,000	222,000,000	217,000,000
Change in plan assets
Fair value of plan assets at beginning of year	213,000,000	186,000,000
Actual return on plan assets	(31,000,000)	30,000,000
Employer contributions	1,000,000	1,000,000
Benefits paid	(5,000,000)	(4,000,000)
Fair value of plan assets at end of year	178,000,000	213,000,000	186,000,000
Accrued asset	15,000,000	(9,000,000)
Southern Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	11,000,000	12,000,000
Service cost	0	0	1,000,000
Interest cost	0	0	0
Benefits paid	(1,000,000)	0
Actuarial gain	(1,000,000)	(1,000,000)
Retiree drug subsidy	0	0
Balance at end of year	9,000,000	11,000,000	12,000,000
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	1,000,000	0
Benefits paid	(1,000,000)	0
Fair value of plan assets at end of year	0	0	0
Accrued asset	(9,000,000)	(11,000,000)
Southern Company Gas | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	1,134,000,000	1,189,000,000
Service cost	34,000,000	37,000,000	33,000,000
Interest cost	28,000,000	24,000,000	31,000,000
Benefits paid	(75,000,000)	(73,000,000)
Actuarial gain	(253,000,000)	(43,000,000)
Balance at end of year	868,000,000	1,134,000,000	1,189,000,000
Change in plan assets
Fair value of plan assets at beginning of year	1,241,000,000	1,123,000,000
Actual return on plan assets	(167,000,000)	181,000,000
Employer contributions	3,000,000	10,000,000
Benefits paid	(75,000,000)	(73,000,000)
Fair value of plan assets at end of year	1,002,000,000	1,241,000,000	1,123,000,000
Accrued asset	134,000,000	107,000,000
Southern Company Gas | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	237,000,000	248,000,000
Service cost	1,000,000	2,000,000	2,000,000
Interest cost	5,000,000	4,000,000	7,000,000
Benefits paid	(18,000,000)	(18,000,000)
Actuarial gain	(46,000,000)	1,000,000
Retiree drug subsidy	0	0
Balance at end of year	179,000,000	237,000,000	248,000,000
Change in plan assets
Fair value of plan assets at beginning of year	143,000,000	128,000,000
Actual return on plan assets	(25,000,000)	18,000,000
Employer contributions	13,000,000	15,000,000
Benefits paid	(18,000,000)	(18,000,000)
Fair value of plan assets at end of year	113,000,000	143,000,000	$ 128,000,000
Accrued asset	(66,000,000)	$ (94,000,000)
Qualified pension plan | Pension plans
Change in benefit obligation
Balance at end of year	11,928,000,000
Change in plan assets
Employer contributions	0
Qualified pension plan | Alabama Power | Pension plans
Change in benefit obligation
Balance at end of year	2,799,000,000
Qualified pension plan | Georgia Power | Pension plans
Change in benefit obligation
Balance at end of year	3,717,000,000
Qualified pension plan | Mississippi Power | Pension plans
Change in benefit obligation
Balance at end of year	540,000,000
Qualified pension plan | Southern Power | Pension plans
Change in benefit obligation
Balance at end of year	140,000,000
Qualified pension plan | Southern Company Gas | Pension plans
Change in benefit obligation
Balance at end of year	819,000,000
Non-qualified pension plan | Pension plans
Change in benefit obligation
Balance at end of year	674,000,000
Non-qualified pension plan | Alabama Power | Pension plans
Change in benefit obligation
Balance at end of year	107,000,000
Non-qualified pension plan | Georgia Power | Pension plans
Change in benefit obligation
Balance at end of year	134,000,000
Non-qualified pension plan | Mississippi Power | Pension plans
Change in benefit obligation
Balance at end of year	28,000,000
Non-qualified pension plan | Southern Power | Pension plans
Change in benefit obligation
Balance at end of year	23,000,000
Non-qualified pension plan | Southern Company Gas | Pension plans
Change in benefit obligation
Balance at end of year	$ 49,000,000