RETIREMENT BENEFITS - Changes in Projected Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	$ 14,788,000,000	$ 12,763,000,000
Dispositions		(509,000,000)
Service cost	376,000,000	292,000,000	$ 359,000,000
Interest cost	432,000,000	492,000,000	464,000,000
Benefits paid	(629,000,000)	(596,000,000)
Actuarial (gain) loss	1,679,000,000	2,346,000,000
Balance at end of year	16,646,000,000	14,788,000,000	12,763,000,000
Change in plan assets
Fair value of plan assets at beginning of year	14,057,000,000	11,611,000,000
Dispositions		(509,000,000)
Actual return (loss) on plan assets	1,881,000,000	2,343,000,000
Employer contributions	58,000,000	1,208,000,000
Benefits paid	(629,000,000)	(596,000,000)
Fair value of plan assets at end of year	15,367,000,000	14,057,000,000	11,611,000,000
Accrued liability	(1,279,000,000)	(731,000,000)
Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	1,985,000,000	1,865,000,000
Dispositions		(69,000,000)
Service cost	22,000,000	18,000,000	24,000,000
Interest cost	54,000,000	69,000,000	75,000,000
Benefits paid	(126,000,000)	(126,000,000)
Actuarial (gain) loss	7,000,000	223,000,000
Retiree drug subsidy	6,000,000	5,000,000
Balance at end of year	1,948,000,000	1,985,000,000	1,865,000,000
Change in plan assets
Fair value of plan assets at beginning of year	1,061,000,000	928,000,000
Dispositions		(18,000,000)
Actual return (loss) on plan assets	145,000,000	189,000,000
Employer contributions	72,000,000	83,000,000
Benefits paid	(120,000,000)	(121,000,000)
Fair value of plan assets at end of year	1,158,000,000	1,061,000,000	928,000,000
Accrued liability	(790,000,000)	(924,000,000)
Alabama Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	3,404,000,000	2,816,000,000
Dispositions		0
Service cost	89,000,000	69,000,000	78,000,000
Interest cost	100,000,000	114,000,000	101,000,000
Benefits paid	(132,000,000)	(125,000,000)
Actuarial (gain) loss	393,000,000	530,000,000
Balance at end of year	3,854,000,000	3,404,000,000	2,816,000,000
Change in plan assets
Fair value of plan assets at beginning of year	3,357,000,000	2,575,000,000
Dispositions		0
Actual return (loss) on plan assets	450,000,000	524,000,000
Employer contributions	9,000,000	383,000,000
Benefits paid	(132,000,000)	(125,000,000)
Fair value of plan assets at end of year	3,684,000,000	3,357,000,000	2,575,000,000
Accrued liability	(170,000,000)	(47,000,000)
Alabama Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	462,000,000	403,000,000
Dispositions		0
Service cost	6,000,000	5,000,000	6,000,000
Interest cost	13,000,000	16,000,000	17,000,000
Benefits paid	(29,000,000)	(27,000,000)
Actuarial (gain) loss	9,000,000	63,000,000
Retiree drug subsidy	2,000,000	2,000,000
Balance at end of year	463,000,000	462,000,000	403,000,000
Change in plan assets
Fair value of plan assets at beginning of year	413,000,000	360,000,000
Dispositions		0
Actual return (loss) on plan assets	60,000,000	76,000,000
Employer contributions	12,000,000	2,000,000
Benefits paid	(27,000,000)	(25,000,000)
Fair value of plan assets at end of year	458,000,000	413,000,000	360,000,000
Accrued liability	(5,000,000)	(49,000,000)
Georgia Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	4,610,000,000	3,905,000,000
Dispositions		0
Service cost	96,000,000	74,000,000	87,000,000
Interest cost	133,000,000	156,000,000	139,000,000
Benefits paid	(202,000,000)	(194,000,000)
Actuarial (gain) loss	490,000,000	669,000,000
Balance at end of year	5,127,000,000	4,610,000,000	3,905,000,000
Change in plan assets
Fair value of plan assets at beginning of year	4,442,000,000	3,663,000,000
Dispositions		0
Actual return (loss) on plan assets	594,000,000	730,000,000
Employer contributions	10,000,000	243,000,000
Benefits paid	(202,000,000)	(194,000,000)
Fair value of plan assets at end of year	4,844,000,000	4,442,000,000	3,663,000,000
Accrued liability	(283,000,000)	(168,000,000)
Georgia Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	742,000,000	675,000,000
Dispositions		0
Service cost	6,000,000	5,000,000	6,000,000
Interest cost	20,000,000	26,000,000	28,000,000
Benefits paid	(46,000,000)	(47,000,000)
Actuarial (gain) loss	(26,000,000)	80,000,000
Retiree drug subsidy	3,000,000	3,000,000
Balance at end of year	699,000,000	742,000,000	675,000,000
Change in plan assets
Fair value of plan assets at beginning of year	403,000,000	344,000,000
Dispositions		0
Actual return (loss) on plan assets	50,000,000	68,000,000
Employer contributions	17,000,000	35,000,000
Benefits paid	(43,000,000)	(44,000,000)
Fair value of plan assets at end of year	427,000,000	403,000,000	344,000,000
Accrued liability	(272,000,000)	(339,000,000)
Mississippi Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	671,000,000	557,000,000
Dispositions		0
Service cost	15,000,000	12,000,000	17,000,000
Interest cost	20,000,000	22,000,000	20,000,000
Benefits paid	(27,000,000)	(26,000,000)
Actuarial (gain) loss	75,000,000	106,000,000
Balance at end of year	754,000,000	671,000,000	557,000,000
Change in plan assets
Fair value of plan assets at beginning of year	641,000,000	505,000,000
Dispositions		0
Actual return (loss) on plan assets	85,000,000	103,000,000
Employer contributions	2,000,000	59,000,000
Benefits paid	(27,000,000)	(26,000,000)
Fair value of plan assets at end of year	701,000,000	641,000,000	505,000,000
Accrued liability	(53,000,000)	(30,000,000)
Mississippi Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	87,000,000	81,000,000
Dispositions		0
Service cost	1,000,000	1,000,000	1,000,000
Interest cost	2,000,000	3,000,000	3,000,000
Benefits paid	(6,000,000)	(6,000,000)
Actuarial (gain) loss	(3,000,000)	8,000,000
Retiree drug subsidy	0	0
Balance at end of year	81,000,000	87,000,000	81,000,000
Change in plan assets
Fair value of plan assets at beginning of year	26,000,000	23,000,000
Dispositions		0
Actual return (loss) on plan assets	3,000,000	4,000,000
Employer contributions	4,000,000	5,000,000
Benefits paid	(6,000,000)	(6,000,000)
Fair value of plan assets at end of year	27,000,000	26,000,000	23,000,000
Accrued liability	(54,000,000)	(61,000,000)
Southern Power | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	185,000,000	123,000,000
Dispositions		0
Service cost	8,000,000	7,000,000	9,000,000
Interest cost	6,000,000	5,000,000	5,000,000
Benefits paid	(6,000,000)	(4,000,000)
Actuarial (gain) loss	24,000,000	54,000,000
Balance at end of year	217,000,000	185,000,000	123,000,000
Change in plan assets
Fair value of plan assets at beginning of year	169,000,000	123,000,000
Dispositions		0
Actual return (loss) on plan assets	22,000,000	43,000,000
Employer contributions	1,000,000	7,000,000
Benefits paid	(6,000,000)	(4,000,000)
Fair value of plan assets at end of year	186,000,000	169,000,000	123,000,000
Accrued liability	(31,000,000)	(16,000,000)
Southern Power | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	11,000,000	9,000,000
Dispositions		0
Service cost	1,000,000	1,000,000	1,000,000
Interest cost	0	0	0
Benefits paid	0	(1,000,000)
Actuarial (gain) loss	0	2,000,000
Retiree drug subsidy	0	0
Balance at end of year	12,000,000	11,000,000	9,000,000
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Dispositions		0
Actual return (loss) on plan assets	0	0
Employer contributions	0	1,000,000
Benefits paid	0	(1,000,000)
Fair value of plan assets at end of year	0	0	0
Accrued liability	(12,000,000)	(11,000,000)
Southern Company Gas | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	1,067,000,000	907,000,000
Dispositions		0
Service cost	33,000,000	25,000,000	34,000,000
Interest cost	31,000,000	36,000,000	39,000,000
Benefits paid	(69,000,000)	(64,000,000)
Actuarial (gain) loss	127,000,000	163,000,000
Balance at end of year	1,189,000,000	1,067,000,000	907,000,000
Change in plan assets
Fair value of plan assets at beginning of year	1,050,000,000	798,000,000
Dispositions		0
Actual return (loss) on plan assets	139,000,000	172,000,000
Employer contributions	3,000,000	144,000,000
Benefits paid	(69,000,000)	(64,000,000)
Fair value of plan assets at end of year	1,123,000,000	1,050,000,000	798,000,000
Accrued liability	(66,000,000)	(17,000,000)
Southern Company Gas | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	250,000,000	244,000,000
Dispositions		0
Service cost	2,000,000	1,000,000	2,000,000
Interest cost	7,000,000	9,000,000	10,000,000
Benefits paid	(17,000,000)	(17,000,000)
Actuarial (gain) loss	6,000,000	13,000,000
Retiree drug subsidy	0	0
Balance at end of year	248,000,000	250,000,000	244,000,000
Change in plan assets
Fair value of plan assets at beginning of year	115,000,000	98,000,000
Dispositions		0
Actual return (loss) on plan assets	18,000,000	21,000,000
Employer contributions	12,000,000	13,000,000
Benefits paid	(17,000,000)	(17,000,000)
Fair value of plan assets at end of year	128,000,000	115,000,000	$ 98,000,000
Accrued liability	(120,000,000)	$ (135,000,000)
Qualified pension plan | Pension plans
Change in benefit obligation
Balance at end of year	15,818,000,000
Change in plan assets
Employer contributions	0
Qualified pension plan | Alabama Power | Pension plans
Change in benefit obligation
Balance at end of year	3,719,000,000
Qualified pension plan | Georgia Power | Pension plans
Change in benefit obligation
Balance at end of year	4,977,000,000
Qualified pension plan | Mississippi Power | Pension plans
Change in benefit obligation
Balance at end of year	718,000,000
Qualified pension plan | Southern Power | Pension plans
Change in benefit obligation
Balance at end of year	187,000,000
Qualified pension plan | Southern Company Gas | Pension plans
Change in benefit obligation
Balance at end of year	1,114,000,000
Non-qualified pension plan | Pension plans
Change in benefit obligation
Balance at end of year	828,000,000
Non-qualified pension plan | Alabama Power | Pension plans
Change in benefit obligation
Balance at end of year	135,000,000
Non-qualified pension plan | Georgia Power | Pension plans
Change in benefit obligation
Balance at end of year	150,000,000
Non-qualified pension plan | Mississippi Power | Pension plans
Change in benefit obligation
Balance at end of year	36,000,000
Non-qualified pension plan | Southern Power | Pension plans
Change in benefit obligation
Balance at end of year	30,000,000
Non-qualified pension plan | Southern Company Gas | Pension plans
Change in benefit obligation
Balance at end of year	$ 75,000,000