RETIREMENT BENEFITS - Changes in Projected Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($)	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	$ 16,646,000,000	$ 14,788,000,000
Service cost	434,000,000	376,000,000	$ 292,000,000
Interest cost	346,000,000	432,000,000	492,000,000
Benefits paid	(651,000,000)	(629,000,000)
Actuarial gain	(393,000,000)	1,679,000,000
Balance at end of year	16,382,000,000	16,646,000,000	14,788,000,000
Change in plan assets
Fair value of plan assets at beginning of year	15,367,000,000	14,057,000,000
Actual return on plan assets	2,449,000,000	1,881,000,000
Employer contributions	60,000,000	58,000,000
Benefits paid	(651,000,000)	(629,000,000)
Fair value of plan assets at end of year	17,225,000,000	15,367,000,000	14,057,000,000
Accrued asset (liability)	843,000,000	(1,279,000,000)
Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	1,948,000,000	1,985,000,000
Service cost	24,000,000	22,000,000	18,000,000
Interest cost	35,000,000	54,000,000	69,000,000
Benefits paid	(105,000,000)	(126,000,000)
Actuarial gain	(54,000,000)	7,000,000
Retiree drug subsidy	1,000,000	6,000,000
Balance at end of year	1,849,000,000	1,948,000,000	1,985,000,000
Change in plan assets
Fair value of plan assets at beginning of year	1,158,000,000	1,061,000,000
Actual return on plan assets	154,000,000	145,000,000
Employer contributions	43,000,000	72,000,000
Benefits paid	(104,000,000)	(120,000,000)
Fair value of plan assets at end of year	1,251,000,000	1,158,000,000	1,061,000,000
Accrued asset (liability)	(598,000,000)	(790,000,000)
Alabama Power \| Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	3,854,000,000	3,404,000,000
Service cost	102,000,000	89,000,000	69,000,000
Interest cost	82,000,000	100,000,000	114,000,000
Benefits paid	(137,000,000)	(132,000,000)
Actuarial gain	(95,000,000)	393,000,000
Balance at end of year	3,806,000,000	3,854,000,000	3,404,000,000
Change in plan assets
Fair value of plan assets at beginning of year	3,684,000,000	3,357,000,000
Actual return on plan assets	586,000,000	450,000,000
Employer contributions	8,000,000	9,000,000
Benefits paid	(137,000,000)	(132,000,000)
Fair value of plan assets at end of year	4,141,000,000	3,684,000,000	3,357,000,000
Accrued asset (liability)	335,000,000	(170,000,000)
Alabama Power \| Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	463,000,000	462,000,000
Service cost	6,000,000	6,000,000	5,000,000
Interest cost	9,000,000	13,000,000	16,000,000
Benefits paid	(22,000,000)	(29,000,000)
Actuarial gain	(16,000,000)	9,000,000
Retiree drug subsidy	0	2,000,000
Balance at end of year	440,000,000	463,000,000	462,000,000
Change in plan assets
Fair value of plan assets at beginning of year	458,000,000	413,000,000
Actual return on plan assets	55,000,000	60,000,000
Employer contributions	(2,000,000)	12,000,000
Benefits paid	(22,000,000)	(27,000,000)
Fair value of plan assets at end of year	489,000,000	458,000,000	413,000,000
Accrued asset (liability)	49,000,000	(5,000,000)
Georgia Power \| Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	5,127,000,000	4,610,000,000
Service cost	112,000,000	96,000,000	74,000,000
Interest cost	104,000,000	133,000,000	156,000,000
Benefits paid	(210,000,000)	(202,000,000)
Actuarial gain	(121,000,000)	490,000,000
Balance at end of year	5,012,000,000	5,127,000,000	4,610,000,000
Change in plan assets
Fair value of plan assets at beginning of year	4,844,000,000	4,442,000,000
Actual return on plan assets	781,000,000	594,000,000
Employer contributions	0	10,000,000
Benefits paid	(210,000,000)	(202,000,000)
Fair value of plan assets at end of year	5,415,000,000	4,844,000,000	4,442,000,000
Accrued asset (liability)	403,000,000	(283,000,000)
Georgia Power \| Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	699,000,000	742,000,000
Service cost	7,000,000	6,000,000	5,000,000
Interest cost	12,000,000	20,000,000	26,000,000
Benefits paid	(36,000,000)	(46,000,000)
Actuarial gain	(26,000,000)	(26,000,000)
Retiree drug subsidy	0	3,000,000
Balance at end of year	656,000,000	699,000,000	742,000,000
Change in plan assets
Fair value of plan assets at beginning of year	427,000,000	403,000,000
Actual return on plan assets	55,000,000	50,000,000
Employer contributions	4,000,000	17,000,000
Benefits paid	(36,000,000)	(43,000,000)
Fair value of plan assets at end of year	450,000,000	427,000,000	403,000,000
Accrued asset (liability)	(206,000,000)	(272,000,000)
Mississippi Power \| Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	754,000,000	671,000,000
Service cost	18,000,000	15,000,000	12,000,000
Interest cost	16,000,000	20,000,000	22,000,000
Benefits paid	(28,000,000)	(27,000,000)
Actuarial gain	(17,000,000)	75,000,000
Balance at end of year	743,000,000	754,000,000	671,000,000
Change in plan assets
Fair value of plan assets at beginning of year	701,000,000	641,000,000
Actual return on plan assets	111,000,000	85,000,000
Employer contributions	2,000,000	2,000,000
Benefits paid	(28,000,000)	(27,000,000)
Fair value of plan assets at end of year	786,000,000	701,000,000	641,000,000
Accrued asset (liability)	43,000,000	(53,000,000)
Mississippi Power \| Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	81,000,000	87,000,000
Service cost	1,000,000	1,000,000	1,000,000
Interest cost	1,000,000	2,000,000	3,000,000
Benefits paid	(5,000,000)	(6,000,000)
Actuarial gain	(2,000,000)	(3,000,000)
Retiree drug subsidy	0	0
Balance at end of year	76,000,000	81,000,000	87,000,000
Change in plan assets
Fair value of plan assets at beginning of year	27,000,000	26,000,000
Actual return on plan assets	4,000,000	3,000,000
Employer contributions	3,000,000	4,000,000
Benefits paid	(5,000,000)	(6,000,000)
Fair value of plan assets at end of year	29,000,000	27,000,000	26,000,000
Accrued asset (liability)	(47,000,000)	(54,000,000)
Southern Power \| Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	217,000,000	185,000,000
Service cost	10,000,000	8,000,000	7,000,000
Interest cost	5,000,000	6,000,000	5,000,000
Benefits paid	(4,000,000)	(6,000,000)
Actuarial gain	(6,000,000)	24,000,000
Balance at end of year	222,000,000	217,000,000	185,000,000
Change in plan assets
Fair value of plan assets at beginning of year	186,000,000	169,000,000
Actual return on plan assets	30,000,000	22,000,000
Employer contributions	1,000,000	1,000,000
Benefits paid	(4,000,000)	(6,000,000)
Fair value of plan assets at end of year	213,000,000	186,000,000	169,000,000
Accrued asset (liability)	(9,000,000)	(31,000,000)
Southern Power \| Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	12,000,000	11,000,000
Service cost	0	1,000,000	1,000,000
Interest cost	0	0	0
Benefits paid	0	0
Actuarial gain	(1,000,000)	0
Retiree drug subsidy	0	0
Balance at end of year	11,000,000	12,000,000	11,000,000
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	0	0
Benefits paid	0	0
Fair value of plan assets at end of year	0	0	0
Accrued asset (liability)	(11,000,000)	(12,000,000)
Southern Company Gas \| Pension plans
Change in benefit obligation
Benefit obligation at beginning of year	1,189,000,000	1,067,000,000
Service cost	37,000,000	33,000,000	25,000,000
Interest cost	24,000,000	31,000,000	36,000,000
Benefits paid	(73,000,000)	(69,000,000)
Actuarial gain	(43,000,000)	127,000,000
Balance at end of year	1,134,000,000	1,189,000,000	1,067,000,000
Change in plan assets
Fair value of plan assets at beginning of year	1,123,000,000	1,050,000,000
Actual return on plan assets	181,000,000	139,000,000
Employer contributions	10,000,000	3,000,000
Benefits paid	(73,000,000)	(69,000,000)
Fair value of plan assets at end of year	1,241,000,000	1,123,000,000	1,050,000,000
Accrued asset (liability)	107,000,000	(66,000,000)
Southern Company Gas \| Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year	248,000,000	250,000,000
Service cost	2,000,000	2,000,000	1,000,000
Interest cost	4,000,000	7,000,000	9,000,000
Benefits paid	(18,000,000)	(17,000,000)
Actuarial gain	1,000,000	6,000,000
Retiree drug subsidy	0	0
Balance at end of year	237,000,000	248,000,000	250,000,000
Change in plan assets
Fair value of plan assets at beginning of year	128,000,000	115,000,000
Actual return on plan assets	18,000,000	18,000,000
Employer contributions	15,000,000	12,000,000
Benefits paid	(18,000,000)	(17,000,000)
Fair value of plan assets at end of year	143,000,000	128,000,000	$ 115,000,000
Accrued asset (liability)	(94,000,000)	$ (120,000,000)
Qualified pension plan \| Pension plans
Change in benefit obligation
Balance at end of year	15,568,000,000
Change in plan assets
Employer contributions	0
Qualified pension plan \| Alabama Power \| Pension plans
Change in benefit obligation
Balance at end of year	3,678,000,000
Qualified pension plan \| Georgia Power \| Pension plans
Change in benefit obligation
Balance at end of year	4,852,000,000
Qualified pension plan \| Mississippi Power \| Pension plans
Change in benefit obligation
Balance at end of year	708,000,000
Qualified pension plan \| Southern Power \| Pension plans
Change in benefit obligation
Balance at end of year	193,000,000
Qualified pension plan \| Southern Company Gas \| Pension plans
Change in benefit obligation
Balance at end of year	1,066,000,000
Non-qualified pension plan \| Pension plans
Change in benefit obligation
Balance at end of year	814,000,000
Non-qualified pension plan \| Alabama Power \| Pension plans
Change in benefit obligation
Balance at end of year	129,000,000
Non-qualified pension plan \| Georgia Power \| Pension plans
Change in benefit obligation
Balance at end of year	160,000,000
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	29,000,000
Non-qualified pension plan \| Southern Company Gas \| Pension plans
Change in benefit obligation
Balance at end of year	$ 68,000,000

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount, before benefit payment, of receipt from prescription drug subsidy which increases benefit obligation of defined benefit postretirement plan.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

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Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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