Retirement Benefits - Changes in Projected Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($)	6 Months Ended	12 Months Ended
Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension plans
Change in benefit obligation
Benefit obligation at beginning of year		$ 13,808,000,000	$ 12,385,000,000
Dispositions		(107,000,000)
Service cost		359,000,000	293,000,000	$ 262,000,000
Interest cost		464,000,000	455,000,000	422,000,000
Benefits paid		(618,000,000)	(596,000,000)
Actuarial (gain) loss		(1,143,000,000)	1,297,000,000
Plan amendments			(26,000,000)
Obligations assumed from employee transfer			0
Balance at end of year	$ 12,385,000,000	12,763,000,000	13,808,000,000	12,385,000,000
Change in plan assets
Fair value of plan assets at beginning of year		12,992,000,000	11,583,000,000
Dispositions		(107,000,000)
Actual return (loss) on plan assets		(711,000,000)	1,953,000,000
Employer contributions		55,000,000	52,000,000
Benefits paid		(618,000,000)	(596,000,000)
Assets assumed from employee transfer			0
Fair value of plan assets at end of year	11,583,000,000	11,611,000,000	12,992,000,000	11,583,000,000
Accrued liability		(1,152,000,000)	(816,000,000)
Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year		2,339,000,000	2,297,000,000
Dispositions		(18,000,000)
Service cost		24,000,000	24,000,000	22,000,000
Interest cost		75,000,000	79,000,000	76,000,000
Benefits paid		(129,000,000)	(136,000,000)
Actuarial (gain) loss		(432,000,000)	65,000,000
Plan amendments			3,000,000
Retiree drug subsidy		6,000,000	7,000,000
Obligations assumed from employee transfer			0
Employee contributions			0
Balance at end of year	2,297,000,000	1,865,000,000	2,339,000,000	2,297,000,000
Change in plan assets
Fair value of plan assets at beginning of year		1,053,000,000	944,000,000
Dispositions		(18,000,000)
Actual return (loss) on plan assets		(57,000,000)	154,000,000
Employer contributions		73,000,000	84,000,000
Employee contributions			0
Benefits paid		(123,000,000)	(129,000,000)
Fair value of plan assets at end of year	944,000,000	928,000,000	1,053,000,000	944,000,000
Accrued liability		(937,000,000)	(1,286,000,000)
Alabama Power \| Pension plans
Change in benefit obligation
Benefit obligation at beginning of year		2,998,000,000	2,663,000,000
Dispositions		0
Service cost		78,000,000	63,000,000	57,000,000
Interest cost		101,000,000	98,000,000	95,000,000
Benefits paid		(124,000,000)	(120,000,000)
Actuarial (gain) loss		(237,000,000)	294,000,000
Plan amendments			0
Obligations assumed from employee transfer			0
Balance at end of year	2,663,000,000	2,816,000,000	2,998,000,000	2,663,000,000
Change in plan assets
Fair value of plan assets at beginning of year		2,836,000,000	2,517,000,000
Dispositions		0
Actual return (loss) on plan assets		(150,000,000)	427,000,000
Employer contributions		13,000,000	12,000,000
Benefits paid		(124,000,000)	(120,000,000)
Assets assumed from employee transfer			0
Fair value of plan assets at end of year	2,517,000,000	2,575,000,000	2,836,000,000	2,517,000,000
Accrued liability		(241,000,000)	(162,000,000)
Alabama Power \| Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year		517,000,000	501,000,000
Dispositions		0
Service cost		6,000,000	6,000,000	5,000,000
Interest cost		17,000,000	17,000,000	18,000,000
Benefits paid		(28,000,000)	(29,000,000)
Actuarial (gain) loss		(111,000,000)	20,000,000
Plan amendments			0
Retiree drug subsidy		2,000,000	2,000,000
Obligations assumed from employee transfer			0
Employee contributions			0
Balance at end of year	501,000,000	403,000,000	517,000,000	501,000,000
Change in plan assets
Fair value of plan assets at beginning of year		406,000,000	367,000,000
Dispositions		0
Actual return (loss) on plan assets		(25,000,000)	60,000,000
Employer contributions		5,000,000	6,000,000
Employee contributions			0
Benefits paid		(26,000,000)	(27,000,000)
Fair value of plan assets at end of year	367,000,000	360,000,000	406,000,000	367,000,000
Accrued liability		(43,000,000)	(111,000,000)
Georgia Power \| Pension plans
Change in benefit obligation
Benefit obligation at beginning of year		4,188,000,000	3,800,000,000
Dispositions		0
Service cost		87,000,000	74,000,000	70,000,000
Interest cost		139,000,000	138,000,000	136,000,000
Benefits paid		(191,000,000)	(187,000,000)
Actuarial (gain) loss		(318,000,000)	363,000,000
Plan amendments			0
Obligations assumed from employee transfer			0
Balance at end of year	3,800,000,000	3,905,000,000	4,188,000,000	3,800,000,000
Change in plan assets
Fair value of plan assets at beginning of year		4,058,000,000	3,621,000,000
Dispositions		0
Actual return (loss) on plan assets		(218,000,000)	610,000,000
Employer contributions		14,000,000	14,000,000
Benefits paid		(191,000,000)	(187,000,000)
Assets assumed from employee transfer			0
Fair value of plan assets at end of year	3,621,000,000	3,663,000,000	4,058,000,000	3,621,000,000
Accrued liability		(242,000,000)	(130,000,000)
Georgia Power \| Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year		863,000,000	847,000,000
Dispositions		0
Service cost		6,000,000	7,000,000	6,000,000
Interest cost		28,000,000	29,000,000	30,000,000
Benefits paid		(47,000,000)	(51,000,000)
Actuarial (gain) loss		(178,000,000)	28,000,000
Plan amendments			0
Retiree drug subsidy		3,000,000	3,000,000
Obligations assumed from employee transfer			0
Employee contributions			0
Balance at end of year	847,000,000	675,000,000	863,000,000	847,000,000
Change in plan assets
Fair value of plan assets at beginning of year		386,000,000	354,000,000
Dispositions		0
Actual return (loss) on plan assets		(20,000,000)	54,000,000
Employer contributions		22,000,000	26,000,000
Employee contributions			0
Benefits paid		(44,000,000)	(48,000,000)
Fair value of plan assets at end of year	354,000,000	344,000,000	386,000,000	354,000,000
Accrued liability		(331,000,000)	(477,000,000)
Mississippi Power \| Pension plans
Change in benefit obligation
Benefit obligation at beginning of year		602,000,000	534,000,000
Dispositions		0
Service cost		17,000,000	15,000,000	13,000,000
Interest cost		20,000,000	20,000,000	19,000,000
Benefits paid		(24,000,000)	(22,000,000)
Actuarial (gain) loss		(58,000,000)	55,000,000
Plan amendments			0
Obligations assumed from employee transfer			0
Balance at end of year	534,000,000	557,000,000	602,000,000	534,000,000
Change in plan assets
Fair value of plan assets at beginning of year		563,000,000	499,000,000
Dispositions		0
Actual return (loss) on plan assets		(37,000,000)	84,000,000
Employer contributions		3,000,000	2,000,000
Benefits paid		(24,000,000)	(22,000,000)
Assets assumed from employee transfer			0
Fair value of plan assets at end of year	499,000,000	505,000,000	563,000,000	499,000,000
Accrued liability		(52,000,000)	(39,000,000)
Mississippi Power \| Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year		97,000,000	97,000,000
Dispositions		0
Service cost		1,000,000	1,000,000	1,000,000
Interest cost		3,000,000	3,000,000	3,000,000
Benefits paid		(5,000,000)	(6,000,000)
Actuarial (gain) loss		(15,000,000)	1,000,000
Plan amendments			0
Retiree drug subsidy		0	1,000,000
Obligations assumed from employee transfer			0
Employee contributions			0
Balance at end of year	97,000,000	81,000,000	97,000,000	97,000,000
Change in plan assets
Fair value of plan assets at beginning of year		25,000,000	23,000,000
Dispositions		0
Actual return (loss) on plan assets		(1,000,000)	3,000,000
Employer contributions		4,000,000	4,000,000
Employee contributions			0
Benefits paid		(5,000,000)	(5,000,000)
Fair value of plan assets at end of year	23,000,000	23,000,000	25,000,000	23,000,000
Accrued liability		(58,000,000)	(72,000,000)
Southern Power \| Pension plans
Change in benefit obligation
Benefit obligation at beginning of year		139,000,000	0
Dispositions		(3,000,000)
Service cost		9,000,000	0
Interest cost		5,000,000	0
Benefits paid		(3,000,000)	0
Actuarial (gain) loss		(24,000,000)	0
Plan amendments			0
Obligations assumed from employee transfer			139,000,000
Balance at end of year	0	123,000,000	139,000,000	0
Change in plan assets
Fair value of plan assets at beginning of year		138,000,000	0
Dispositions		(3,000,000)
Actual return (loss) on plan assets		(9,000,000)	0
Employer contributions		0	0
Benefits paid		(3,000,000)	0
Assets assumed from employee transfer			138,000,000
Fair value of plan assets at end of year	0	123,000,000	138,000,000	0
Accrued liability		0	(1,000,000)
Southern Power \| Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year		11,000,000	0
Dispositions		0
Service cost		1,000,000	0
Interest cost		0	0
Benefits paid		(1,000,000)	0
Actuarial (gain) loss		(2,000,000)	0
Plan amendments			0
Retiree drug subsidy		0	0
Obligations assumed from employee transfer			11,000,000
Employee contributions			0
Balance at end of year	0	9,000,000	11,000,000	0
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		1,000,000	0
Employee contributions			0
Benefits paid		(1,000,000)	0
Fair value of plan assets at end of year	0	0	0	0
Accrued liability		(9,000,000)	(11,000,000)
Southern Company Gas \| Pension plans
Change in benefit obligation
Benefit obligation at beginning of year		1,184,000,000	1,133,000,000
Dispositions		(104,000,000)
Service cost	15,000,000	34,000,000	23,000,000
Interest cost	20,000,000	39,000,000	42,000,000
Benefits paid		(98,000,000)	(91,000,000)
Actuarial (gain) loss		(148,000,000)	103,000,000
Plan amendments			(26,000,000)
Obligations assumed from employee transfer			0
Balance at end of year	1,133,000,000	907,000,000	1,184,000,000	1,133,000,000
Change in plan assets
Fair value of plan assets at beginning of year		1,068,000,000	983,000,000
Dispositions		(104,000,000)
Actual return (loss) on plan assets		(70,000,000)	175,000,000
Employer contributions		2,000,000	1,000,000
Benefits paid		(98,000,000)	(91,000,000)
Assets assumed from employee transfer			0
Fair value of plan assets at end of year	983,000,000	798,000,000	1,068,000,000	983,000,000
Accrued liability		(109,000,000)	(116,000,000)
Southern Company Gas \| Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year		310,000,000	308,000,000
Dispositions		(18,000,000)
Service cost	1,000,000	2,000,000	2,000,000
Interest cost	5,000,000	10,000,000	10,000,000
Benefits paid		(17,000,000)	(19,000,000)
Actuarial (gain) loss		(43,000,000)	3,000,000
Plan amendments			3,000,000
Retiree drug subsidy		0	0
Obligations assumed from employee transfer			0
Employee contributions			3,000,000
Balance at end of year	308,000,000	244,000,000	310,000,000	308,000,000
Change in plan assets
Fair value of plan assets at beginning of year		125,000,000	105,000,000
Dispositions		(18,000,000)
Actual return (loss) on plan assets		(5,000,000)	20,000,000
Employer contributions		13,000,000	17,000,000
Employee contributions			3,000,000
Benefits paid		(17,000,000)	(20,000,000)
Fair value of plan assets at end of year	$ 105,000,000	98,000,000	125,000,000	$ 105,000,000
Accrued liability		(146,000,000)	$ (185,000,000)
Qualified Pension Plans \| Pension plans
Change in benefit obligation
Balance at end of year		12,135,000,000
Change in plan assets
Employer contributions		0
Qualified Pension Plans \| Alabama Power \| Pension plans
Change in benefit obligation
Balance at end of year		2,692,000,000
Qualified Pension Plans \| Georgia Power \| Pension plans
Change in benefit obligation
Balance at end of year		3,757,000,000
Qualified Pension Plans \| Mississippi Power \| Pension plans
Change in benefit obligation
Balance at end of year		527,000,000
Qualified Pension Plans \| Southern Power \| Pension plans
Change in benefit obligation
Balance at end of year		122,000,000
Qualified Pension Plans \| Southern Company Gas \| Pension plans
Change in benefit obligation
Balance at end of year		866,000,000
Non Qualified Pension Plans \| Pension plans
Change in benefit obligation
Balance at end of year		629,000,000
Non Qualified Pension Plans \| Alabama Power \| Pension plans
Change in benefit obligation
Balance at end of year		124,000,000
Non Qualified Pension Plans \| Georgia Power \| Pension plans
Change in benefit obligation
Balance at end of year		148,000,000
Non Qualified Pension Plans \| Mississippi Power \| Pension plans
Change in benefit obligation
Balance at end of year		30,000,000
Non Qualified Pension Plans \| Southern Power \| Pension plans
Change in benefit obligation
Balance at end of year		1,000,000
Non Qualified Pension Plans \| Southern Company Gas \| Pension plans
Change in benefit obligation
Balance at end of year		$ 41,000,000

Defined Benefit Plan, Benefit Obligation, Obligations Assumed From Employee Transfer

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Defined Benefit Plan, Plan Assets, Assets Assumed From Employee Transfer

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- Definition

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 of contributions received by defined benefit plan from participant which increase benefit obligation.

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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 decrease in benefit obligation of defined benefit plan from disposal or classification as held-for-sale.

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Amount of decrease in plan assets of defined benefit plan from disposal or classification as held-for-sale.

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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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