Retirement Benefits - Changes in Projected Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($)		6 Months Ended		12 Months Ended
	Dec. 19, 2016	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension plans
Change in benefit obligation
Benefit obligation at beginning of year			$ 10,542,000,000	$ 12,385,000,000	$ 10,542,000,000
Acquisitions				0	1,244,000,000
Service cost				293,000,000	262,000,000	$ 257,000,000
Interest cost				455,000,000	422,000,000	445,000,000
Benefits paid				(596,000,000)	(466,000,000)
Actuarial loss (gain)				1,297,000,000	342,000,000
Plan amendments				(26,000,000)	39,000,000
Balance at end of year		$ 12,385,000,000		13,808,000,000	12,385,000,000	10,542,000,000
Change in plan assets
Fair value of plan assets at beginning of year			9,234,000,000	11,583,000,000	9,234,000,000
Acquisitions				0	837,000,000
Actual return (loss) on plan assets				1,953,000,000	902,000,000
Employer contributions				52,000,000	1,076,000,000
Benefits paid				(596,000,000)	(466,000,000)
Fair value of plan assets at end of year		11,583,000,000		12,992,000,000	11,583,000,000	9,234,000,000
Accrued liability		(802,000,000)		(816,000,000)	(802,000,000)
Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year			1,989,000,000	2,297,000,000	1,989,000,000
Acquisitions				0	338,000,000
Service cost				24,000,000	22,000,000	23,000,000
Interest cost				79,000,000	76,000,000	78,000,000
Benefits paid				(136,000,000)	(119,000,000)
Actuarial loss (gain)				65,000,000	(16,000,000)
Plan amendments				3,000,000	0
Retiree drug subsidy				7,000,000	7,000,000
Balance at end of year		2,297,000,000		2,339,000,000	2,297,000,000	1,989,000,000
Change in plan assets
Fair value of plan assets at beginning of year			833,000,000	944,000,000	833,000,000
Acquisitions				0	100,000,000
Actual return (loss) on plan assets				154,000,000	58,000,000
Employer contributions				84,000,000	65,000,000
Benefits paid				(129,000,000)	(112,000,000)
Fair value of plan assets at end of year		944,000,000		1,053,000,000	944,000,000	833,000,000
Accrued liability		(1,353,000,000)		(1,286,000,000)	(1,353,000,000)
GEORGIA POWER CO | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year			3,615,000,000	3,800,000,000	3,615,000,000
Service cost				74,000,000	70,000,000	73,000,000
Interest cost				138,000,000	136,000,000	154,000,000
Benefits paid				(187,000,000)	(164,000,000)
Actuarial loss (gain)				363,000,000	143,000,000
Balance at end of year		3,800,000,000		4,188,000,000	3,800,000,000	3,615,000,000
Change in plan assets
Fair value of plan assets at beginning of year			3,196,000,000	3,621,000,000	3,196,000,000
Actual return (loss) on plan assets				610,000,000	288,000,000
Employer contributions	$ 0			14,000,000	301,000,000
Benefits paid				(187,000,000)	(164,000,000)
Fair value of plan assets at end of year		3,621,000,000		4,058,000,000	3,621,000,000	3,196,000,000
Accrued liability		(179,000,000)		(130,000,000)	(179,000,000)
GEORGIA POWER CO | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year			854,000,000	847,000,000	854,000,000
Service cost				7,000,000	6,000,000	7,000,000
Interest cost				29,000,000	30,000,000	34,000,000
Benefits paid				(51,000,000)	(45,000,000)
Actuarial loss (gain)				28,000,000	(1,000,000)
Retiree drug subsidy				3,000,000	3,000,000
Balance at end of year		847,000,000		863,000,000	847,000,000	854,000,000
Change in plan assets
Fair value of plan assets at beginning of year			358,000,000	354,000,000	358,000,000
Actual return (loss) on plan assets				54,000,000	21,000,000
Employer contributions				26,000,000	17,000,000
Benefits paid				(48,000,000)	(42,000,000)
Fair value of plan assets at end of year		354,000,000		386,000,000	354,000,000	358,000,000
Accrued liability		(493,000,000)		(477,000,000)	(493,000,000)
ALABAMA POWER CO | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year			2,506,000,000	2,663,000,000	2,506,000,000
Service cost				63,000,000	57,000,000	59,000,000
Interest cost				98,000,000	95,000,000	106,000,000
Benefits paid				(120,000,000)	(109,000,000)
Actuarial loss (gain)				294,000,000	114,000,000
Balance at end of year		2,663,000,000		2,998,000,000	2,663,000,000	2,506,000,000
Change in plan assets
Fair value of plan assets at beginning of year			2,279,000,000	2,517,000,000	2,279,000,000
Actual return (loss) on plan assets				427,000,000	206,000,000
Employer contributions	0			12,000,000	141,000,000
Benefits paid				(120,000,000)	(109,000,000)
Fair value of plan assets at end of year		2,517,000,000		2,836,000,000	2,517,000,000	2,279,000,000
Accrued liability		(146,000,000)		(162,000,000)	(146,000,000)
ALABAMA POWER CO | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year			505,000,000	501,000,000	505,000,000
Service cost				6,000,000	5,000,000	6,000,000
Interest cost				17,000,000	18,000,000	20,000,000
Benefits paid				(29,000,000)	(28,000,000)
Actuarial loss (gain)				20,000,000	(1,000,000)
Retiree drug subsidy				2,000,000	2,000,000
Balance at end of year		501,000,000		517,000,000	501,000,000	505,000,000
Change in plan assets
Fair value of plan assets at beginning of year			363,000,000	367,000,000	363,000,000
Actual return (loss) on plan assets				60,000,000	23,000,000
Employer contributions				6,000,000	7,000,000
Benefits paid				(27,000,000)	(26,000,000)
Fair value of plan assets at end of year		367,000,000		406,000,000	367,000,000	363,000,000
Accrued liability		(134,000,000)		(111,000,000)	(134,000,000)
GULF POWER CO
Change in benefit obligation
Service cost				13,000,000	12,000,000	12,000,000
Interest cost				19,000,000	19,000,000	20,000,000
GULF POWER CO | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year			480,000,000	517,000,000	480,000,000
Service cost				13,000,000	12,000,000
Interest cost				19,000,000	19,000,000
Benefits paid				(20,000,000)	(17,000,000)
Actuarial loss (gain)				58,000,000	23,000,000
Balance at end of year		517,000,000		587,000,000	517,000,000	480,000,000
Change in plan assets
Fair value of plan assets at beginning of year			420,000,000	491,000,000	420,000,000
Actual return (loss) on plan assets				81,000,000	39,000,000
Employer contributions	0			1,000,000	49,000,000
Benefits paid				(20,000,000)	(17,000,000)
Fair value of plan assets at end of year		491,000,000		553,000,000	491,000,000	420,000,000
Accrued liability		(26,000,000)		(34,000,000)	(26,000,000)
GULF POWER CO | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year			81,000,000	83,000,000	81,000,000
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)	(4,000,000)
Actuarial loss (gain)				1,000,000	2,000,000
Balance at end of year		83,000,000		83,000,000	83,000,000	81,000,000
Change in plan assets
Fair value of plan assets at beginning of year			17,000,000	18,000,000	17,000,000
Actual return (loss) on plan assets				3,000,000	2,000,000
Employer contributions				4,000,000	3,000,000
Benefits paid				(5,000,000)	(4,000,000)
Fair value of plan assets at end of year		18,000,000		20,000,000	18,000,000	17,000,000
Accrued liability		(65,000,000)		(63,000,000)	(65,000,000)
MISSISSIPPI POWER CO
Change in benefit obligation
Service cost				15,000,000	13,000,000	13,000,000
Interest cost				20,000,000	19,000,000	21,000,000
MISSISSIPPI POWER CO | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year			500,000,000	534,000,000	500,000,000
Service cost				15,000,000	13,000,000
Interest cost				20,000,000	19,000,000
Benefits paid				(22,000,000)	(20,000,000)
Actuarial loss (gain)				55,000,000	22,000,000
Balance at end of year		534,000,000		602,000,000	534,000,000	500,000,000
Change in plan assets
Fair value of plan assets at beginning of year			430,000,000	499,000,000	430,000,000
Actual return (loss) on plan assets				84,000,000	39,000,000
Employer contributions	$ 0			2,000,000	50,000,000
Benefits paid				(22,000,000)	(20,000,000)
Fair value of plan assets at end of year		499,000,000		563,000,000	499,000,000	430,000,000
Accrued liability		(35,000,000)		(39,000,000)	(35,000,000)
MISSISSIPPI POWER CO | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year			97,000,000	97,000,000	97,000,000
Service cost				1,000,000	1,000,000	1,000,000
Interest cost				3,000,000	3,000,000	4,000,000
Benefits paid				(6,000,000)	(6,000,000)
Actuarial loss (gain)				1,000,000	1,000,000
Retiree drug subsidy				1,000,000	1,000,000
Balance at end of year		97,000,000		97,000,000	97,000,000	97,000,000
Change in plan assets
Fair value of plan assets at beginning of year			23,000,000	23,000,000	23,000,000
Actual return (loss) on plan assets				3,000,000	1,000,000
Employer contributions				4,000,000	4,000,000
Benefits paid				(5,000,000)	(5,000,000)
Fair value of plan assets at end of year		23,000,000		25,000,000	23,000,000	23,000,000
Accrued liability		(74,000,000)		(72,000,000)	(74,000,000)
SOUTHERN Co GAS
Change in benefit obligation
Employee contributions		1,000,000		3,000,000
SOUTHERN Co GAS | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year		1,244,000,000		1,133,000,000
Service cost		15,000,000		23,000,000
Interest cost		20,000,000		42,000,000
Benefits paid		(31,000,000)		(91,000,000)
Actuarial loss (gain)		(115,000,000)		103,000,000
Plan amendments		0		(26,000,000)
Balance at end of year		1,133,000,000	1,244,000,000	1,184,000,000	1,133,000,000
Change in plan assets
Fair value of plan assets at beginning of year		837,000,000		983,000,000
Actual return (loss) on plan assets		48,000,000		175,000,000
Employer contributions		129,000,000		1,000,000
Benefits paid		(31,000,000)		(91,000,000)
Fair value of plan assets at end of year		983,000,000	837,000,000	1,068,000,000	983,000,000
Accrued liability		(150,000,000)		(116,000,000)	(150,000,000)
SOUTHERN Co GAS | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year		338,000,000		308,000,000
Service cost		1,000,000		2,000,000
Interest cost		5,000,000		10,000,000
Benefits paid		(11,000,000)		(19,000,000)
Actuarial loss (gain)		(26,000,000)		3,000,000
Plan amendments		0		3,000,000
Balance at end of year		308,000,000	338,000,000	310,000,000	308,000,000
Change in plan assets
Fair value of plan assets at beginning of year		100,000,000		105,000,000
Actual return (loss) on plan assets		4,000,000		20,000,000
Employee contributions		1,000,000		3,000,000
Employer contributions		11,000,000		17,000,000
Benefits paid		(11,000,000)		(20,000,000)
Fair value of plan assets at end of year		105,000,000	100,000,000	125,000,000	105,000,000
Accrued liability		(203,000,000)		(185,000,000)	(203,000,000)
Predecessor | SOUTHERN Co GAS
Change in benefit obligation
Employee contributions			1,000,000
Predecessor | SOUTHERN Co GAS | Pension plans
Change in benefit obligation
Benefit obligation at beginning of year		1,244,000,000	1,067,000,000		1,067,000,000
Service cost			13,000,000			28,000,000
Interest cost			21,000,000			45,000,000
Benefits paid			(26,000,000)
Actuarial loss (gain)			169,000,000
Plan amendments			0
Balance at end of year			1,244,000,000			1,067,000,000
Change in plan assets
Fair value of plan assets at beginning of year		837,000,000	847,000,000		847,000,000
Actual return (loss) on plan assets			15,000,000
Employer contributions			1,000,000
Benefits paid			(26,000,000)
Fair value of plan assets at end of year			837,000,000			847,000,000
Accrued liability			(407,000,000)
Predecessor | SOUTHERN Co GAS | Other postretirement benefit plans
Change in benefit obligation
Benefit obligation at beginning of year		338,000,000		318,000,000
Service cost			1,000,000			2,000,000
Interest cost			5,000,000			$ 13,000,000
Benefits paid			(11,000,000)
Actuarial loss (gain)			24,000,000
Plan amendments			0
Balance at end of year		318,000,000	338,000,000		318,000,000
Change in plan assets
Fair value of plan assets at beginning of year		100,000,000		$ 99,000,000
Actual return (loss) on plan assets			1,000,000
Employee contributions			1,000,000
Employer contributions			10,000,000
Benefits paid			(11,000,000)
Fair value of plan assets at end of year		$ 99,000,000	100,000,000		$ 99,000,000
Accrued liability			$ (238,000,000)