Retirement Benefits - Textual (Details)			3 Months Ended	6 Months Ended		12 Months Ended
	Jan. 01, 2018	Dec. 19, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2017 USD ($) Employee	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Aug. 29, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Annual increase or decrease in assumed medical care cost trend rate						1.00%
Total accumulated benefit obligation for the pension plans				$ (11,300,000,000)		$ (12,600,000,000)	$ (11,300,000,000)
Period over which company has elected to amortize changes in the market value of all plan assets						5 years
Actual plan asset allocations (as percent)						100.00%
Target plan asset allocations (as percent)						100.00%
Defined contribution plan, cost						$ 118,000,000	105,000,000	$ 92,000,000
Defined contribution plan, employer discretionary contribution amount				0	$ 2,000,000	2,000,000		2,000,000
Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations						13,200,000,000
Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations						$ 652,000,000
Southern Company Services, Inc.
Defined Benefit Plan Disclosure [Line Items]
Number of employee transfers | Employee						538
GEORGIA POWER CO
Defined Benefit Plan Disclosure [Line Items]
Expected postretirement trust contributions						$ 0
Annual increase or decrease in assumed medical care cost trend rate						1.00%
Period over which company has elected to amortize changes in the market value of all plan assets						5 years
GEORGIA POWER CO | Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations						$ 4,000,000,000
GEORGIA POWER CO | Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations						$ 153,000,000
GEORGIA POWER CO | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						6.00%
GEORGIA POWER CO | Maximum | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Maximum limit of contribution of employees base salary						5.10%
SOUTHERN Co GAS
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans				(1,100,000,000)		$ (1,100,000,000)	(1,100,000,000)
SOUTHERN Co GAS | Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations						1,100,000,000
SOUTHERN Co GAS | Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations						$ 44,000,000
SOUTHERN Co GAS | Employee Savings Plan Option One Contribution Percentage
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						65.00%
SOUTHERN Co GAS | Employee Savings Plan First Matching Contribution Percentage
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						100.00%
SOUTHERN Co GAS | Employee Saving Plan
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						75.00%
SOUTHERN Co GAS | Pension and Other Postretirement Plans Costs
Defined Benefit Plan Disclosure [Line Items]
Regulatory asset			$ 437,000,000
SOUTHERN Co GAS | Maximum | Employee Savings Plan Option One Contribution Percentage
Defined Benefit Plan Disclosure [Line Items]
Maximum limit of contribution of employees base salary						8.00%
SOUTHERN Co GAS | Maximum | Employee Savings Plan First Matching Contribution Percentage
Defined Benefit Plan Disclosure [Line Items]
Maximum limit of contribution of employees base salary						3.00%
SOUTHERN Co GAS | Maximum | Employee Saving Plan
Defined Benefit Plan Disclosure [Line Items]
Maximum limit of contribution of employees base salary						3.00%
ALABAMA POWER CO
Defined Benefit Plan Disclosure [Line Items]
Expected postretirement trust contributions						$ 0
Annual increase or decrease in assumed medical care cost trend rate						1.00%
Total accumulated benefit obligation for the pension plans				(2,400,000,000)		$ (2,700,000,000)	(2,400,000,000)
Period over which company has elected to amortize changes in the market value of all plan assets						5 years
Defined contribution plan, cost						$ 23,000,000	23,000,000	22,000,000
ALABAMA POWER CO | Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations						2,900,000,000
ALABAMA POWER CO | Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations						$ 126,000,000
ALABAMA POWER CO | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						6.00%
Maximum limit of contribution of employees base salary						5.10%
GULF POWER CO
Defined Benefit Plan Disclosure [Line Items]
Annual increase or decrease in assumed medical care cost trend rate						1.00%
Period over which company has elected to amortize changes in the market value of all plan assets						5 years
Regulatory asset									$ 63,000,000
GULF POWER CO | Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Expected postretirement trust contributions						$ 0
Projected benefit obligations						563,000,000
GULF POWER CO | Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations						$ 25,000,000
GULF POWER CO | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						6.00%
Maximum limit of contribution of employees base salary						5.10%
MISSISSIPPI POWER CO
Defined Benefit Plan Disclosure [Line Items]
Annual increase or decrease in assumed medical care cost trend rate						1.00%
Total accumulated benefit obligation for the pension plans				$ (479,000,000)		$ (541,000,000)	$ (479,000,000)
Period over which company has elected to amortize changes in the market value of all plan assets						5 years
MISSISSIPPI POWER CO | Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Expected postretirement trust contributions						$ 0
Projected benefit obligations						571,000,000
MISSISSIPPI POWER CO | Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations						$ 31,000,000
MISSISSIPPI POWER CO | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						6.00%
Maximum limit of contribution of employees base salary						5.10%
SOUTHERN POWER CO
Defined Benefit Plan Disclosure [Line Items]
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax						$ 36,000,000
AGL Resources Inc. Retirement Plan | SOUTHERN Co GAS
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
AGL Resources Inc. Retirement Plan | SOUTHERN Co GAS | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				20.00%		19.00%	20.00%
Target plan asset allocations (as percent)						15.00%
AGL Resources Inc. Retirement Plan | SOUTHERN Co GAS | Other Types Of Investments
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				10.00%		10.00%	10.00%
Target plan asset allocations (as percent)						30.00%
AGL Resources Inc. Retirement Plan | SOUTHERN Co GAS | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				69.00%		65.00%	69.00%
Target plan asset allocations (as percent)						53.00%
AGL Resources Inc. Retirement Plan | SOUTHERN Co GAS | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				1.00%		6.00%	1.00%
Target plan asset allocations (as percent)						2.00%
Employee Savings Plan | GEORGIA POWER CO
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost						$ 26,000,000	$ 27,000,000	26,000,000
Employee Savings Plan | SOUTHERN Co GAS
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost						17,000,000	8,000,000
Employee Savings Plan | SOUTHERN Co GAS | Predecessor
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost					10,000,000			16,000,000
Employee Savings Plan | GULF POWER CO
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost						5,000,000	5,000,000	4,000,000
Employee Savings Plan | MISSISSIPPI POWER CO
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost						5,000,000	5,000,000	5,000,000
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Benefit Payments, 2019						637,000,000
Benefit Payments, 2018						634,000,000
Employer contributions						52,000,000	1,076,000,000
Defined benefit plan, AOCI, gain (loss), before tax				$ (3,165,000,000)		(3,244,000,000)	(3,165,000,000)
Projected benefit obligations				$ 12,385,000,000		$ 13,808,000,000	$ 12,385,000,000	$ 10,542,000,000
Annual salary increase on net periodic benefit costs						4.37%	4.37%	3.59%
Actual plan asset allocations (as percent)				100.00%			100.00%
Target plan asset allocations (as percent)				100.00%			100.00%
Benefit Payments, 2020						$ 663,000,000
Benefit Payments, 2021						681,000,000
Benefit Payments, 2022						704,000,000
Benefit Payments, 2023 to 2027						$ 3,836,000,000
Pension plans | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				29.00%		24.00%	29.00%
Target plan asset allocations (as percent)				23.00%		23.00%	23.00%
Pension plans | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax				$ 3,120,000,000		$ 3,155,000,000	$ 3,120,000,000	$ 2,998,000,000
Defined benefit plan, AOCI, gain (loss), before tax				(3,069,000,000)		(3,140,000,000)	(3,069,000,000)
Defined Benefit Plan, Amortization of Gain (Loss)						155,000,000	145,000,000
Pension plans | GEORGIA POWER CO
Defined Benefit Plan Disclosure [Line Items]
Benefit Payments, 2019						201,000,000
Benefit Payments, 2018						196,000,000
Employer contributions		$ 0				14,000,000	301,000,000
Defined benefit plan, AOCI, gain (loss), before tax				(1,112,000,000)		(1,091,000,000)	(1,112,000,000)
Expected postretirement trust contributions						0
Total accumulated benefit obligation for the pension plans				(3,500,000,000)		(3,800,000,000)	(3,500,000,000)
Projected benefit obligations				$ 3,800,000,000		$ 4,188,000,000	$ 3,800,000,000	$ 3,615,000,000
Annual salary increase on net periodic benefit costs						4.46%	4.46%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Benefit Payments, 2020						$ 207,000,000
Benefit Payments, 2021						210,000,000
Benefit Payments, 2022						216,000,000
Benefit Payments, 2023 to 2027						$ 1,156,000,000
Pension plans | GEORGIA POWER CO | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				29.00%		24.00%	29.00%
Target plan asset allocations (as percent)						23.00%
Pension plans | GEORGIA POWER CO | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax				$ 1,129,000,000		$ 1,105,000,000	$ 1,129,000,000	$ 1,076,000,000
Defined Benefit Plan, Amortization of Gain (Loss)						57,000,000	55,000,000
Pension plans | SOUTHERN Co GAS
Defined Benefit Plan Disclosure [Line Items]
Benefit Payments, 2019						77,000,000
Benefit Payments, 2018						100,000,000
Employer contributions				129,000,000		1,000,000
Defined benefit plan, AOCI, gain (loss), before tax				(226,000,000)		(155,000,000)	(226,000,000)
Projected benefit obligations				$ 1,133,000,000	1,244,000,000	$ 1,184,000,000	1,133,000,000
Annual salary increase on net periodic benefit costs				3.50%		3.50%
Regulatory asset					334,000,000
Defined benefit plan, benefit obligation, period increase (decrease)			177,000,000
Defined benefit plan, plan assets, period increase (decrease)			(10,000,000)
Benefit Payments, 2020						$ 79,000,000
Benefit Payments, 2021						79,000,000
Benefit Payments, 2022						80,000,000
Benefit Payments, 2023 to 2027						392,000,000
Defined Benefit Plan, Amortization of Gain (Loss)				$ (14,000,000)		(18,000,000)
Pension plans | SOUTHERN Co GAS | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax				267,000,000	368,000,000	217,000,000	267,000,000
Defined benefit plan, AOCI, gain (loss), before tax				(269,000,000)		(197,000,000)	(269,000,000)
Defined Benefit Plan, Amortization of Gain (Loss)				15,000,000		18,000,000
Pension plans | SOUTHERN Co GAS | Predecessor
Defined Benefit Plan Disclosure [Line Items]
Employer contributions					1,000,000
Projected benefit obligations					$ 1,244,000,000			$ 1,067,000,000
Annual salary increase on net periodic benefit costs					3.70%			3.70%
Defined Benefit Plan, Amortization of Gain (Loss)					$ (13,000,000)			$ (31,000,000)
Pension plans | SOUTHERN Co GAS | Predecessor | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax					84,000,000			88,000,000
Defined Benefit Plan, Amortization of Gain (Loss)					4,000,000
Pension plans | ALABAMA POWER CO
Defined Benefit Plan Disclosure [Line Items]
Benefit Payments, 2019						134,000,000
Benefit Payments, 2018						129,000,000
Employer contributions		0				12,000,000	141,000,000
Expected postretirement trust contributions						0
Projected benefit obligations				$ 2,663,000,000		$ 2,998,000,000	$ 2,663,000,000	$ 2,506,000,000
Annual salary increase on net periodic benefit costs						4.46%	4.46%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Benefit Payments, 2020						$ 139,000,000
Benefit Payments, 2021						143,000,000
Benefit Payments, 2022						148,000,000
Benefit Payments, 2023 to 2027						$ 807,000,000
Pension plans | ALABAMA POWER CO | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				29.00%		24.00%	29.00%
Target plan asset allocations (as percent)						23.00%
Pension plans | ALABAMA POWER CO | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax				$ 870,000,000		$ 890,000,000	$ 870,000,000	$ 822,000,000
Defined benefit plan, AOCI, gain (loss), before tax				(860,000,000)		(882,000,000)	(860,000,000)
Defined Benefit Plan, Amortization of Gain (Loss)						42,000,000	40,000,000
Pension plans | GULF POWER CO
Defined Benefit Plan Disclosure [Line Items]
Benefit Payments, 2019						23,000,000
Benefit Payments, 2018						22,000,000
Employer contributions		0				1,000,000	49,000,000
Defined benefit plan, AOCI, gain (loss), before tax				(150,000,000)		(158,000,000)	(150,000,000)
Total accumulated benefit obligation for the pension plans				(460,000,000)		(524,000,000)	(460,000,000)
Projected benefit obligations				$ 517,000,000		$ 587,000,000	$ 517,000,000	$ 480,000,000
Annual salary increase on net periodic benefit costs						4.46%	4.46%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Benefit Payments, 2020						$ 25,000,000
Benefit Payments, 2021						26,000,000
Benefit Payments, 2022						28,000,000
Benefit Payments, 2023 to 2027						$ 155,000,000
Pension plans | GULF POWER CO | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				29.00%		24.00%	29.00%
Target plan asset allocations (as percent)						23.00%
Pension plans | GULF POWER CO | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax				$ 153,000,000		$ 160,000,000	$ 153,000,000	$ 142,000,000
Defined Benefit Plan, Amortization of Gain (Loss)						7,000,000	6,000,000
Pension plans | MISSISSIPPI POWER CO
Defined Benefit Plan Disclosure [Line Items]
Benefit Payments, 2019						24,000,000
Benefit Payments, 2018						23,000,000
Employer contributions		$ 0				2,000,000	50,000,000
Defined benefit plan, AOCI, gain (loss), before tax				(151,000,000)		(155,000,000)	(151,000,000)
Projected benefit obligations				$ 534,000,000		$ 602,000,000	$ 534,000,000	$ 500,000,000
Annual salary increase on net periodic benefit costs						4.46%	4.46%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Benefit Payments, 2020						$ 26,000,000
Benefit Payments, 2021						27,000,000
Benefit Payments, 2022						28,000,000
Benefit Payments, 2023 to 2027						$ 164,000,000
Pension plans | MISSISSIPPI POWER CO | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				29.00%		24.00%	29.00%
Target plan asset allocations (as percent)						23.00%
Pension plans | MISSISSIPPI POWER CO | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax				$ 154,000,000		$ 158,000,000	$ 154,000,000	$ 144,000,000
Defined Benefit Plan, Amortization of Gain (Loss)						7,000,000	7,000,000
Pension plans | SOUTHERN POWER CO
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, AOCI, gain (loss), before tax						(32,000,000)
Total accumulated benefit obligation for the pension plans						(111,000,000)
Projected benefit obligations						$ 139,000,000
Actual plan asset allocations (as percent)						100.00%
Target plan asset allocations (as percent)						100.00%
Assets for Plan Benefits, Defined Benefit Plan						$ 138,000,000
Pension plans | SOUTHERN POWER CO | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)						24.00%
Target plan asset allocations (as percent)						23.00%
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Benefit Payments, 2019						$ 148,000,000
Benefit Payments, 2018						144,000,000
Employer contributions						84,000,000	65,000,000
Defined benefit plan, AOCI, gain (loss), before tax				(360,000,000)		(324,000,000)	(360,000,000)
Expected postretirement trust contributions						0
Projected benefit obligations				$ 2,297,000,000		$ 2,339,000,000	$ 2,297,000,000	$ 1,989,000,000
Annual salary increase on net periodic benefit costs						4.37%	4.37%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)				100.00%		100.00%	100.00%
Benefit Payments, 2020						$ 151,000,000
Benefit Payments, 2021						154,000,000
Benefit Payments, 2022						156,000,000
Benefit Payments, 2023 to 2027						$ 780,000,000
Other postretirement benefit plans | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				31.00%		29.00%	31.00%
Target plan asset allocations (as percent)				29.00%		30.00%	29.00%
Other postretirement benefit plans | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax				$ 378,000,000		$ 341,000,000	$ 378,000,000	$ 411,000,000
Defined benefit plan, AOCI, gain (loss), before tax				(353,000,000)		(320,000,000)	(353,000,000)
Defined Benefit Plan, Amortization of Gain (Loss)						13,000,000	14,000,000
Other postretirement benefit plans | GEORGIA POWER CO
Defined Benefit Plan Disclosure [Line Items]
Benefit Payments, 2019						55,000,000
Benefit Payments, 2018						55,000,000
Employer contributions						26,000,000	17,000,000
Defined benefit plan, AOCI, gain (loss), before tax				(207,000,000)		(197,000,000)	(207,000,000)
Projected benefit obligations				$ 847,000,000		$ 863,000,000	$ 847,000,000	$ 854,000,000
Annual salary increase on net periodic benefit costs						4.46%	4.46%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Benefit Payments, 2020						$ 56,000,000
Benefit Payments, 2021						57,000,000
Benefit Payments, 2022						58,000,000
Benefit Payments, 2023 to 2027						288,000,000
Other postretirement benefit plans | GEORGIA POWER CO | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax				$ 213,000,000		202,000,000	$ 213,000,000	$ 223,000,000
Defined Benefit Plan, Amortization of Gain (Loss)						8,000,000	9,000,000
Other postretirement benefit plans | SOUTHERN Co GAS
Defined Benefit Plan Disclosure [Line Items]
Benefit Payments, 2019						20,000,000
Benefit Payments, 2018						20,000,000
Employer contributions				11,000,000		17,000,000
Defined benefit plan, AOCI, gain (loss), before tax				61,000,000		44,000,000	61,000,000
Expected postretirement trust contributions						0
Projected benefit obligations				$ 308,000,000	338,000,000	$ 310,000,000	$ 308,000,000
Annual salary increase on net periodic benefit costs				3.50%		3.50%
Regulatory asset					77,000,000
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Defined benefit plan, benefit obligation, period increase (decrease)			20,000,000
Defined benefit plan, plan assets, period increase (decrease)			$ 1,000,000
Benefit Payments, 2020						$ 21,000,000
Benefit Payments, 2021						21,000,000
Benefit Payments, 2022						22,000,000
Benefit Payments, 2023 to 2027						105,000,000
Defined Benefit Plan, Amortization of Gain (Loss)				$ 0		$ (4,000,000)
Other postretirement benefit plans | SOUTHERN Co GAS | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				23.00%		20.00%	23.00%
Target plan asset allocations (as percent)						24.00%
Other postretirement benefit plans | SOUTHERN Co GAS | Other Types Of Investments
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				2.00%		2.00%	2.00%
Target plan asset allocations (as percent)						3.00%
Other postretirement benefit plans | SOUTHERN Co GAS | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				74.00%		76.00%	74.00%
Target plan asset allocations (as percent)						72.00%
Other postretirement benefit plans | SOUTHERN Co GAS | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				1.00%		2.00%	1.00%
Target plan asset allocations (as percent)						1.00%
Other postretirement benefit plans | SOUTHERN Co GAS | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax				$ 52,000,000	77,000,000	$ 46,000,000	$ 52,000,000
Defined benefit plan, AOCI, gain (loss), before tax				64,000,000		47,000,000	64,000,000
Defined Benefit Plan, Amortization of Gain (Loss)				3,000,000		4,000,000
Other postretirement benefit plans | SOUTHERN Co GAS | Predecessor
Defined Benefit Plan Disclosure [Line Items]
Employer contributions					10,000,000
Projected benefit obligations				318,000,000	$ 338,000,000		318,000,000
Annual salary increase on net periodic benefit costs					3.70%			3.70%
Defined Benefit Plan, Amortization of Gain (Loss)					$ (2,000,000)			$ (6,000,000)
Other postretirement benefit plans | SOUTHERN Co GAS | Predecessor | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax					30,000,000			30,000,000
Defined Benefit Plan, Amortization of Gain (Loss)					$ 1,000,000
Other postretirement benefit plans | ALABAMA POWER CO
Defined Benefit Plan Disclosure [Line Items]
Benefit Payments, 2019						32,000,000
Benefit Payments, 2018						31,000,000
Employer contributions						6,000,000	7,000,000
Projected benefit obligations				$ 501,000,000		$ 517,000,000	$ 501,000,000	$ 505,000,000
Annual salary increase on net periodic benefit costs						4.46%	4.46%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Benefit Payments, 2020						$ 33,000,000
Benefit Payments, 2021						34,000,000
Benefit Payments, 2022						35,000,000
Benefit Payments, 2023 to 2027						173,000,000
Other postretirement benefit plans | ALABAMA POWER CO | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax				$ 76,000,000		56,000,000	$ 76,000,000	$ 82,000,000
Defined benefit plan, AOCI, gain (loss), before tax				(61,000,000)		(45,000,000)	(61,000,000)
Defined Benefit Plan, Amortization of Gain (Loss)						1,000,000	2,000,000
Other postretirement benefit plans | GULF POWER CO
Defined Benefit Plan Disclosure [Line Items]
Benefit Payments, 2019						5,000,000
Benefit Payments, 2018						5,000,000
Employer contributions						4,000,000	3,000,000
Defined benefit plan, AOCI, gain (loss), before tax				7,000,000		6,000,000	7,000,000
Expected postretirement trust contributions						0
Projected benefit obligations				$ 83,000,000		$ 83,000,000	$ 83,000,000	$ 81,000,000
Annual salary increase on net periodic benefit costs						4.46%	4.46%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Benefit Payments, 2020						$ 5,000,000
Benefit Payments, 2021						6,000,000
Benefit Payments, 2022						6,000,000
Benefit Payments, 2023 to 2027						28,000,000
Other postretirement benefit plans | GULF POWER CO | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax				$ 7,000,000		6,000,000	$ 7,000,000	$ 5,000,000
Other postretirement benefit plans | MISSISSIPPI POWER CO
Defined Benefit Plan Disclosure [Line Items]
Benefit Payments, 2019						6,000,000
Benefit Payments, 2018						6,000,000
Employer contributions						4,000,000	4,000,000
Expected postretirement trust contributions						0
Projected benefit obligations				$ 97,000,000		$ 97,000,000	$ 97,000,000	$ 97,000,000
Annual salary increase on net periodic benefit costs						4.46%	4.46%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Benefit Payments, 2020						$ 6,000,000
Benefit Payments, 2021						7,000,000
Benefit Payments, 2022						7,000,000
Benefit Payments, 2023 to 2027						$ 34,000,000
Other postretirement benefit plans | MISSISSIPPI POWER CO | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				43.00%		38.00%	43.00%
Target plan asset allocations (as percent)						37.00%
Other postretirement benefit plans | MISSISSIPPI POWER CO | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax				$ 19,000,000		$ 17,000,000	$ 19,000,000	$ 18,000,000
Defined Benefit Plan, Amortization of Gain (Loss)						1,000,000	$ 1,000,000
Other postretirement benefit plans | SOUTHERN POWER CO
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax						3,000,000
Benefit Payments, 2019						4,000,000
Benefit Payments, 2018						4,000,000
Defined benefit plan, AOCI, gain (loss), before tax						(3,000,000)
Total accumulated benefit obligation for the pension plans						(11,000,000)
Projected benefit obligations						11,000,000
Benefit Payments, 2020						4,000,000
Benefit Payments, 2021						4,000,000
Benefit Payments, 2022						4,000,000
Benefit Payments, 2023 to 2027						27,000,000
Subsequent Event
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of match	6.00%
Defined contribution, maximum match of base salary	5.10%
Qualified Plan | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions						0
Expected postretirement trust contributions						0
Qualified Plan | Pension plans | SOUTHERN Co GAS
Defined Benefit Plan Disclosure [Line Items]
Employer contributions						0
Expected postretirement trust contributions						$ 0