Retirement Benefits - Textual (Details) - USD ($)			3 Months Ended	6 Months Ended		12 Months Ended
	Dec. 19, 2016	Sep. 12, 2016	Sep. 30, 2016	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Aug. 29, 2016
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		$ 11,300,000,000	$ 9,600,000,000
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%
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 900,000,000					$ 1,076,000,000	$ 45,000,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax				(3,165,000,000)		(3,165,000,000)	(3,093,000,000)
Expected postretirement trust contributions						0
Projected benefit obligations				12,385,000,000		$ 12,385,000,000	$ 10,542,000,000	$ 10,909,000,000
Annual salary increase on net periodic benefit costs						4.37%	3.59%	3.59%
Actual plan asset allocations (as percent)							100.00%
Target plan asset allocations (as percent)							100.00%
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions						$ 65,000,000	$ 39,000,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax				(360,000,000)		(360,000,000)	(387,000,000)
Expected postretirement trust contributions						0
Projected benefit obligations				$ 2,297,000,000		$ 2,297,000,000	$ 1,989,000,000	$ 1,986,000,000
Annual salary increase on net periodic benefit costs						4.37%	3.59%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%	100.00%
Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations				$ 11,800,000,000		$ 11,800,000,000
Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations				$ 627,000,000		$ 627,000,000
Fixed income | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				29.00%		29.00%	23.00%
Target plan asset allocations (as percent)						23.00%	23.00%
Fixed income | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				31.00%		31.00%	30.00%
Target plan asset allocations (as percent)						29.00%	28.00%
Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Cost Recognized						$ 105,000,000	$ 92,000,000	$ 87,000,000
Georgia Power [Member]
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 [Member] | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	287,000,000					$ 301,000,000	14,000,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax				$ (1,112,000,000)		(1,112,000,000)	(1,068,000,000)
Expected postretirement trust contributions						0
Total accumulated benefit obligation for the pension plans				3,500,000,000		3,500,000,000	3,300,000,000
Projected benefit obligations				$ 3,800,000,000		$ 3,800,000,000	$ 3,615,000,000	$ 3,781,000,000
Annual salary increase on net periodic benefit costs						4.46%	3.59%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Georgia Power [Member] | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions						$ 17,000,000	$ 10,000,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax				$ (207,000,000)		(207,000,000)	(215,000,000)
Projected benefit obligations				$ 847,000,000		$ 847,000,000	$ 854,000,000	$ 864,000,000
Annual salary increase on net periodic benefit costs						4.46%	3.59%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Georgia Power [Member] | Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations				$ 3,600,000,000		$ 3,600,000,000
Georgia Power [Member] | Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations				$ 152,000,000		$ 152,000,000
Georgia Power [Member] | Fixed income | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				29.00%		29.00%	23.00%
Target plan asset allocations (as percent)						23.00%
Georgia Power [Member] | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						85.00%
Georgia Power [Member] | Maximum [Member] | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Maximum limit of contribution of employees base salary						6.00%
Georgia Power [Member] | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Cost Recognized						$ 27,000,000	$ 26,000,000	$ 25,000,000
Southern Company Gas [Member]
Defined Benefit Plan Disclosure [Line Items]
Annual increase or decrease in assumed medical care cost trend rate						1.00%
Southern Company Gas [Member] | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Regulatory asset					$ 369,000,000
Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease)			$ 177,000,000
Defined Benefit Plan, Fair Value of Plan Assets, Period Increase (Decrease)			(10,000,000)
Southern Company Gas [Member] | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Regulatory asset					77,000,000
Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease)			20,000,000
Defined Benefit Plan, Fair Value of Plan Assets, Period Increase (Decrease)			1,000,000
Southern Company Gas [Member] | Employee Savings Plan Option One Contribution Percentage
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						65.00%
Southern Company Gas [Member] | Employee Savings Plan First Matching Contribution Percentage
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						100.00%
Southern Company Gas [Member] | Employee Saving Plan
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						75.00%
Southern Company Gas [Member] | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans				$ 1,100,000,000		$ 1,100,000,000
Southern Company Gas [Member] | Successor [Member] | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		$ 125,000,000		129,000,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax				(226,000,000)		(226,000,000)
Expected postretirement trust contributions						0
Projected benefit obligations				$ 1,133,000,000	1,244,000,000	1,133,000,000
Annual salary increase on net periodic benefit costs				3.50%
Southern Company Gas [Member] | Successor [Member] | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions				$ 11,000,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax				(61,000,000)		(61,000,000)
Expected postretirement trust contributions						0
Projected benefit obligations				$ 308,000,000	338,000,000	308,000,000
Annual salary increase on net periodic benefit costs				3.50%
Southern Company Gas [Member] | Successor [Member] | Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations				$ 1,100,000,000		1,100,000,000
Southern Company Gas [Member] | Successor [Member] | Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations				$ 39,000,000		$ 39,000,000
Southern Company Gas [Member] | Successor [Member] | Fixed income | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				23.00%		23.00%
Target plan asset allocations (as percent)						24.00%
Southern Company Gas [Member] | Successor [Member] | Other Types Of Investments [Member] | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				2.00%		2.00%
Target plan asset allocations (as percent)						3.00%
Southern Company Gas [Member] | Successor [Member] | Equity Securities | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				74.00%		74.00%
Target plan asset allocations (as percent)						72.00%
Southern Company Gas [Member] | Successor [Member] | Cash and Cash Equivalents [Member] | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				1.00%		1.00%
Target plan asset allocations (as percent)						1.00%
Southern Company Gas [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Total accumulated benefit obligation for the pension plans							1,000,000,000
Southern Company Gas [Member] | Predecessor [Member] | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions					1,000,000		2,000,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax							(374,000,000)
Projected benefit obligations					$ 1,244,000,000		$ 1,067,000,000	$ 1,098,000,000
Annual salary increase on net periodic benefit costs					3.70%		3.70%	3.70%
Actual plan asset allocations (as percent)							100.00%
Southern Company Gas [Member] | Predecessor [Member] | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions					$ 10,000,000		$ 17,000,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax							(81,000,000)
Projected benefit obligations					$ 338,000,000		$ 318,000,000	$ 334,000,000
Annual salary increase on net periodic benefit costs					3.70%		3.70%	3.70%
Actual plan asset allocations (as percent)							100.00%
Southern Company Gas [Member] | Predecessor [Member] | Fixed income | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)							29.00%
Southern Company Gas [Member] | Predecessor [Member] | Fixed income | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)							24.00%
Southern Company Gas [Member] | Predecessor [Member] | Other Types Of Investments [Member] | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)							12.00%
Southern Company Gas [Member] | Predecessor [Member] | Other Types Of Investments [Member] | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)							0.00%
Southern Company Gas [Member] | Predecessor [Member] | Equity Securities | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)							59.00%
Southern Company Gas [Member] | Predecessor [Member] | Equity Securities | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)							75.00%
Southern Company Gas [Member] | Predecessor [Member] | Cash and Cash Equivalents [Member] | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)							0.00%
Southern Company Gas [Member] | Predecessor [Member] | Cash and Cash Equivalents [Member] | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)							1.00%
Southern Company Gas [Member] | Pension and Other Postretirement Plans Costs [Member] | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory asset			$ 437,000,000
Southern Company Gas [Member] | Maximum [Member] | Employee Savings Plan Option One Contribution Percentage
Defined Benefit Plan Disclosure [Line Items]
Maximum limit of contribution of employees base salary						8.00%
Southern Company Gas [Member] | Maximum [Member] | Employee Savings Plan First Matching Contribution Percentage
Defined Benefit Plan Disclosure [Line Items]
Maximum limit of contribution of employees base salary						3.00%
Southern Company Gas [Member] | Maximum [Member] | Employee Saving Plan
Defined Benefit Plan Disclosure [Line Items]
Maximum limit of contribution of employees base salary						3.00%
Southern Company Gas [Member] | Employee Savings Plan | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Cost Recognized						$ 8,000,000
Southern Company Gas [Member] | Employee Savings Plan | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Cost Recognized					$ 10,000,000		$ 16,000,000	$ 14,000,000
Southern Company Gas [Member] | AGL Resources Inc. Retirement Plan | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Cost Recognized					$ 2,000,000		2,000,000	1,000,000
Alabama Power [Member]
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		$ 2,400,000,000	2,300,000,000
Period over which company has elected to amortize changes in the market value of all plan assets						5 years
Alabama Power [Member] | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	129,000,000					$ 141,000,000	12,000,000
Expected postretirement trust contributions						0
Projected benefit obligations				$ 2,663,000,000		$ 2,663,000,000	$ 2,506,000,000	$ 2,592,000,000
Annual salary increase on net periodic benefit costs						4.46%	3.59%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Alabama Power [Member] | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions						$ 7,000,000	$ 1,000,000
Projected benefit obligations				$ 501,000,000		$ 501,000,000	$ 505,000,000	$ 503,000,000
Annual salary increase on net periodic benefit costs						4.46%	3.59%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Alabama Power [Member] | Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations				$ 2,500,000,000		$ 2,500,000,000
Alabama Power [Member] | Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations				$ 124,000,000		$ 124,000,000
Alabama Power [Member] | Fixed income | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				29.00%		29.00%	23.00%
Target plan asset allocations (as percent)						23.00%
Alabama Power [Member] | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						85.00%
Maximum limit of contribution of employees base salary						6.00%
Alabama Power [Member] | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Cost Recognized						$ 23,000,000	$ 22,000,000	$ 21,000,000
Gulf Power [Member]
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 [Member] | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	48,000,000					$ 49,000,000	1,000,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax				$ (150,000,000)		(150,000,000)	(140,000,000)
Total accumulated benefit obligation for the pension plans				460,000,000		460,000,000	424,000,000
Projected benefit obligations				$ 517,000,000		$ 517,000,000	$ 480,000,000	$ 491,000,000
Annual salary increase on net periodic benefit costs						4.46%	3.59%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Gulf Power [Member] | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions						$ 3,000,000	$ 3,000,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax				$ 7,000,000		7,000,000	5,000,000
Expected postretirement trust contributions						0
Projected benefit obligations				$ 83,000,000		$ 83,000,000	$ 81,000,000	$ 78,000,000
Annual salary increase on net periodic benefit costs						4.46%	3.59%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Gulf Power [Member] | Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Expected postretirement trust contributions						$ 0
Projected benefit obligations				$ 494,000,000		494,000,000
Gulf Power [Member] | Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations				$ 23,000,000		$ 23,000,000
Gulf Power [Member] | Fixed income | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				29.00%		29.00%	23.00%
Target plan asset allocations (as percent)						23.00%
Gulf Power [Member] | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						85.00%
Maximum limit of contribution of employees base salary						6.00%
Gulf Power [Member] | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Cost Recognized						$ 5,000,000	$ 4,000,000	$ 4,000,000
Mississippi Power [Member]
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		$ 479,000,000	447,000,000
Period over which company has elected to amortize changes in the market value of all plan assets						5 years
Mississippi Power [Member] | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 47,000,000					$ 50,000,000	2,000,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax				(151,000,000)		(151,000,000)	(142,000,000)
Projected benefit obligations				$ 534,000,000		$ 534,000,000	$ 500,000,000	$ 513,000,000
Annual salary increase on net periodic benefit costs						4.46%	3.59%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Mississippi Power [Member] | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions						$ 4,000,000	$ 3,000,000
Expected postretirement trust contributions						0
Projected benefit obligations				$ 97,000,000		$ 97,000,000	$ 97,000,000	$ 96,000,000
Annual salary increase on net periodic benefit costs						4.46%	3.59%	3.59%
Actual plan asset allocations (as percent)				100.00%		100.00%	100.00%
Target plan asset allocations (as percent)						100.00%
Mississippi Power [Member] | Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Expected postretirement trust contributions						$ 0
Projected benefit obligations				$ 504,000,000		504,000,000
Mississippi Power [Member] | Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations				$ 30,000,000		$ 30,000,000
Mississippi Power [Member] | Fixed income | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				29.00%		29.00%	23.00%
Target plan asset allocations (as percent)						23.00%
Mississippi Power [Member] | Fixed income | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				43.00%		43.00%	38.00%
Target plan asset allocations (as percent)						38.00%
Mississippi Power [Member] | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Matching limit of contribution by employer						85.00%
Maximum limit of contribution of employees base salary						6.00%
Mississippi Power [Member] | Employee Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Cost Recognized						$ 5,000,000	$ 5,000,000	$ 5,000,000
AGL Resources Inc. Retirement Plan | Southern Company Gas [Member] | Successor [Member] | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				20.00%		20.00%
Target plan asset allocations (as percent)						15.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas [Member] | Successor [Member] | Other Types Of Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				10.00%		10.00%
Target plan asset allocations (as percent)						30.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas [Member] | Successor [Member] | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				69.00%		69.00%
Target plan asset allocations (as percent)						53.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas [Member] | Successor [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)				1.00%		1.00%
Target plan asset allocations (as percent)						2.00%	10.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas [Member] | Predecessor [Member] | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)							28.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas [Member] | Predecessor [Member] | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations (as percent)							72.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas [Member] | Minimum [Member] | Successor [Member] | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as percent)							5.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas [Member] | Minimum [Member] | Successor [Member] | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as percent)							70.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas [Member] | Maximum [Member] | Successor [Member] | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as percent)							20.00%
AGL Resources Inc. Retirement Plan | Southern Company Gas [Member] | Maximum [Member] | Successor [Member] | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations (as percent)							95.00%