Contingencies and Regulatory Matters - Narrative (Details)													1 Months Ended								3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended					24 Months Ended	29 Months Ended	31 Months Ended
	Nov. 02, 2015 USD ($)	Oct. 13, 2015 MW	Aug. 24, 2015 USD ($) MW	Jun. 03, 2015 USD ($)	Apr. 16, 2015 USD ($) MW	Apr. 15, 2015 USD ($)	Apr. 01, 2015 USD ($)	Mar. 19, 2015 USD ($)	Mar. 10, 2015 USD ($)	Feb. 25, 2015 USD ($)	Jan. 01, 2014 USD ($)	Mar. 19, 2013	Sep. 30, 2015 USD ($) lawsuit mi	Jun. 30, 2015 USD ($)	Apr. 30, 2015 MW	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Mar. 31, 2013 USD ($)	Feb. 28, 2013 USD ($)	Jan. 31, 2013 USD ($)	Sep. 30, 2015 USD ($) lawsuit mi	Sep. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Sep. 30, 2015 USD ($) clause lawsuit mi MW	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2010	Dec. 31, 2008 USD ($)	Dec. 31, 2014 USD ($)	Jul. 20, 2015 USD ($)	Sep. 30, 2015 USD ($) lawsuit mi	Jan. 01, 2016 USD ($)	Oct. 27, 2015 USD ($)	Oct. 06, 2015 USD ($)	Sep. 18, 2015 USD ($)	Sep. 01, 2015 USD ($)	Aug. 31, 2015 USD ($)	Aug. 13, 2015 USD ($)	Jul. 10, 2015 USD ($)	May. 20, 2015 USD ($)	Oct. 06, 2014
Loss Contingencies [Line Items]
Revenues																					$ 5,401,000,000	$ 5,339,000,000		$ 13,921,000,000	$ 14,450,000,000
Return Of Interest Bearing Refundable Deposits Related to Assets Sale Plus Accrued Interest				$ 301,000,000
Unrecognized Tax Benefits													$ 657,000,000			$ 170,000,000					657,000,000			657,000,000		$ 170,000,000					$ 170,000,000		$ 657,000,000
Kemper IGCC [Member]
Loss Contingencies [Line Items]
Pre-Tax Charge To Income																					150,000,000	418,000,000		182,000,000	798,000,000	868,000,000	$ 1,200,000,000
After Tax Charge To Income																					93,000,000	258,000,000		112,000,000	493,000,000	536,000,000	729,000,000
Unrecognized Tax Benefits													$ 414,000,000								$ 414,000,000			$ 414,000,000									$ 414,000,000
AGL Resources [Member]
Loss Contingencies [Line Items]
Number of Class Action Lawsuits | lawsuit													4								4			4									4
Parent Company and Merger Sub [Member]
Loss Contingencies [Line Items]
Number of Class Action Lawsuits | lawsuit													2								2			2									2
Alabama Power [Member]
Loss Contingencies [Line Items]
Civil Penalties Payment Related to NSR			$ 100,000
Civil Penalties Required Investment In Electric Transportation Infrastructure Projects			$ 1,500,000
Period to Invest in Electric Transportation Infrastructure Projects			3 years
Claims Awarded to Companies Related to Nuclear Fuel Disposal Litigation																26,000,000
Non-environmental Costs																								$ 50,000,000
Recovery Of Non-environmental Costs																								50,000,000
Electric Generating Units, Capacity (in MW's) | MW															12,200
Revenues																					$ 1,695,000,000	1,669,000,000		4,551,000,000	4,614,000,000
Other Regulatory Assets Deferred													$ 1,055,000,000			1,063,000,000					1,055,000,000			1,055,000,000		1,063,000,000					1,063,000,000		$ 1,055,000,000
Projected Balance Of Regulatory Assets													118,000,000			84,000,000					118,000,000			$ 118,000,000		84,000,000					84,000,000		118,000,000
Alabama Power [Member] | Plant Gorgas Units 6 and 7 [Member]
Loss Contingencies [Line Items]
Capacity Of Units Included In Request For Decertification Of Units (in MWs) | MW															200
Alabama Power [Member] | Plant Barry Units 1 And 2 [Member]
Loss Contingencies [Line Items]
Capacity Of Units Included In Request For Decertification Of Units (in MWs) | MW															250
Alabama Power [Member] | Plant Greene County Units 1 And 2 [Member]
Loss Contingencies [Line Items]
Capacity Of Units Included In Request For Decertification Of Units (in MWs) | MW																								300
Alabama Power [Member] | Plant Barry Unit 3 [Member]
Loss Contingencies [Line Items]
Capacity Of Units Included In Request For Decertification Of Units (in MWs) | MW			225
Georgia Power [Member]
Loss Contingencies [Line Items]
Environmental Exit Costs, Assets Previously Disposed, Liability for Remediation													29,000,000								29,000,000			$ 29,000,000									29,000,000
Civil penalties per day violation rate													$ 37,500								$ 37,500			$ 37,500									$ 37,500
Number of times of punitive damages in comparison to cost incurred by Environmental Protection Agency													3								3			3									3
Claims Awarded to Companies Related to Nuclear Fuel Disposal Litigation								$ 18,000,000
Approved Increase (Decrease) in Annual Billing Based on Fuel Cost Recovery Rate																																					$ (268,000,000)
Additional Construction Capital Costs																							$ 148,000,000								3,000,000,000
Percentage of Proportionate Share Owed in Consortium Agreement																														45.70%
Increase (Decrease) In Projected Certified Construction Capital Costs																								5.00%
Delay Of Estimated In-service Date						18 months
Revenues																					$ 2,691,000,000	2,631,000,000		$ 6,685,000,000	7,086,000,000
Other Regulatory Assets Deferred													$ 2,075,000,000			1,753,000,000					2,075,000,000			2,075,000,000		1,753,000,000					1,753,000,000		$ 2,075,000,000
Projected Balance Of Regulatory Assets													124,000,000			136,000,000					124,000,000			124,000,000		136,000,000					136,000,000		124,000,000
Georgia Power [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Additional Construction Capital Costs																														$ 114,000,000
Georgia Power [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Estimated In-service Capital Cost						$ 4,400,000,000
Georgia Power [Member] | Minimum [Member] | Plant Vogtle Units 3 And 4 [Member] | Pending Litigation [Member]
Loss Contingencies [Line Items]
Estimated Adjustment to Contract Price Related to Issues that May Impact Project Budget and Schedule									$ 591,000,000					$ 714,000,000										118,000,000		113,000,000				$ 425,000,000
Georgia Power [Member] | Other deferred charges and assets [Member]
Loss Contingencies [Line Items]
Fuel cost recovery balance													41,000,000			199,000,000					41,000,000			41,000,000		199,000,000					199,000,000		41,000,000
Georgia Power [Member] | Subsequent Event [Member]
Loss Contingencies [Line Items]
Increase in Tariff Rate Three																																		$ 49,000,000
Estimated Increase In ECCR Tariff Three																																		75,000,000
Estimated Increase In Demand Side Management Tariffs Two																																		7,000,000
Estimated Increase In Municipal Franchise Fee Tariff Three																																		13,000,000
Increase In NCCR Tariff																																		$ 19,000,000
Georgia Power [Member] | Subsequent Event [Member] | Plant Vogtle Units 3 And 4 [Member] | Pending Litigation [Member]
Loss Contingencies [Line Items]
Project Settlement Cost To Be Capitalized																																			$ 350,000,000
Project Settlement Cost To Be Capitalized, Amount Paid To Date																																			$ 120,000,000
Georgia Power [Member] | Plant Branch Units 1 and 3 and 4 [Member]
Loss Contingencies [Line Items]
Capacity Of Units Included In Request For Decertification Of Units (in MWs) | MW					1,266
Georgia Power [Member] | Plant Yates Units 1 through 5 [Member]
Loss Contingencies [Line Items]
Capacity Of Units Included In Request For Decertification Of Units (in MWs) | MW					579
Georgia Power [Member] | Plant McManus Units 1 and 2 [Member]
Loss Contingencies [Line Items]
Capacity Of Units Included In Request For Decertification Of Units (in MWs) | MW					122
Georgia Power [Member] | Plant Mitchell Unit 3 [Member]
Loss Contingencies [Line Items]
Electric Generating Units, Capacity (in MW's) | MW					155
Georgia Power [Member] | Plant Kraft Units One Through Four [Member] | Subsequent Event [Member]
Loss Contingencies [Line Items]
Capacity Of Units Included In Request For Decertification Of Units (in MWs) | MW		316
Georgia Power [Member] | Plant Vogtle Units 3 And 4 [Member]
Loss Contingencies [Line Items]
Estimated In-service Capital Cost						5,000,000,000
Monthly Operational Readiness Costs						$ 10,000,000
Monthly Financing Costs																								30,000,000
Construction Financing Costs																								2,400,000,000
Southern Company [Member]
Loss Contingencies [Line Items]
Line Of Credit Expire Year Five													1,000,000,000.0								1,000,000,000.0			1,000,000,000.0									1,000,000,000.0						$ 1,000,000,000.0
Revenues																					5,401,000,000	5,339,000,000		13,921,000,000	14,450,000,000
Other Regulatory Assets Deferred													4,733,000,000			4,334,000,000					4,733,000,000			4,733,000,000		4,334,000,000					4,334,000,000		4,733,000,000
Projected Balance Of Regulatory Assets													421,000,000			346,000,000					421,000,000			421,000,000		346,000,000					346,000,000		421,000,000
Interest Bearing Refundable Deposit Related to Assets Sale													$ 0			275,000,000					0			0		275,000,000					275,000,000		0
Bank Loans Period Of Extension													18 months
Gulf Power [Member]
Loss Contingencies [Line Items]
Environmental Exit Costs, Assets Previously Disposed, Liability for Remediation													$ 46,000,000								46,000,000			46,000,000									46,000,000
Reduction In Depreciation Expense																	$ 62,500,000
Reduction In Depreciation Expense Year One																								$ 20,500,000		8,400,000
Number Of Regulatory Clauses | clause																								4
Ownership percentage in scrubber project																												50.00%
Revenues																					429,000,000	438,000,000		$ 1,170,000,000	1,229,000,000
Other Regulatory Assets Deferred													430,000,000			416,000,000					430,000,000			430,000,000		416,000,000					416,000,000		430,000,000
Projected Balance Of Regulatory Assets													81,000,000			74,000,000					81,000,000			81,000,000		74,000,000					74,000,000		81,000,000
Gulf Power [Member] | Subsequent Event [Member]
Loss Contingencies [Line Items]
Annual Retail Revenues	$ 49,000,000
Mississippi Power [Member]
Loss Contingencies [Line Items]
Environmental Exit Costs, Assets Previously Disposed, Liability for Remediation													300,000								300,000			300,000									300,000
AFUDC Cost							$ 13,000,000
Over Recovered Fuel Cost													44,000,000								44,000,000			44,000,000									44,000,000
PSC Retail Rate Increase (Decrease)																																						$ (2,000,000)
Ownership percentage in scrubber project																												50.00%
Estimated cost of scrubber project																												$ 330,000,000
Scrubber project expenditures amount													$ 320,000,000								320,000,000			320,000,000									$ 320,000,000
Allowance For Funds Used During Construction Cost																								32,000,000
Revenues										$ 8,000,000											$ 341,000,000	$ 355,000,000		$ 893,000,000	997,000,000
Under recovered regulatory clause revenues																2,000,000										2,000,000					2,000,000
Percentage Of PSC Retail Rate Increase (Decrease)																																						(0.35%)
Plant Capacity Under Coal Gasification Combined Cycle Technology (in MWs) | MW																								582
Co Two Pipeline Infrastructure (in miles) | mi													61								61			61									61
Costs associated with CCP12 grant funds																												$ 245,000,000
Other Property And Investments													$ 2,000,000								$ 2,000,000			$ 2,000,000									$ 2,000,000
Lignite Mining Costs													62,000,000								62,000,000			62,000,000									62,000,000
Materials, Supplies, and Other													43,000,000								43,000,000			43,000,000									43,000,000
Cost deferred in other regulatory assets													50,000,000								50,000,000			50,000,000									50,000,000
Other Regulatory Assets Deferred													460,000,000			385,000,000					460,000,000			460,000,000		385,000,000					385,000,000		460,000,000
Other deferred charges and assets													15,000,000								15,000,000			15,000,000									15,000,000
Increase Retail Rates In Year One												15.00%
Increase Retail Rates In Year Two											3.00%
Settlement Agreement Collection Amount To Mitigate Rate Impact Year Two											$ 156,000,000
Retail Rate Recovery																																$ 342,000,000
Carrying Costs Associated With Retail Rate Recovery																																	27,000,000
Projected Balance Of Regulatory Assets													$ 119,000,000			73,000,000					$ 119,000,000			$ 119,000,000		73,000,000					73,000,000		$ 119,000,000
Percentage of Carbon Dioxide Captured from Project													70.00%								70.00%			70.00%									70.00%
Percentage of Contract to Purchase Carbon Dioxide from Project													30.00%								30.00%			30.00%									30.00%
Reduced percentage interest transferred under asset purchase agreement																												15.00%
Interest Bearing Refundable Deposit Related to Assets Sale													$ 0			275,000,000					$ 0			$ 0		275,000,000					275,000,000		$ 0									$ 275,000,000
Bank Loans Period Of Extension				18 months										18 months
Issuance Of Promissory Note From Parent In Exchange For Repayment Of Interest-bearing Refundable Deposit And Accrued Interest				$ 301,000,000										$ 301,000,000										301,000,000	$ 0
Internal Revenue Code Section 48A tax credits (Phase II)																								$ 279,000,000
Minimum percentage of carbon dioxide that must be capture and sequester to remain eligible for the phase II tax credits																								65.00%
Tax credit carryforward													276,000,000								276,000,000			$ 276,000,000									276,000,000
Tax credit carryforward, utilized													235,000,000								235,000,000			235,000,000									235,000,000
Unrecognized Tax Benefits													648,000,000			165,000,000					648,000,000			648,000,000		165,000,000					165,000,000		648,000,000
Mississippi Power [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Costs Due To Extension Of In-service Date																								30,000,000
Mississippi Power [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Costs Due To Extension Of In-service Date																								25,000,000
Mississippi Power [Member] | Construction in Progress [Member]
Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss													2,230,000,000								2,230,000,000			2,230,000,000									2,230,000,000
Mississippi Power [Member] | Subsequent Event [Member]
Loss Contingencies [Line Items]
Psc Approved Annual Property Damage Reserve Accrual																																				$ 3,000,000
Mississippi Power [Member] | MRA Revenue [Member]
Loss Contingencies [Line Items]
Over Recovered Fuel Cost													14,000,000			$ 200,000					$ 14,000,000			$ 14,000,000		200,000					$ 200,000		14,000,000
Mississippi Power [Member] | Kemper IGCC [Member]
Loss Contingencies [Line Items]
Period of Amortization of Regulatory Assets																					24 months			9 months
AFUDC Cost							$ 10,000,000
Estimated Cost																		$ 2,880,000,000		$ 2,880,000,000				$ 4,660,000,000				$ 2,880,000,000
Other Regulatory Assets Deferred													$ 158,000,000								$ 158,000,000			158,000,000									$ 158,000,000
Pre-Tax Charge To Income																					150,000,000			182,000,000		868,000,000	1,100,000,000	78,000,000
After Tax Charge To Income																					93,000,000			112,000,000		$ 536,000,000	$ 681,000,000	48,000,000
Monthly Charge Of Allowance For Equity Funds Used During Construction																								$ 12,000,000
Monthly Cost Regulatory Assets Deferred																					$ 6,000,000
Purchase of Interest													100.00%								100.00%			100.00%									100.00%										15.00%
Maximum Cap Construction Cost																								$ 5,800,000,000
Regulatory Assets													$ 117,000,000								$ 117,000,000			117,000,000									$ 117,000,000
Projected Balance Of Regulatory Assets													91,000,000								91,000,000			91,000,000									91,000,000
Unrecognized Tax Benefits													414,000,000								414,000,000			414,000,000									414,000,000
Mississippi Power [Member] | Kemper IGCC [Member] | Current Estimate [Member]
Loss Contingencies [Line Items]
Estimated Cost																								5,110,000,000
Maximum Cap Construction Cost																								6,430,000,000
Mississippi Power [Member] | Kemper IGCC [Member] | In-Service Asset Proposal [Member]
Loss Contingencies [Line Items]
PSC Retail Rate Increase (Decrease)													28,000,000								28,000,000			28,000,000									28,000,000							$ 159,000,000	$ 159,000,000
Mississippi Power [Member] | Kemper IGCC [Member] | Property, Plant and Equipment [Member]
Loss Contingencies [Line Items]
Estimated Cost																								3,450,000,000
Mississippi Power [Member] | Plant Watson Units 4 And 5 [Member]
Loss Contingencies [Line Items]
Net Book Value Of Units Included In Request For Decertification Of Units					$ 32,000,000
Mississippi Power [Member] | Electricity Generation Plant, Non-Nuclear [Member]
Loss Contingencies [Line Items]
Estimated Cost																				$ 2,400,000,000								2,400,000,000
Alternate Financing																			$ 1,000,000,000
Mississippi Power [Member] | Mine [Member]
Loss Contingencies [Line Items]
Term of Management Fee Contract																													40 years
Gulf Power and Mississippi Power [Member]
Loss Contingencies [Line Items]
Estimated cost of scrubber project																												$ 660,000,000
Scrubber project expenditures amount													$ 626,000,000								$ 626,000,000			$ 626,000,000									$ 626,000,000