Contingencies and Regulatory Matters - Narrative (Details)																					1 Months Ended						3 Months Ended		4 Months Ended	6 Months Ended	12 Months Ended												35 Months Ended	48 Months Ended	57 Months Ended	63 Months Ended	90 Months Ended
	Jul. 01, 2017 USD ($)	Apr. 04, 2017 intervenor	Mar. 31, 2017 USD ($) mi MW	Mar. 30, 2017	Mar. 27, 2017	Mar. 10, 2017 USD ($)	Feb. 21, 2017 USD ($)	Dec. 20, 2016 USD ($)	Nov. 17, 2016 USD ($)	Jun. 09, 2016 USD ($)	Jun. 03, 2016	May 01, 2016 USD ($)	Mar. 31, 2016 USD ($)	Jan. 01, 2016	Dec. 05, 2015 USD ($)	Dec. 03, 2015	Jul. 07, 2015 USD ($)	Jun. 03, 2015 USD ($)	Jan. 01, 2014 USD ($)	Mar. 19, 2013	Jun. 30, 2019 USD ($)	Apr. 28, 2017 USD ($)	Sep. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Jan. 31, 2016 USD ($)	Jan. 31, 2013 USD ($)	Mar. 31, 2017 USD ($) clause mi MW	Mar. 31, 2016 USD ($)	Apr. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018	Aug. 31, 2018	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($) mi MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2010	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Mar. 31, 2017 USD ($) mi MW	Dec. 31, 2020 USD ($)	Sep. 30, 2016 USD ($)	Mar. 31, 2017 USD ($) mi MW	Jun. 30, 2016 USD ($)	Jan. 01, 2121	Apr. 11, 2017 USD ($)	Apr. 07, 2017 USD ($)	Mar. 17, 2017 USD ($)	Nov. 30, 2016 USD ($)	Aug. 31, 2016 USD ($)	May 20, 2015 USD ($)
Loss Contingencies [Line Items]
Lawsuit deferral period					30 days
Maximum borrowing capacity			$ 8,443,000,000																								$ 8,443,000,000							$ 8,443,000,000									$ 8,443,000,000			$ 8,443,000,000
Stockholders' equity, including portion attributable to noncontrolling interest			(26,996,000,000)										$ (22,184,000,000)											$ (22,184,000,000)			(26,996,000,000)	$ (22,184,000,000)		$ (26,612,000,000)				(26,996,000,000)	$ (26,612,000,000)	$ (21,982,000,000)							(26,996,000,000)			(26,996,000,000)
Construction work in progress			9,465,000,000																								9,465,000,000			8,977,000,000				9,465,000,000	8,977,000,000								9,465,000,000			9,465,000,000
Property, plant and equipment, gross			99,774,000,000																								99,774,000,000			98,416,000,000				99,774,000,000	98,416,000,000								99,774,000,000			99,774,000,000
Allowance for equity funds used during construction																											57,000,000	53,000,000
Other regulatory assets, current			560,000,000																								560,000,000			581,000,000				560,000,000	581,000,000								560,000,000			560,000,000
Georgia Power [Member]
Loss Contingencies [Line Items]
Environmental remediation liability			13,000,000																								13,000,000							13,000,000									13,000,000			13,000,000
Cost recovery, new nuclear			50,000,000																								$ 50,000,000							50,000,000									50,000,000			50,000,000
Liquidated damages, percentage																											40.00%															10.00%
Additional construction capital costs								$ 3,300,000,000																						222,000,000																	$ 3,700,000,000
Percentage of proportionate share owed in consortium agreement																																										45.70%
Maximum borrowing capacity			1,750,000,000																								$ 1,750,000,000							1,750,000,000									1,750,000,000			1,750,000,000
Period of notice required for contract termination				5 days
Aggregate liability under contract agreement and removal of liens			215,000,000																								215,000,000							215,000,000									215,000,000			215,000,000
Estimated in-service capital cost																																									$ 4,418,000,000
Estimate of possible loss								$ 240,000,000
Retail rate of return on common equity																																						10.95%
Public utilities, approved return on equity percentage														10.00%
Construction work in progress			5,183,000,000																								5,183,000,000			4,939,000,000				5,183,000,000	4,939,000,000								5,183,000,000			5,183,000,000
Wholesale revenues, non-affiliates																											39,000,000	41,000,000
Property, plant and equipment, gross			34,059,000,000																								34,059,000,000			33,841,000,000				34,059,000,000	33,841,000,000								34,059,000,000			34,059,000,000
Allowance for equity funds used during construction																											13,000,000	14,000,000
Other regulatory assets, current			195,000,000																								195,000,000			193,000,000				195,000,000	193,000,000								195,000,000			195,000,000
Georgia Power [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Additional construction capital costs																																										$ 114,000,000
Georgia Power [Member] | Other Current Liabilities [Member]
Loss Contingencies [Line Items]
Over recovered fuel balance			18,000,000																								18,000,000							18,000,000									18,000,000			18,000,000
Georgia Power [Member] | Other deferred credits and liabilities [Member]
Loss Contingencies [Line Items]
Over recovered fuel balance																														84,000,000					84,000,000
Gulf Power [Member]
Loss Contingencies [Line Items]
Environmental remediation liability			52,000,000																								52,000,000							52,000,000									52,000,000			52,000,000
Maximum borrowing capacity			$ 280,000,000																								$ 280,000,000							$ 280,000,000									$ 280,000,000			$ 280,000,000
Retail rate of return on common equity			10.25%																								10.25%							10.25%									10.25%			10.25%
Construction work in progress			$ 67,000,000																								$ 67,000,000			51,000,000				$ 67,000,000	51,000,000								$ 67,000,000			$ 67,000,000
Reduction in depreciation expense																											25,500,000.0								0	$ 20,100,000	$ 8,400,000	$ 62,500,000
Wholesale revenues, non-affiliates																											17,000,000	16,000,000
Loss on Plant Scherer Unit 3																											$ 33,000,000	0
Number of regulatory clauses | clause																											4
Property, plant and equipment, gross			5,110,000,000																								$ 5,110,000,000			5,140,000,000				5,110,000,000	5,140,000,000								5,110,000,000			5,110,000,000
Other regulatory assets, current			$ 50,000,000																								$ 50,000,000			44,000,000				$ 50,000,000	44,000,000								$ 50,000,000			$ 50,000,000
Gulf Power [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Retail rate of return on common equity			9.25%																								9.25%							9.25%									9.25%			9.25%
Gulf Power [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Retail rate of return on common equity			11.25%																								11.25%							11.25%									11.25%			11.25%
Southern Company Gas [Member]
Loss Contingencies [Line Items]
Environmental remediation liability			$ 409,000,000																								$ 409,000,000							$ 409,000,000									$ 409,000,000			$ 409,000,000
Maximum borrowing capacity			2,000,000,000																								2,000,000,000							2,000,000,000									2,000,000,000			2,000,000,000
Stockholders' equity, including portion attributable to noncontrolling interest			(9,244,000,000)																								(9,244,000,000)			(9,109,000,000)				(9,244,000,000)	(9,109,000,000)								(9,244,000,000)			(9,244,000,000)
Construction work in progress			625,000,000																								625,000,000			496,000,000				625,000,000	496,000,000								625,000,000			625,000,000
Property, plant and equipment, gross			14,660,000,000																								14,660,000,000			14,508,000,000				14,660,000,000	14,508,000,000								14,660,000,000			14,660,000,000
Other regulatory assets, current			60,000,000																								60,000,000			81,000,000				60,000,000	81,000,000								60,000,000			60,000,000
Mississippi Power [Member]
Loss Contingencies [Line Items]
Increase in base rate under cost based electric tariff due to settlement												$ 7,000,000
Fuel cost recovery			27,000,000																								27,000,000			37,000,000				27,000,000	37,000,000								27,000,000			27,000,000
Maximum borrowing capacity			173,000,000																								173,000,000							173,000,000									173,000,000			173,000,000
Construction work in progress			2,570,000,000																								2,570,000,000			2,545,000,000				2,570,000,000	2,545,000,000								2,570,000,000			2,570,000,000
Wholesale revenues, non-affiliates																											62,000,000	60,000,000
Gain contingency, surcharge revenue																																			5,000,000
PSC retail rate increase (decrease)																																																			$ 2,000,000	$ 1,000,000
Property, plant and equipment, gross			4,963,000,000																								$ 4,963,000,000			4,865,000,000				4,963,000,000	4,865,000,000								4,963,000,000			4,963,000,000
Plant capacity under coal gasification combined cycle technology (in MWs) | MW																											582
Additional DOE grants																																							$ 245,000,000
Maximum cap construction cost																																							2,880,000,000
Reduced percentage interest transferred under asset purchase agreement																				15.00%																				15.00%
Approved rate increase (decrease) percentage																			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																	$ 29,000,000
Allowance for equity funds used during construction																											$ 35,000,000	29,000,000
Other regulatory assets, current			113,000,000																								113,000,000			115,000,000				113,000,000	115,000,000								113,000,000			113,000,000
Regulatory liabilities			8,000,000																								8,000,000							8,000,000									8,000,000			8,000,000
Interest bearing refundable deposit related to assets sale																																																						$ 275,000,000
Promissory note																		$ 301,000,000
Mississippi Power [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Costs due to extension of in-service date																											25,000,000
Mississippi Power [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Costs due to extension of in-service date																											35,000,000
Mississippi Power [Member] | Construction in Progress [Member]
Loss Contingencies [Line Items]
Estimate of possible loss			2,950,000,000																								2,950,000,000							$ 2,950,000,000									2,950,000,000			2,950,000,000
Atlanta Gas Light [Member]
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage							10.75%
Approved increase (decrease) in fuel rates amount							$ 20,000,000
Elizabethtown Gas [Member]
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), Amount																							$ 19,000,000
Test year duration																																		12 months
Public utilities, requested return on equity, percentage																							10.25%
Nicor Gas [Member]
Loss Contingencies [Line Items]
Maximum borrowing capacity			$ 700,000,000																								700,000,000							$ 700,000,000									700,000,000			700,000,000
Public utilities, requested rate increase (decrease), Amount						$ 208,000,000
Public utilities, requested return on equity, percentage						10.70%
Statutory time limit						11 months
Virginia Natural Gas [Member]
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage			10.25%
Florida City Gas [Member]
Loss Contingencies [Line Items]
Infrastructure replacement program, petitioned investment amount			$ 3,000,000																								3,000,000							3,000,000									3,000,000			3,000,000
Location One [Member] | Southern Company Gas [Member]
Loss Contingencies [Line Items]
Environmental remediation liability			$ 5,000,000																								5,000,000							$ 5,000,000									$ 5,000,000			$ 5,000,000
Kemper Igcc [Member]
Loss Contingencies [Line Items]
Pre-tax charge to income																											108,000,000	53,000,000
After tax charge to income																											$ 67,000,000	$ 33,000,000
Kemper Igcc [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Period of amortization of regulatory assets													36 months
Pipeline Infrastructure | mi			61																								61							61									61			61
Estimated cost									$ 68,000,000																		$ 5,570,000,000												2,880,000,000
Additional DOE grants																											(140,000,000)
Maximum cap construction cost																											$ 6,930,000,000
Purchase of interest			100.00%																								100.00%							100.00%									100.00%			100.00%
Pre-tax charge to income																											$ 108,000,000																			$ 2,870,000,000
After tax charge to income																											67,000,000																		$ 1,770,000,000
Recoverable cost above maximum cap construction cost																											3,370,000,000
Project improvement costs																														12,000,000					12,000,000
Monthly charge of allowance for equity funds used during construction																											16,000,000																$ 16,000,000
Monthly cost regulatory assets deferred																											3,000,000
Costs subject to cost cap																											2,010,000,000
Cost cap exceptions																											$ 1,500,000,000
Period of commercial operations established by discovery docket																											5 years
Average annual increase (decrease) in operations and maintenance expenses																											$ 105,000,000
Average annual increase (decrease) in maintenance capital																											44,000,000
Allowance for equity funds used during construction																																											445,000,000
Regulatory assets			$ 86,000,000																								86,000,000							$ 86,000,000									86,000,000			86,000,000
Other regulatory assets, current			111,000,000																								$ 111,000,000							111,000,000									111,000,000			111,000,000
Kemper Igcc [Member] | Mississippi Power [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Amortization period of regulatory assets and liabilities																											2 years
Kemper Igcc [Member] | Mississippi Power [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Amortization period of regulatory assets and liabilities																											10 years
Kemper Igcc [Member] | Mississippi Power [Member] | Pending Litigation [Member]
Loss Contingencies [Line Items]
Loss contingency damages sought										$ 100,000,000
Kemper Igcc [Member] | Mississippi Power [Member] | Property, Plant and Equipment [Member]
Loss Contingencies [Line Items]
Estimated cost																											$ 3,730,000,000
Kemper Igcc [Member] | Mississippi Power [Member] | Other Property And Investments [Member]
Loss Contingencies [Line Items]
Estimated cost																											6,000,000
Kemper Igcc [Member] | Mississippi Power [Member] | Fossil Fuel Stock [Member]
Loss Contingencies [Line Items]
Estimated cost																											64,000,000
Kemper Igcc [Member] | Mississippi Power [Member] | Materials And Supplies [Member]
Loss Contingencies [Line Items]
Estimated cost																											48,000,000
Kemper Igcc [Member] | Mississippi Power [Member] | Other regulatory assets current [Member]
Loss Contingencies [Line Items]
Estimated cost																											24,000,000
Kemper Igcc [Member] | Mississippi Power [Member] | Other regulatory assets, deferred [Member]
Loss Contingencies [Line Items]
Estimated cost																											173,000,000
Kemper Igcc [Member] | Mississippi Power [Member] | Other current assets [Member]
Loss Contingencies [Line Items]
Estimated cost																											1,000,000
Kemper Igcc [Member] | Mississippi Power [Member] | Other deferred charges and assets [Member]
Loss Contingencies [Line Items]
Estimated cost																											17,000,000
Kemper Igcc [Member] | Mississippi Power [Member] | Assets, Current [Member]
Loss Contingencies [Line Items]
Regulatory assets			24,000,000																								24,000,000							24,000,000									24,000,000			24,000,000
Plant Vogtle Units 3 And 4 [Member] | Georgia Power [Member]
Loss Contingencies [Line Items]
Liquidated damages, monetary amount																											1,700,000,000															$ 920,000,000
Maximum borrowing capacity																									$ 920,000,000
Construction work in progress			4,100,000,000																								4,100,000,000							4,100,000,000									4,100,000,000			4,100,000,000
Construction financing costs			1,300,000,000
Requested rate increase (decrease) amount																											400,000,000
Plant Vogtle Units 3 And 4 [Member] | Georgia Power [Member] | Pending Litigation [Member]
Loss Contingencies [Line Items]
Project settlement cost to be capitalized			350,000,000																								350,000,000							350,000,000									350,000,000			350,000,000
Plant Scherer Unit 3 [Member]
Loss Contingencies [Line Items]
Loss on Plant Scherer Unit 3																											33,000,000
After tax charge to income																											$ 20,000,000
Plant Smith Units 1 and 2 [Member] | Gulf Power [Member]
Loss Contingencies [Line Items]
Capacity of units included in request for decertification of units | MW																											357
Plant Scherer Unit Three [Member] | Gulf Power [Member]
Loss Contingencies [Line Items]
Loss on Plant Scherer Unit 3																											$ 32,500,000
Electricity Generation Plant, Non-Nuclear [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Estimated cost																										$ 2,400,000,000													$ 2,400,000,000
Mine [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Term of management fee contract																																								40 years
Subsequent Event [Member] | Georgia Power [Member]
Loss Contingencies [Line Items]
Weekly payment of services under contract agreement																						$ 5,400,000
Subsequent Event [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
AFUDC cost																													$ 18,000,000
PSC retail rate increase (decrease)																																																		$ 8,000,000
Percentage of PSC retail rate increase																																																		0.85%
Subsequent Event [Member] | Plant Scherer Unit 3 [Member] | Gulf Power [Member]
Loss Contingencies [Line Items]
Number of Intervenors | intervenor		3
MRA Revenue [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Fuel cost recovery			12,000,000																								12,000,000			13,000,000				12,000,000	13,000,000								12,000,000			12,000,000
MB Revenue [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Fuel cost recovery			1,000,000																								1,000,000			$ 1,000,000				1,000,000	$ 1,000,000								1,000,000			1,000,000
Scenario, Forecast [Member] | Georgia Power [Member]
Loss Contingencies [Line Items]
Additional construction capital costs																																												$ 5,440,000,000
Stockholders' equity, including portion attributable to noncontrolling interest																																																	$ 1,900,000,000
Amendment to estimated In-service capital cost																																												$ 5,680,000,000
Percent of basis points																																												0.95%				0.30%
Increase (decrease) in basis points																																												$ 8,000,000
Scenario, Forecast [Member] | Gulf Power [Member]
Loss Contingencies [Line Items]
Overall net customer impact from rate case settlement agreement																																	$ 54,300,000
Increase (decrease) in revenue to be received from base rate change	$ 62,000,000
Wholesale revenues, non-affiliates																																	$ 7,700,000
Retail regulatory equity ratio	0.525
Scenario, Forecast [Member] | Virginia Natural Gas [Member]
Loss Contingencies [Line Items]
Test year duration																																12 months
Scenario, Forecast [Member] | Plant Vogtle Units 3 And 4 [Member] | Georgia Power [Member]
Loss Contingencies [Line Items]
Construction financing costs																					$ 30,000,000
Monthly operational readiness costs																					$ 10,000,000
FFB Loan [Member] | Georgia Power [Member]
Loss Contingencies [Line Items]
Maximum borrowing capacity			$ 2,600,000,000.0																								$ 2,600,000,000.0							$ 2,600,000,000.0									$ 2,600,000,000.0			$ 2,600,000,000.0
Retirement Of Plant [Member] | Scenario, Forecast [Member] | Gulf Power [Member]
Loss Contingencies [Line Items]
Regulatory asset, amortization period																															15 years
Plant Scherer Unit Three [Member] | Gulf Power [Member]
Loss Contingencies [Line Items]
Electric generating units, capacity | MW			205																								205							205									205			205
Regulatory Infrastructure Program [Member] | Nicor Gas [Member]
Loss Contingencies [Line Items]
Program duration period																																					9 years
Property, plant and equipment, gross			$ 24,000,000																								$ 24,000,000							$ 24,000,000									$ 24,000,000			$ 24,000,000
STRIDE [Member] | Atlanta Gas Light [Member]
Loss Contingencies [Line Items]
Approved infrastructure replacement program			38,000,000																								$ 38,000,000							38,000,000									38,000,000			38,000,000
Integrated System Reinforcement Program [Member] | Atlanta Gas Light [Member]
Loss Contingencies [Line Items]
Infrastructure replacement program, petitioned investment amount																																																					$ 177,000,000
Integrated System Reinforcement Program [Member] | Atlanta Gas Light [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Program duration period																											4 years
Aging Infrastructure Replacement Program [Member] | Elizabethtown Gas [Member]
Loss Contingencies [Line Items]
Approved infrastructure replacement program			3,000,000																								$ 3,000,000							3,000,000									3,000,000			3,000,000
SAVE Program [Member] | Virginia Natural Gas [Member]
Loss Contingencies [Line Items]
Regulated operation, allowable cost recovery			13,000,000
Infrastructure replacement program, petitioned investment amount			44,000,000																								44,000,000							44,000,000									44,000,000			44,000,000
Approved infrastructure replacement program			$ 7,000,000																								7,000,000							$ 7,000,000									$ 7,000,000			$ 7,000,000
Cost Estimate Extension [Member] | Kemper Igcc [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Estimated cost																											67,000,000
Start-Up Fuel [Member] | Kemper Igcc [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Estimated cost																											23,000,000
Outage Work and Operational Maintenance and Improvements [Member] | Kemper Igcc [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Estimated cost																											$ 18,000,000
Wholesale [Member] | Kemper Igcc [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Recoverable cost above maximum cap construction cost, percent																											29.00%
Retail [Member] | Kemper Igcc [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Recoverable cost above maximum cap construction cost, percent																											71.00%
2017 Accounting Order Request [Member] | Kemper Igcc [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Monthly charge of allowance for equity funds used during construction																											$ 12,000,000
Monthly cost, regulatory assets and other																											25,000,000
2017 Rate Case [Member] | Kemper Igcc [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Pre-tax charge to income																											80,000,000
Alternate financing																																						$ 1,000,000,000
In-Service Asset Proposal [Member] | Kemper Igcc [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage																9.225%
Approved increase (decrease) in fuel rates amount															$ 126,000,000												$ 159,000,000
Public utilities approved equity capital structure percentage																49.733%
Customer refund																								$ 11,000,000
Denbury Onshore, Contractor [Member] | Kemper Igcc [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Long-term contract for purchase of percentage of carbon dioxide captured from plant			70.00%								100.00%																70.00%							70.00%									70.00%			70.00%
Treetop Midstream Services, LLC, Contractor [Member] | Kemper Igcc [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Long-term contract for purchase of percentage of carbon dioxide captured from plant			30.00%																								30.00%							30.00%									30.00%			30.00%
Denbury Onshore, Contractor [Member] | Electricity Generation Plant, Non-Nuclear [Member] | Mississippi Power [Member]
Loss Contingencies [Line Items]
Long-term contract for purchase of carbon dioxide term											16 years
Westinghouse [Member] | Plant Vogtle Units 3 And 4 [Member] | Georgia Power [Member]
Loss Contingencies [Line Items]
Period of notice required in the event letters of credit are not renewed																									60 days
Vogtle Owners [Member]
Loss Contingencies [Line Items]
Aggregate liability under contract agreement and removal of liens			$ 470,000,000																								$ 470,000,000							$ 470,000,000									$ 470,000,000			$ 470,000,000
Aggregate liability under contract agreement and removal of liens, amount paid or accrued to date			$ 245,000,000																								$ 245,000,000							$ 245,000,000									$ 245,000,000			$ 245,000,000