Contingencies and Regulatory Matters - Narrative (Details)																				1 Months Ended		3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended										38 Months Ended	48 Months Ended	96 Months Ended	105 Months Ended
	Nov. 01, 2017 USD ($)	Oct. 23, 2017 USD ($)	Oct. 03, 2017 USD ($)	Oct. 02, 2017 USD ($)	Aug. 21, 2017 USD ($)	Aug. 15, 2017	Jul. 01, 2017	Jun. 30, 2017 USD ($)	Jun. 09, 2017 USD ($)	May 04, 2017 USD ($)	Apr. 04, 2017 intervenor	Mar. 31, 2017 USD ($)	Mar. 27, 2017	Mar. 10, 2017 USD ($)	Feb. 21, 2017 USD ($)	Dec. 20, 2016 USD ($)	May 01, 2016 USD ($)	Mar. 31, 2016	Jan. 01, 2016	Sep. 30, 2016 USD ($)	Jan. 31, 2016 USD ($)	Sep. 30, 2017 USD ($) MW	Sep. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) clause MW	Sep. 30, 2016 USD ($)	Dec. 31, 2018 USD ($)	Aug. 31, 2018	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2017 USD ($) MW	Jan. 01, 2121	Sep. 28, 2017 USD ($)	Jul. 06, 2017 USD ($)	Jun. 23, 2017 USD ($)	May 31, 2017 USD ($)	Mar. 07, 2017 USD ($)	Feb. 17, 2017 USD ($)	Aug. 31, 2016 USD ($)
Loss Contingencies [Line Items]
Lawsuit deferral period						30 days							30 days
Maximum borrowing capacity																						$ 8,145,000,000			$ 8,145,000,000															$ 8,145,000,000
Stockholders' equity, including portion attributable to noncontrolling interest																				$ (26,180,000,000)		(25,939,000,000)	$ (26,180,000,000)		(25,939,000,000)	$ (26,180,000,000)					$ (26,612,000,000)	$ (21,982,000,000)							$ (26,612,000,000)	(25,939,000,000)
Property, plant and equipment, gross																						102,014,000,000			102,014,000,000						98,416,000,000								98,416,000,000	102,014,000,000
SOUTHERN POWER CO
Loss Contingencies [Line Items]
Maximum borrowing capacity																						750,000,000			750,000,000															750,000,000					$ 600,000,000
Stockholders' equity, including portion attributable to noncontrolling interest																						(5,843,000,000)			(5,843,000,000)						(5,675,000,000)								(5,675,000,000)	(5,843,000,000)
Wholesale revenues, non-affiliates																						510,000,000	387,000,000		1,293,000,000	866,000,000
Property, plant and equipment, gross																						13,734,000,000			13,734,000,000						12,728,000,000								12,728,000,000	13,734,000,000
GEORGIA POWER CO
Loss Contingencies [Line Items]
Environmental remediation liability																						26,000,000			$ 26,000,000						17,000,000								17,000,000	26,000,000
Cost recovery, new nuclear																																														$ (50,000,000)
Percentage of Proportionate share owed in consortium agreement																																				45.70%
Liquidated damages, percentage																									40.00%											10.00%
Maximum borrowing capacity																						1,750,000,000			$ 1,750,000,000															1,750,000,000
Estimated in-service capital cost																																			$ 4,418,000,000
Financing costs collected, net of tax																									1,500,000,000
Additional construction capital costs																$ 3,300,000,000								$ 542,000,000															3,900,000,000
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%
Financing costs incurred																																								1,500,000,000
Estimated cancellation costs																																								350,000,000
Wholesale revenues, non-affiliates																						45,000,000	49,000,000		124,000,000	131,000,000
Property, plant and equipment, gross																						34,589,000,000			34,589,000,000						33,841,000,000								33,841,000,000	34,589,000,000
GEORGIA POWER CO | Minimum
Loss Contingencies [Line Items]
Estimated financing costs																																								3,400,000,000
GEORGIA POWER CO | Other current liabilities
Loss Contingencies [Line Items]
Over recovered fuel balance																						100,000,000			100,000,000															100,000,000
GEORGIA POWER CO | Other deferred credits and liabilities
Loss Contingencies [Line Items]
Over recovered fuel balance																															84,000,000								84,000,000
GULF POWER CO
Loss Contingencies [Line Items]
Environmental remediation liability																						53,000,000			53,000,000						44,000,000								44,000,000	53,000,000
Accrued storm reserve damaged property costs																						40,000,000			40,000,000															40,000,000
Maximum borrowing capacity																						$ 280,000,000			$ 280,000,000															$ 280,000,000
Retail rate of return on common equity																						10.25%			10.25%															10.25%
Reduction in depreciation expense																								34,000,000							0	$ 20,100,000	$ 8,400,000	$ 62,500,000
Wholesale revenues, non-affiliates																						$ 14,000,000	17,000,000		$ 44,000,000	48,000,000
Retail regulatory equity ratio							0.525
Loss on Plant Scherer Unit 3																						0	0		$ 33,000,000	0
Number of regulatory clauses | clause																									4
Property, plant and equipment, gross																						$ 5,181,000,000			$ 5,181,000,000						5,140,000,000								5,140,000,000	$ 5,181,000,000
GULF POWER CO | Minimum
Loss Contingencies [Line Items]
Retail rate of return on common equity																						9.25%			9.25%															9.25%
GULF POWER CO | Maximum
Loss Contingencies [Line Items]
Retail rate of return on common equity																						11.25%			11.25%															11.25%
SOUTHERN Co GAS
Loss Contingencies [Line Items]
Environmental remediation liability																						$ 399,000,000			$ 399,000,000						426,000,000								426,000,000	$ 399,000,000
Maximum borrowing capacity																						1,900,000,000			1,900,000,000															1,900,000,000
Stockholders' equity, including portion attributable to noncontrolling interest																						(9,167,000,000)			(9,167,000,000)						(9,109,000,000)								(9,109,000,000)	(9,167,000,000)
Property, plant and equipment, gross																						15,383,000,000			15,383,000,000						14,508,000,000								14,508,000,000	15,383,000,000
MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Gain contingency, surcharge revenue																															5,000,000
Increase in base rate under cost based electric tariff due to settlement																	$ 7,000,000
AFUDC cost																									22,000,000
Fuel cost recovery																						2,000,000			2,000,000						37,000,000								37,000,000	2,000,000
Percentage of PSC retail rate increase																																											(0.85%)
Maximum borrowing capacity																						100,000,000			100,000,000															100,000,000
Wholesale revenues, non-affiliates																						72,000,000	78,000,000		196,000,000	198,000,000
PSC retail rate increase (decrease)																																											$ 8,000,000
Maximum annual percentage increase in revenue										2.00%
Maximum annual increase in revenue										$ 18,000,000
Deferred revenue										$ 26,000,000
Property, plant and equipment, gross																						4,836,000,000			4,836,000,000						4,865,000,000								4,865,000,000	4,836,000,000
ALABAMA POWER CO
Loss Contingencies [Line Items]
Maximum borrowing capacity																						1,335,000,000			1,335,000,000															1,335,000,000
Wholesale revenues, non-affiliates																						77,000,000	$ 82,000,000		210,000,000	$ 211,000,000
Property, plant and equipment, gross																						26,619,000,000			26,619,000,000						26,031,000,000								26,031,000,000	26,619,000,000
ALABAMA POWER CO | Regulatory assets, deferred
Loss Contingencies [Line Items]
Over (under) recovered environmental clause																																															$ 36,000,000
Atlanta Gas Light
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage															10.75%
Annual proceeds from strategic economic development projects															$ 15,000,000
Approved increase (decrease) in fuel rates amount															$ 20,000,000
Elizabethtown Gas
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amount								$ 13,000,000												$ 19,000,000
Test year duration																														12 months
Public utilities, requested return on equity, percentage								9.60%												10.25%
Public utilities, property, plant and equipment, accumulated depreciation								$ 3,000,000																$ 3,000,000													$ 3,000,000
Nicor Gas
Loss Contingencies [Line Items]
Maximum borrowing capacity																						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%
Virginia Natural Gas
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage												10.25%
RE Roserock, LLC | SOUTHERN POWER CO
Loss Contingencies [Line Items]
Ownership percentage by parent																																51.00%
Loss contingency, payments																									26,000,000
Location One | SOUTHERN Co GAS
Loss Contingencies [Line Items]
Environmental remediation liability																						5,000,000			5,000,000															5,000,000
Natural Gas Storage
Loss Contingencies [Line Items]
Public utilities, property, plant and equipment, net																						111,000,000			$ 111,000,000															111,000,000
Natural Gas Storage - Salt Dome Caverns
Loss Contingencies [Line Items]
Public utilities, property, plant and equipment, percentage of total assets																									20.00%
Kemper IGCC | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Estimated Cost Incurred After in Service Date																																													$ 175,000,000
Period of amortization of regulatory assets																		36 months
AFUDC cost																																					$ 494,000,000
Approved increase (decrease) in fuel rates amount					$ 126,000,000
Transmission and Distribution Facilities | GEORGIA POWER CO
Loss Contingencies [Line Items]
Regulatory assets related to storm damage																						360,000,000			$ 360,000,000															360,000,000
Asset recovery damaged property costs																						150,000,000			150,000,000															150,000,000
Accrued storm reserve damaged property costs																						$ 145,000,000			145,000,000															$ 145,000,000
Annual accrual for asset recovery damaged property costs																									30,000,000
Plant Vogtle Units 3 And 4
Loss Contingencies [Line Items]
Guarantor obligations									$ 3,680,000,000.00
Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Loss Contingencies [Line Items]
Liquidated damages, monetary amount																									$ 1,700,000,000											$ 920,000,000
Liquidated damages, monetary amount Received, based on percentage of ownership																																				$ 420,000,000
Maximum borrowing capacity																					$ 920,000,000
Percentage of ownership interest required for voting for continuing construction																						90.00%			90.00%															90.00%
Public utilities, increase in construction budget																									$ 1,000,000,000
Extension in project schedule																									1 year
Percentage of approval required to change primary construction contractor																						90.00%			90.00%															90.00%
Percentage of approval required for material amendments																						67.00%			67.00%															67.00%
Guarantor obligations									$ 1,700,000,000.0
Requested rate increase (decrease) amount																									$ (400,000,000)
Plant Scherer Unit 3
Loss Contingencies [Line Items]
Loss on Plant Scherer Unit 3																									$ 33,000,000
Plant Scherer Unit 3 | GULF POWER CO
Loss Contingencies [Line Items]
Number of intervenors | intervenor											3
Plant Smith Units 1 and 2 | GULF POWER CO
Loss Contingencies [Line Items]
Capacity of units included in request for decertification of units | MW																									357
Plant Scherer Unit Three | GULF POWER CO
Loss Contingencies [Line Items]
Loss on Plant Scherer Unit 3																									$ 32,500,000
MRA Revenue | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Fuel cost recovery																						$ 3,000,000			3,000,000						$ 13,000,000								$ 13,000,000	$ 3,000,000
Westinghouse | Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Loss Contingencies [Line Items]
Period of notice required in the event letters of credit are not renewed																					60 days
Vogtle Owners | Removal of Subcontractor Liens
Loss Contingencies [Line Items]
Loss contingency, payments																									340,000,000
Estimate of possible loss																						386,000,000			386,000,000															386,000,000
Vogtle Owners | GEORGIA POWER CO | Removal of Subcontractor Liens
Loss Contingencies [Line Items]
Estimate of possible loss																						176,000,000			176,000,000															176,000,000
Vogtle Owners | Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Loss Contingencies [Line Items]
Period of notice required in the event letters of credit are not renewed									30 days
Restatement Adjustment | Toshiba
Loss Contingencies [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest																																												$ 4,500,000,000
Scenario, Forecast | GEORGIA POWER CO
Loss Contingencies [Line Items]
Additional construction capital costs																																						$ 5,440,000,000
Percent of basis points																																						0.95%			0.30%
Decrease in basis points																																						$ 8,000,000
Amendment to estimated in-service capital cost																																						$ 5,680,000,000
Scenario, Forecast | GULF POWER CO
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																											$ 63,000,000		62,000,000
Wholesale revenues, non-affiliates																													$ 7,700,000
Scenario, Forecast | Virginia Natural Gas
Loss Contingencies [Line Items]
Test year duration																												12 months
Scenario, Forecast | Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Loss Contingencies [Line Items]
Increase in NCCR tariff	$ 90,000,000
FFB Loan | GEORGIA POWER CO
Loss Contingencies [Line Items]
Long-term line of credit																						2,600,000,000			2,600,000,000															2,600,000,000
Maximum borrowing capacity																						$ 3,460,000,000.00			$ 3,460,000,000.00															$ 3,460,000,000.00
Conditional borrowing commitment																																										$ 1,670,000,000
Retirement Of Plant | Scenario, Forecast | GULF POWER CO
Loss Contingencies [Line Items]
Regulatory asset, amortization period																											15 years
Plant Scherer Unit Three | GULF POWER CO
Loss Contingencies [Line Items]
Electric generating units, capacity | MW																						205			205															205
Energy Efficiency Cost Rider | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
PSC retail rate increase (decrease)																																											$ 2,000,000
SAVE Program | Virginia Natural Gas
Loss Contingencies [Line Items]
Infrastructure replacement program, petitioned investment amount												$ 44,000,000																		$ 44,000,000
Regulated operation, allowable cost recovery												$ 13,000,000
Approved infrastructure replacement program																						$ 21,000,000			$ 21,000,000															$ 21,000,000
Regulatory Infrastructure Program | Nicor Gas
Loss Contingencies [Line Items]
Program duration period																																	9 years
Property, plant and equipment, gross																						178,000,000			178,000,000															178,000,000
STRIDE | Atlanta Gas Light
Loss Contingencies [Line Items]
Approved infrastructure replacement program																						127,000,000			$ 127,000,000															127,000,000
Integrated System Reinforcement Program | Atlanta Gas Light
Loss Contingencies [Line Items]
Infrastructure replacement program, petitioned investment amount																																																$ 177,000,000
Integrated System Reinforcement Program | Atlanta Gas Light | Minimum
Loss Contingencies [Line Items]
Program duration period																									4 years
Aging Infrastructure Replacement Program | Elizabethtown Gas
Loss Contingencies [Line Items]
Approved infrastructure replacement program																						16,000,000			$ 16,000,000															16,000,000
Safety, Access, and Facility Enhancement Program | Florida City Gas
Loss Contingencies [Line Items]
Approved infrastructure replacement program																						$ 9,000,000			$ 9,000,000															$ 9,000,000
Subsequent Event | Virginia Natural Gas | Minimum
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage			9.00%
Subsequent Event | Virginia Natural Gas | Maximum
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage			10.00%
Subsequent Event | Virginia Natural Gas | Midpoint
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage			9.50%
Subsequent Event | Florida City Gas
Loss Contingencies [Line Items]
Public utilities, requested rate increase (decrease), amount		$ 19,000,000
Public utilities, requested return on equity, percentage		11.25%
Regulated operation, allowable cost recovery		$ 3,000,000
Public utilities, interim rate		$ 5,000,000
Subsequent Event | Plant Vogtle Units 3 And 4
Loss Contingencies [Line Items]
Proceeds from guarantor obligations				$ 300,000,000
Subsequent Event | Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Loss Contingencies [Line Items]
Proceeds from guarantor obligations				$ 137,000,000
Subsequent Event | SAVE Program | Virginia Natural Gas
Loss Contingencies [Line Items]
Infrastructure replacement program, petitioned investment amount			$ 34,000,000
Regulated operation, allowable cost recovery			$ 13,000,000