Contingencies and Regulatory Matters - Textual - Gas (Details)									1 Months Ended					12 Months Ended					48 Months Ended
	Feb. 15, 2018 USD ($)	Jan. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)	Dec. 21, 2017 USD ($)	Dec. 01, 2017 USD ($)	Oct. 23, 2017 USD ($)	Jun. 30, 2017 USD ($)	Feb. 21, 2017 USD ($)	Oct. 31, 2017 USD ($)	Oct. 31, 2016 USD ($)	Aug. 31, 2016 USD ($)	Mar. 31, 2016 USD ($) mi	Oct. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) state utility defendant site cavern pipeline mi	Dec. 31, 2015 USD ($) station mi	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2017 USD ($) state utility site cavern	Dec. 31, 2016 USD ($)	Feb. 28, 2015 USD ($)
Schedule of Regulatory Assets and Liabilities [Line Items]
Property, plant and equipment														$ 79,872,000,000					$ 79,872,000,000	$ 78,446,000,000
SOUTHERN Co GAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Number of states in which entity operates | state														7					7
Number of natural gas distribution utilities | utility														7					7
Environmental exit costs, costs accrued to date														$ 388,000,000					$ 388,000,000	426,000,000
Property, plant and equipment														11,728,000,000					$ 11,728,000,000	$ 10,565,000,000
Impairment of long-lived assets														0
Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Loss contingency, damages sought, value															$ 300,000
Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Annual proceeds from strategic economic development projects														$ 15,000,000
Public utilities, approved rate increase (decrease), amount				$ 34,000,000			$ 13,000,000	$ 20,000,000
Public utilities, approved rate increase (decrease) amount recovery of investments				$ 13,000,000
Public utilities, approved return on equity				9.50%			9.60%	10.75%
Reduction in depreciation expense							$ 3,000,000
Public utilities requested rate increase (decrease)					$ 22,000,000
Florida City Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities requested rate increase (decrease)						$ 19,000,000
Public utilities, requested return on equity percentage						11.25%
Regulated operation, allowable cost recovery						$ 3,000,000
Public utilities, interim rate increase (decrease) amount						$ 5,000,000
Pending Litigation | SOUTHERN Co GAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Number of states in which entity operates | state														1					1
Number of natural gas distribution utilities | utility														1					1
Loss contingency, number of defendants | defendant														1
Minimum | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity				9.00%				10.55%
Minimum | Pending Litigation | SOUTHERN Co GAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Loss contingency, estimate of possible loss														$ 11,000,000					$ 11,000,000
Maximum | Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Infrastructure investment, annual customer rate increase																	4.00%
Infrastructure investment, annual customer rate increase in any given year																	5.50%
Maximum | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity				10.00%				10.95%
Manufactured Gas Plants [Member] | SOUTHERN Co GAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Number of states in which entity operates | state														5					5
Number of sites | site														46					46
Location One | SOUTHERN Co GAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Environmental exit costs, assets previously disposed, liability for remediation														$ 2,000,000					$ 2,000,000
Regulatory Infrastructure Program | Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period																9 years
AIR | Elizabethtown Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period																	4 years
Fair value inputs, discount rate																	6.65%
Approved infrastructure replacement program																	$ 115,000,000
Safety, Modernization and Reliability Tariff (SMART) Plan | Elizabethtown Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Infrastructure replacement program, petitioned investment amount (more than)															$ 1,100,000,000
Regulated operations, natural gas pipeline length, considered for replacement | mi															630
Number of regulator stations, considered for replacement | station															240
SAVE | Virginia Natural Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period																		5 years
Regulated operations, natural gas pipeline length, approved for replacement | mi												200
SAVE | Maximum | Virginia Natural Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, amount per year																		$ 25,000,000
Approved infrastructure replacement program																		$ 105,000,000
SAVE | Maximum | Virginia Natural Gas | Year 2016
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year												$ 30,000,000
SAVE | Maximum | Virginia Natural Gas | Year 2017
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year												35,000,000
SAVE | Maximum | Virginia Natural Gas | Year 2018
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year												35,000,000
SAVE | Maximum | Virginia Natural Gas | Year 2019
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year												35,000,000
SAVE | Maximum | Virginia Natural Gas | Year 2020
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year												35,000,000
SAVE | Maximum | Virginia Natural Gas | Year 2021
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year												$ 35,000,000
SAFE | Florida City Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period															10 years
Approved infrastructure replacement program															$ 105,000,000
energySMART [Member] | Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period								4 years
Approved infrastructure replacement program																$ 113,000,000
Energy efficiency and term reduction expenditures																			107,000,000
Midstream Operations | Gas pipeline | SOUTHERN Co GAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Number of gas construction projects | pipeline														2
i-SRP | STRIDE | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period											4 years
Infrastructure replacement program, petitioned investment amount (more than)											$ 177,000,000
Approved infrastructure replacement program														$ 445,000,000					445,000,000
i-CGP | STRIDE | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program														91,000,000					91,000,000
i-VPR | STRIDE | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program														$ 275,000,000					275,000,000
Regulated operations, natural gas pipeline length, approved for replacement | mi														756
Regulated operations, natural gas pipeline length, considered for replacement | mi														3,300
Favorable Regulatory Action | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Litigation settlement															144,000,000
Gain contingency, surcharge revenue													$ 15,000,000
Gain contingency, surcharge revenue, annual increase									$ 15,000,000	$ 15,000,000
Proceeds from legal settlements														$ 20,000,000
True Up Recovery | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Gain contingency, unrecorded amount														187,000,000	34,000,000				187,000,000		$ 178,000,000
Interest expense															1,000,000
Selling, general and administrative expense															$ 5,000,000
True-Up Recovery, Unrecognized Equity | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Gain contingency, unrecorded amount														$ 104,000,000					$ 104,000,000
Subsequent Event | SOUTHERN Co GAS
Schedule of Regulatory Assets and Liabilities [Line Items]
Tax reform legislation, adjustment to base rate	$ 0
Subsequent Event | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount		$ 137,000,000
Public utilities, approved rate increase (decrease) amount recovery of investments		$ 93,000,000
Public utilities, approved return on equity		9.80%
Subsequent Event | Chattanooga Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities requested rate increase (decrease)	$ 7,000,000
Public utilities, requested return on equity percentage	11.25%
Subsequent Event | energySMART [Member] | Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period			4 years
Approved infrastructure replacement program			$ 160,000,000
Natural Gas Storage - Salt Dome Caverns
Schedule of Regulatory Assets and Liabilities [Line Items]
Number of salt dome caverns | cavern														2					2
Property, plant and equipment														$ 112,000,000					$ 112,000,000
Property, Plant and Equipment | Natural Gas Storage - Salt Dome Caverns
Schedule of Regulatory Assets and Liabilities [Line Items]
Concentration risk (as percent)														20.00%